Home

Induction motors fed by PWM frequency converters

1. ot YV time tN E E time So the rise time tr has a direct influence on the insulation life because the faster the pulse wavefront grows the greater the av dt ratio over the first coil and the higher the levels of voltage between turns causing the insulation system to wear more quickly away Thus the motor insulation system should present superior dielectric characteristics in order to stand the elevated voltage gradients occurring on PWM environment 6 5 1 1 Normative considerations about rise time The definitions of rise time tr according to NEMA and to IEC Standards differ as shown below allowing for interpretation divergences and conflicts between manufacturers and users of motors and drives NEMA MG1 Part 30 Uy peak ON motor terminals UDG tink trs tr time needed for the voltage to rise from 10 to 90 of the DC link voltage 1 41V ateg Induction motors fed by PWM frequency inverters 15 lu P www weg net NEMA definition of dV dt Supposing the motor voltage Vrated 460 V Vin DC 1 41 x 460 648 6 V AV 0 8 x 648 6 518 9 V Assuming that rise time 0 1us At O 1us dV _ AV 252 sigo dt At 0 1 us IEC 60034 25 LE 100 907 10 0 Hz tr time needed for the voltage to rise from 10 to 90 of the peak voltage at motor terminals IEC definition of dV dt Supposing the motor voltage V_ 4 460 V with incidence of 1200 V peaks AV 0 8 x 1200
2. 960 V Assuming tr 0 25us Oe Oe A 3840 us dt At 0 25 NOTE Due to the cable the rise time is higher at the motor terminals than at the inverter terminals However a very common mistake in the dV dt calculation is to consider the rise time at the inverter terminals and the voltage peak at the motor terminals resulting in an unlikely dV dt value For instance considering tr 0 1 us typical value found at the inverter in the case above it would result dV dt 9600 V us Owing to the differences existing between the rise time definitions given by NEMA and IEC misunderstandings often happen when calculating the voltage gradient dV dt 16 According to NEMA criterion the DC link voltage 1 41 Vin must be taken as 100 voltage reference for the determination of rise time and the calculation of dV dt According to IEC criterion however the peak voltage arriving at the motor terminals is what must be taken as 100 voltage reference Due to the cable the rise time to be considered in IEC criterion will be normally higher than the one considered in NEMA criterion which is the value informed by the inverter manufacturer Thus depending on the criteria considered throughout the calculations pretty different values of dV dt are likely to be attributed to the same situation The insulation criteria defined for WEG motors are based on NEMA in order not to depend on the final customer installation Furthermore the
3. Nantong Economic amp Technical Development Zone Nantong Jiangsu Province Phone s 86 0513 85989333 Fax 86 0513 85922161 info cn weg net www weg net cn COLOMBIA WEG COLOMBIA LTDA Calle 46A N82 54 Porter a Il Bodega 7 San Cayetano Il Bogota Phone s 57 1 416 0166 Fax 57 1 416 2077 info co weg net www weg net co WEG Equipamentos El tricos S A International Division FRANCE WEG FRANCE SAS Z de Chenes Le Loup 13 Rue du Morellon BP 738 38297 Saint Quentin Fallavier Phone s 33 0 4 74 99 11 35 Fax 33 0 4 74 99 11 44 info fr weg net www weg net fr GERMANY WEG GERMANY GmbH Industriegebiet Turnich 3 GeigerstraBe 7 50169 Kerpen Turnich Phone s 49 0 2237 9291 0 Fax 49 0 2237 9292 200 info de weg net www weg net de INDIA WEG Electric India Pvt Ltd 38 Ground Floor 1st Main Road Lower Palace Orchards Bangalore 560 003 Phone s 91 80 4128 2007 91 80 4128 2006 Fax 91 80 2336 7624 info in weg net www weg net in ITALY WEG ITALIA S R L V le Brianza 20 20092 Cinisello Balsamo Milano Phone s 39 02 6129 3535 Fax 39 02 6601 3738 info it weg net www weg net it JAPAN WEG ELECTRIC MOTORS JAPAN CO LTD Matsumoto Bldg 2F 3 23 7 Kamata Ohta ku Tokyo Japan 144 0052 Phone s 81 3 3736 2998 Fax 81 3 3736 2995 info jp weg net www weg net jp MEXICO WEG MEXICO S A DE C V Carretera Jorobas Tula Km 3 5 Manzana 5 Lot
4. lso maximum short current current at PCC maximum demand load current fundamental frequency component at PCC ISC IL HWeh H7 lt h lt 23 lt h lt 39m TDD 23 35 ar sas CA 20 lt 50 0 3 5 2 5 ocw oo as 40 is o7 120 00100 120 ss so 20 10 150 1000 foso zo 60 All power generation equipment is limited to these values of current distortion regardless of actual lz li SJ The documents mentioned from IEC however do not set limits for the harmonic distortion injected by inverters into the power line 5 2 Line reactor DC bus choke Harmonic currents which circulate through the power line impedances and depend on the rectifier input output impedance values cause harmonic voltage drops that distort the power supply voltage of the inverter and other loads connected to this line These harmonic current and voltage distortions may increase the electrical losses in the installation lowering the power factor and overheating components such as cables transformers capacitor banks motors etc The addition of a line reactor and or a DC bus choke reduces the harmonic content of the current and increase the power factor The DC bus choke has the advantage of not introducing a motor voltage drop but depending on the combination of its value with the power line impedance and the DC link capacitance values it may result in undesirable resonances within the overall system On the other hand t
5. of the PWM voltage waveform at the motor terminals gives an idea about the pulses quality at the motor terminals For a better verification of the consistency of these pulses the visualization of two or three cycles is recommended once it evidences the repetitiveness of such pulses A detailed analysis of a single pulse finally allows that conclusions about the rise time and the intensity of the peak voltages be found 9 5 2 Oscilloscope scale setting The better choice of which scale should be adopted while taking measurements will evidently depend on the magnitudes of the electrical quantities being measured However the ranges shown in the table below are commonly suitable for 50 60 Hz measurements and can be used as a first orientation Suggestions of oscilloscope s scale setting 1 lt 2ms div 100 500V div 5 lt 10ms div 100 lt gt 500V div 0 1 lt 10us div 100 lt 500V div 9 5 3 Triggering Oscilloscopes are instruments ordinarily employed for metering and not for monitoring electrical quantities In spite of that the trigger of some modern oscilloscopes can be suitably set so that it is enabled to hold data of particular interest for instance waveforms of voltage peaks taken during transient conditions such as acceleration and deceleration periods Further information on this topic can be found in the User s Manual of the instrument Technical guide Induction motors fed by PWM frequency invert
6. NEMA criterion seems appropriate for considering just the linear stretch of the curve to approximate the derivative dV dt AV At The IEC criterion considers the peak voltage at the motor terminals something extremely complicated to be predicted or estimated a priori The rise time at the motor terminals is increased by the cable high frequency impedance The dV dt ratio at the motor terminals milder than at the drive terminals can be also calculated but it requires a reliable measurement of the voltage pulses at the motor leads and most of times this is not easily accomplished or not even feasible demanding a technician familiar with such applications equipped with a good oscilloscope 6 5 2 Cable length Beside the rise time the cable length is a predominant factor influencing the voltage peaks occurrence at the inverter fed motor terminals The cable can be considered a transmission line with impedances distributed in sections of inductances capacitances series parallel connected At each pulse the inverter delivers energy to the cable charging those reactive elements CONVERTER CABLE L L L a ds dei nul dins om mes co a E G Pay c i or E E P IUS The signal arriving at the motor through the cable is partially reflected causing overvoltage because the motor high frequency impedance is greater than the cable impedance Excessively long leads increase the overshoots at the motor terminals According
7. Non Po P N sist gt ar P P P E 1 conv 1 mot P P N conv Some practical values found by means of the input output measurement method are shown below for standard motors Motor 75 HP 55 kW 6 poles 400 V 50 Hz ae Efficiency of the system converter motor 95 90 85 80 75 70 Eff 0 25 50 75 100 125 150 Load 12 5Hz 25Hz 4 50Hz 67 5Hz s 50Hz sen Motor 15 HP 11 kW 4 poles 400 V 50 Hz Efficiency of the system converter motor e S w pe y 75 100 125 150 Load 9 e 12 5Hz 2 25H 50Hz lt 62 5Hz 0 50Hz sen 6 2 2 Normative considerations about the efficiency of inverter fed motors NEMA MG1 Part 30 Efficiency will be reduced when a motor is operated on a bus with harmonic content The harmonics present will increase the electrical losses which in turn decrease efficiency This increase in losses will also result in an increase in motor temperature which further reduces efficiency Po P constant P gt gt P __ gt Losses constant y a J P 12 Technical guide Induction motors fed by PWM frequency inverters NEMA MG1 Part 31 Performance tests when required shall be conducted on a sinusoidal power supply unless otherwise specified by mutual agreement between the manufacturer and the user NEMA Application Guide for AC ASD Systems The overall efficiency of an ASD is
8. dV dt according to each institution One can notice that the insulation criteria from both IEC and GAMBICA take into account the cable length information which WEG also considers relevant 6 8 Recommendations for the cables connecting WEG motors to inverters As already mentioned the maximum peak voltage appearing at the terminals of the inverter fed motor depends on many factors predominantly the cable length When supplying WEG motors with inverters the following practical rules are suggested for the evaluation of the need of using filters between motor and inverter 100m Not needed Output reactor needed 100 m lt L lt 300m at least 2 voltage drop L gt 300m Special filters needed contact WEG The output reactor is necessary for the eddy current that flows from inverter to earth to be limited The input line reactor prevents the inverter ground fault from tripping The output reactor design must take account of additional losses occurring due to current ripple and current leakage to earth which increases as cable length rises For long cables and reactors designed for small currents there will be great influence of the leakage currents on the reactor losses and heating The cooling system of the inverter panel must also take the reactors additional losses into account for a safe temperature operation to be assured The output reactor must be installed near the inverter as shown below Technical guide In
9. keep their efficiency higher compared to standard motors when both are fed by inverters Technical guide Induction motors fed by PWM frequency inverters 11 lu P www weg net 6 2 1 The influence of the speed variation on the motor efficiency The effects of speed variation on the motor efficiency can be understood from the analysis of the behavior of the inverter fed motor output power as a function of its operation speed Fout WV out fe f op Hz Supposing for instance a 60 Hz frequency base for the situations outlined above P 60Hz Pi Prone Py 05 P 60 30 Considering that the motor losses are essentially comprised of Joule losses P and iron losses P and assuming that the Joule losses prevail then the motor efficiency fall at low speeds where the motor output power is reduced and despite the slight decrease of the iron losses frequency dependant the Joule losses current square dependant are kept nearly constant for a constant torque load so that after all there is no significant variation of the overall losses The equations next explain that Defining efficiency as on ar n EET P P T gt Losses in And according to the exposed above X Losses P P U Eo iron Then the following situation results from speed reduction 6 2 2 1 Numerical example converter P 1 Li P e aa a A F e i P ete gk 7 yo 3 2 ea ls HH motor E E A e y P A Bg E
10. of the frame sizes 447 and 449 587 Ka G 60 120 Hz G Induction motors fed by PWM frequency inverters 13 lu gt www weg net NOTE 1 The speed ranges stated above are related to the motor thermal capability only Speed regulation will depend on VFD mode of operation and proper adjustment 2 W21 and NEMA PREMIUM EFFICIENCY WEG MOTORS of all frame sizes can also be blower cooled under request In such case the motor will be suitable for variable and constant torque applications rated up to 1000 1 with any drive 3 W21 and NEMA PREMIUM EFFICIENCY WEG MOTORS comply with those maximum safe operating speeds established in NEMA MG1 Parts 30 and 31 2003 The relations set above describe operation speed ranges Supposing for instance a 60 Hz base frequency the following equivalence is valid 6 4 1 2 IEC market Constant flux condition Encompassed motor lines Totally enclosed off the shelf motors attending IE1 as per IEC 60034 30 or higher efficiency levels Tr Torque reduction p u 00 01 02 03 04 05 06 07 O8 09 10 11 12 13 14 15 16 17 18 19 20 2 1 f m Frequency p u for temperature nse of thermal class F 105 K for temperature nse of thermal c hss B 80 K Optimal flux condition Encompassed motor lines Totally enclosed off the shelf motors attending IE2 as per IEC 60034 30 or higher efficiency levels The patented WEG Optimal Flux solution was develope
11. optimization providing a final product of better quality Aloof control the control can be installed remotely at a suitable location keeping just the motor in the processing area on the contrary of hydraulic and mechanical varying speed systems torque demand at low speeds the motor voltage is decreased to compensate for the efficiency reduction normally resultant from load reduction With constant torque or constant power loads the system efficiency improvement comes from the feasibility of continuous adjustment of speed with no need to use multiple motors or mechanical variable soeed systems Such as pulleys and gears which introduce additional losses 4 Characteristics of PWM frequency inverters 4 1 General PWM voltage source static frequency inverters presently comprehend the most used equipments to feed low voltage industrial motors in applications that involve speed variation They work as an interface between the energy source AC power line and the induction motor constant amplitude and frequency coming from the power grid DC link or filter Regulation smoothing of the rectified signal with energy storage through a capacitor bank In order to obtain an output signal of desired voltage and frequency the input signal must accomplish three stages within a frequency inverter IGBT power transistors Inversion of the voltage coming from the link DC into an alternate signal of variable amplitude and fr
12. to the NEMA Application Guide for AC ASD Systems with the modern IGBT controls overshoots begin to occur with a cable length of a few feet and can reach 2 times the control DC bus voltage at a length less than 50 feet In some cases however very long cables in excess of 400 feet for example can result in a situation where the overshoot does not decay quickly enough In this case the voltage peak at the motor terminals can ring up well beyond 2 times the inverter DC link voltage This behavior is a function of the PWM pulse pattern the rise time and the very Technical guide Induction motors fed by PWM frequency inverters cable type Voltage measurements realized at the inverter terminals O ft cable and at the motor V_ g 400 V terminals with different cable lengths are presented next The overshoots also depend on the type of cable used in the installation therefore the waveforms shown below are illustrative only Converter terminals 65 5 ft cable 1 Ref A 200 Volt Zus 1 Ref B 200 Volt 2us Voed 560 V Nee 630 Y 98 5 ft cable 328 ft cable 1 Ref B 200 Volt 2 us 1 Ref B 200 Volt 2 us Vopak 90V Voen 990 V 6 5 2 1 Corona effect Depending on the quality homogenelty of the impregnation the impregnating material may contain voids cavities in which the failure mechanism of the interturn insulation develops The deterioration of the motor insulating system due to the voltage overshoots occurs by means of Par
13. torque Centrifugal pumps present a torque that characteristically varies at a rate proportional to the square of the speed while the horsepower varies as the cube of the speed In this case the motor must be dimensioned for the highest speed within the operation range of the pump because the maximum torque demand for the motor happens there The figure next shows that this example allows two alternatives for the dimensioning a 2 pole motor or else a 4 pole motor The 2 pole motor would operate at the constant torque region below base speed while the 4 pole motor would operate at field weakening region above base speed Technical guide Induction motors fed by PWM frequency inverters 27 lu P www weg net Torque Nm Consulting the WEG NEMA Motor Catalog the most the 10 hp 7 5 kW 4 poles 60 Hz frame 215 T Premium Efficiency 8 2 2 2 Regarding the insulation system 1800 2700 3600 Speed rpm Voltage V The torque required by the pump at maximum speed is given i below y y C aaaa ANA 1 41 460 650 5V Pp p non de N DC link 16P Np T 716 10 2 09 kgfm 25 99 Nm 0 9 650 5 5855V Peo f f oo T kgfm AL n rom e 2700 2 pole motor 0 1 650 5 65 05 V 2 00 rom 0 75 p u gt 45 Hz II time s According to the derating criteria of WEG NEMA Premium 0 14s a Efficiency TEFC motors subclause 6 4 1 1 any WEG NPE m
14. 0 P 17 58 kW 23 58 hp 9 51 960 Consulting the WEG Electric Motors Manual for 3600 rpm and 60 Hz the ideal motor for this application is a three phase 18 5 kW 25 hp 2 poles 60 Hz frame IEC 160M NEMA 284T TEFC It was thereby shown that the optimal flux solution provides a better utilization of the energy allowing for a smaller frame motor to attend the application needs yet not using forced ventilation or oversizing 9 Recommendations for the measurement of PWM waveforms 9 1 Warning The measurements dealt with in this clause involve potentially lethal voltage and current levels Only qualified individuals familiar with the construction and operation of the equipment and hazards involved should take these measurements 9 2 Instrumentation Frequency inverters supply motors with PWM voltage which is non sinusoidal Measurements of such voltages must be taken with proper equipments in order to be reliable Modern digital measurement instruments that are able to read true rms values must be used Some of them will not read the fundamental component of a PWM waveform though Harmonic measurement instruments with fast enough sampling rate are capable of reading both rms and fundamental values of voltage current and power n oscilloscope with isolated probes and proper bandwidth is appropriate in most cases Technical guide Induction motors fed by PWM frequency inverters 9 3 Parameter measurements According to th
15. 2 1 0 0 8 0 6 0 4 Upk kV O25 tr us IEC 368 07 A Valid for motors up to 500 Vac without filters B Valid for motors up to 690 Vac without filters C Measured results at 415 Vac supply with different cable lengths GAMBICA REMA the European association of motors REMA and inverters GAMBICA manufacturers set the criteria shown next based on its members experience www weg net p MOTOR PULSE WITHSTAND CHARACTERISTIC CURVES PEAK VOLTAGE RISE TIME Motor Cable Lengths mm Typical test results with SWA motor cable lengths 5 20 30 50 and 100m Curve B 415V a c supply E 480V a c supply A 690V a c supply Voltage rise time withstand requirements for motors fed from PWM drives with supply of lt 500V a c Curve A 500 690V a c Curve B S E o o lu O gt x o O A Voltage rise time withstand capability given in IEC 60034 17 1998 0 4 0 6 0 8 1 0 12 Voltage Pulse Rise Time us otes i Motor pulse withstand requirements on 415V supply generally exceed the minimum capability specified in IEC 60034 17 if Pulse risetime is in accordance with the IEC 60034 17 definition These curves are based on the practical experience of GAMBICA and REMA members It is remarkable the similarities existing between IEC and GAMBICA criteria as well as their disparity with respect to NEMA criteria This results from the particular definitions of rise time and
16. 3 kHz For lower switching frequencies the noise increase may be tremendous up to 15 dB A by experience In some circumstances it may be necessary to create skip bands in the operating speed range in order to avoid specific resonance conditions due to the fundamental frequency 6 15 Influence of the inverter on the mechanical vibration of the motor Interactions between currents and flux harmonics may result in stray forces actuating over the motor causing mechanical vibration and further contributing to increase the overall noise levels This mechanism gains importance especially when amplified by mechanical resonances within the motor or the driven machine If any of the non fundamental harmonics is near the natural frequencies of the motor the forces produced can excite vibration modes Such effects can be attenuated with a careful design of the motor with respect to the stator and rotor slots lamination and frame always looking out for simplifying the mechanical system thus reducing the possibility of exciting natural frequencies that develops modes of vibration within the motor Modern frequency inverters are also provided with tools to 24 get those problems around so that for instance specific frequencies within the operating range can be skipped and the acceleration deceleration times can be conveniently adjusted 6 16 Criteria regarding the vibration levels presented by WEG motors on VSD applications Tests re
17. KW 0 95 www weg net lu gt Consulting the WEG Stock Products Catalog the standard motor which better fits the application has 22 kW and 2 poles If the duty cycle were continuous with full load full time and no speed variation the dimensioning would be well done so and already concluded Nevertheless the actual duty cycle embraces speed changes and different load percentages Therefore in order to achieve a suitable thermal dimensioning the load demand at every operating condition must be analyzed so that a motor equivalent torque can be finally calculated considering the whole duty cycle Once obtained the equivalent torque it must be assured that the chosen motor will be able to provide the maximum horsepower demanded throughout the operation duty Assuming that the temperature rise is directly proportional to the losses and that the Joule losses comprise the prevailing component of motor losses then the losses vary as the square of the speed and the equation below is true where Toy equivalent torque of the motor T torque demanded by the load at each operating speed df derating factor to be applied at each operating speed due to the temperature rise increase occasioned by both harmonics and ventilation reduction t period of each duty stretch considered as below Lalat t k t time intervals with motor running either loaded or not t SUM of time intervals with motor stopped kv constant value that depen
18. Motors Energy Automation Coatings Induction motors fed by Technical guide PWM frequency inverters IE BREED JEA EO E lu P www weg net Table of contents 2 1 Ze 2 2 4 4 1 4 2 5 1 9 1 1 5 2 6 1 6 1 1 6 2 O23 O22 6 3 6 4 6 4 1 6 4 2 6 4 3 6 5 6 5 1 6 5 2 6 5 3 6 5 4 6 5 5 6 6 6 6 8 6 8 1 6 9 6 9 1 0 9 2 6 9 3 6 10 6 11 6 12 6 13 6 14 2 LE EELEE E EE eE EE BP PO o AEE AEA AEA AA AAE EAE EEA 4 NO Maier earn ine vee ston cae R E eee aan een eee 5 NEMA MG1 Motors and generators United States ccccescsssssssssssssscsssssesseesseessesseesseeseeeseeeseesseeseeeseessaessaeseeeserssrsanssaeess 5 NEMA Application Guide for AC Adjustable Speed Drive SySteMS ccoccccnoccnoonononononconanonenonnnnonnono nono nnonanono nono nonenrnnnronnnonanons 9 IEC 60034 Rotating Electrical Machines INternaCiOnall ccccccccsscssssssssscssscssesssessesseesesessssseesssessaessaeseeeseessaeseaesensseessaesaass 5 Other technical documents Of FEfEFENCE ceccsescssssssssscsssecssesecssecssessesseessaessuecseeseeseaecaesseeseessaeseaesaueseessaeseaesussaessaeseaesesesenssaesags 5 INnauctuon MACHINES speed Va rasa ias d Gnaractensuos of PWM freguency Mvet Susana e EEA ENE E EA 4 Eras Emenee ce een eet a RT fal eR oe ee ee ee ee eee eee ee ee eS cn AA ra Ei a 8 Interaction between inverter and AC power liN ccocccccnoccnocncnonononanonnconnnononononononon
19. able torque loads are good candidates to apply VSDs for energy savings once that the mechanical power available at the motor output will not be constant it will actually vary Suitably in accordance with the load demand as shown before in Clause 3 of this technical guide Squared torque variation Es p 2 in o o z Per Unit Torque or HP of Driven Load N 0 00 0 25 0 50 0 75 1 00 1 25 Per Unit of Base Speed www weg net lu gt Torque varies at a rate proportional to the square of the speed Horsepower varies as the cube of the speed 100 load torque and horsepower at base speed Linear torque variation 1 25 a o ah un o a uo o Per Unit Torque and HP of Driven Load 0 00 0 25 0 50 0 75 1 00 1 25 Per Unit of Base Speed Torque varies linearly with soeed Horsepower varies as the square of the soeed 100 load torque and horsepower at base speed 7 1 2 Constant torque loads Typical examples Screw compressors Reciprocating compressors Positive displacement pumps Extruders Crushers Ball mills Conveyors Augers Process lines strip web sheet Machines that are high impact loads intermittent torque loading not as function of speed requiring that the motor and control combination produce sufficient accelerating torque to return the load to the required speed prior to the beginning of the next work stroke or duty cycle loads discrete loads at changing or constant speeds applied for defin
20. age measurements presented above show that there is a succession of peaks in the voltage waveform delivered by the drive and arriving at the motor terminals This signal propagates trough the cable at a determined velocity Depending on the winding characteristics and with respect to the waveform on the minimum time between successive pulses the voltage appearing between turns may vary sensibly The average voltage applied at the motor terminals is controlled by the width of the pulses and by the time between them The overshoots get worse with shorter times between pulses This condition is most likely to occur at high peak or high output voltages and during transient conditions such as acceleration or deceleration If the time between pulses is less than three times the resonant period of the cable typically 0 2 to 2 us for industrial cable then additional overshoot will occur The only way to be sure that this condition does not exist is by measuring the pulses directly or by contacting the control manufacturer Technical guide Induction motors fed by PWM frequency inverters 17 li gt www weg net When the time between successive pulses is less than 6 us particularly when the first and the last turns of a single coil of a random winding are side by side it may be assumed that the voltage between adjacent conductors is the peak to peak value between pulses This fact results from the rapidity of the pulse propagation withi
21. al guide Induction motors fed by PWM frequency inverters 6 10 Criteria regarding protection against bearing currents shaft voltage of WEG motors on VSD applications mod lt 315 IEC No protection Please contact WEG mod lt 504 NEMA Insulated bearing in any or both motor ends Earthing system with 315 and 355 IEC No protection 504 5 and 586 7 NEMA slip ring and graphite brush between frame and shaft Earthing system with slip ring and graphite Insulated DE bearing brush between frame and shaft Insulated NDE bearing Earthing system with NEMA 280 lt mod lt 1800 IEC 440 lt mod lt 2800 NEMA slip ring and graphite Insulated DE bearing brush between frame and shaft w21 W22 Insulated NDE bearing 315 lt mod lt 630 IEC HGF 500 lt mod lt 1040 For Inverter Duty line motors the earthing system is standard NOTE Applications with motors rated for use in hazardous areas must be particularly evaluated in such case please contact WEG 6 11 Normative considerations about the current flowing through the bearings of inverter fed motors NEMA MG1 Part 31 with sinusoidal supply shaft voltages may be present usually in motors of frame 500 and larger More recently for some inverter types and application methods potentially destructive bearing currents have occasionally occurred in much smaller motors The current path could be through either or both bearings
22. alized with several motors and inverters following the procedures recommended by IEC 60034 14 confirmed that the vibration levels of induction motors increase when these are fed by frequency inverters Furthermore the observed increment on vibration speeds generally were lower with higher switching frequencies that is switching frequency increases tend to reduce the mechanical vibration of the inverter fed motor In any case even when operating above the base speed WEG motors presented RMS vibration velocity values mm s below the maximum limits established by both the IEC 60034 14 and the NEMA MG1 Part 7 standards thus attending the criteria required 6 17 Normative considerations about mechanical vibration of inverter fed motors NEMA MG1 Part 30 When an induction motor is operated from a control torque ripple at various frequencies may exist over the operating speed range lt is of particular importance that the equipment not be operated longer than momentarily at a speed where a resonant condition exists between the torsional system and the electrical system i e the motor electrical torque It also is possible that some speeds within the operating range may correspond to the natural mechanical frequencies of the load or support structure and operation other than momentarily could be damaging to the motor and or load and should be avoided at those speeds NEMA MG1 Part 31 Machine sound and vibration are i
23. ands and is particularly interesting when there are multiple motors connected to a single drive The control is open loop and the speed precision obtained is a function of the motor slip which depends on the load since the frequency is imposed on the stator windings In order to improve the performance of the motor at low speeds some drives make use of special functions such as slip Compensation attenuation of the speed variation as function of load and torque boost increase of the V f ratio to compensate for the voltage drop due to the stator resistance so that the torque capacity of the motor is maintained This is the most used control type owing to its simplicity and also to the fact that the majority of applications do not require high precision or fast responses of the speed control The vector control enables fast responses and high level of precision on the motor speed and torque control Essentially the motor current is decoupled into two vectors one to produce the magnetizing flux and the other to produce torque each of them regulated separately lt can be open loop sensorless or closed loop feedback Speed feedback a speed sensor for instance an incremental encoder is required on the motor This control mode provides great accuracy on both torque and speed of the motor even at very low and zero speeds Sensorless simpler than the closed loop control but its action is limited particularly at very low spee
24. ans of input output measurement for motors gt 150 kW is also applicable under agreement between manufacturer and user In this case however the motor efficiency shall not be determined separately 6 3 Influence of the inverter on the temperature rise of the windings Induction motors may heat up more when fed by frequency inverter than when fed by sinusoidal supply This higher temperature rise results from the motor losses growth owing to the high frequency components of the PWM signal and the often reduced heat transfer resulting from speed variation The voltage harmonic distortion contributes to increase the motor losses once that creates minor hysteretic loops in the lamination steel increasing the effective saturation of the magnetic core and giving rise to high frequency harmonic currents which bring about additional Joule losses Nevertheless these high frequency components do not contribute to the production of torque at steady operation of the motor since they do not increase the airgap fundamental flux which rotates at synchronous speed The operation at low speeds causes the ventilation over the self ventilated motor frame to decrease consequently lowering the motor cooling and raising in this way the thermal stabilization temperature Technical guide www weg net p Therefore when operating with frequency inverters both the effects mentioned above must be considered There are basically the following solu
25. ations 23 Normative considerations about the current flowing through the bearings of inverter fed MoOtoOFS occoccccccnccncconcnnnnos 23 Influence of the inverter on the motor ACOUSTIC NOISE ccsccessessesssscsscesseesseecseeesseesenscsueesseeseecsueesseesenessueeseesensssueesnsessnseaes 23 Criteria regarding the noise emitted by WEG motors on VSD AppliCAtioNS ccccsesscsscseessesesrscsseessescsescseessstsetsensesseeeeas 2a Normative considerations about the noise of inverter fed MOtOTS ss ssresresrrsrrsrrsrrernsrnsrrerrsnrnsrnrnsrrnnrnnrnsrnsrnennnnensrnnrene 24 Technical guide Induction motors fed by PWM frequency inverters www weg net gt Weg 6 15 Influence of the inverter on the mechanical vibration Of the MOTONM cccceccesccsscssssesssecssesseecseesseecseeseesceeesseescerssseeestestess 24 6 16 Criteria regarding the vibration levels presented by WEG motors ON VSD applicatiONS c occccccccnocanonnnonnnonnnonanonanonononnos 24 6 17 Normative considerations about mechanical vibration of inverter fed MOTtOFS oocoococcioconocncnoncnonnnnnnononononnnnn nono ncnonnonancnnnos 24 Interaction between motor and driven lOA ccccssscssesssesseecssesseessescsusesseecsnscsueeseeessuscsueesseecsuscsuseseeesnscsueeseeesensesaeesensssnsesaeesenes 25 LA E PP ooo e Ei UE o O E E ts dante ee 25 71 1 Variable torque lO S ccccccescccsstesssseecsseecsseecssescseeeeeesseeeseseeecssesesseses
26. based on the total losses of the control the motor and any auxiliary equipment The motor efficiency when operated on a control is slightly less than when operated on sinewave power Overall system efficiency is often increased when used an ASD Traditional methods of changing speed such as gears or belts introduce additional losses which reduce efficiency IEC 60034 17 The performance characteristics and operating data for drives with inverter fed cage induction motors are influenced by the complete system comprising supply system inverter induction motor mechanical shafting and control equipment Each of these components exists in numerous technical types Any values quoted in this technical specification are thus indicative only There is no simple method to calculate the additional losses and no general statement can be made about their value Their dependence upon the different physical quantities is very complex Also there is a great variety both of inverters and of motors IEC 60034 25 The recommended methods to determine the motor efficiency are given in IEC 60034 2 summation of losses method for motors gt 150 kW and input output measurement for motors lt 150 kW The no load losses including the additional losses should be measured at the same pulse pattern and pulse frequency that the inverter will produce at rated load The determination of the overall efficiency of the system motor inverter by me
27. be 2700 rpm 1 50 p u gt 90 Hz used The switching frequency defined for this example 2 5 kHz thus attends WEG s recommendation According to the derating criteria of WEG NEMA Premium Therefore the motor designed fully attend this application s Efficiency TEFC motors subclause 6 4 1 1 any WEG NPE demands with regard to the insulation system motor is able to support constant horsepower from 60 to 90 However it will not be possible to evaluate the matter on the Hz with variable torque loads Then at 90 Hz the derating point of view of IEC because it requires the measurement of factor will be 1 5 the voltage at the motor terminals As the VSD system is still at the dimensioning stage and there is no actual motor at the T application it is understood that the final motor environment A is not defined yet so that measures are still made unfeasible Lap df 20 99 Nm 38 99 Nm 28 75 loft and the actual voltage peak and rise time values at the motor 1 terminals are unknown Such values will depend on type and 15 length of the cable employed at the end installation 28 Technical guide Induction motors fed by PWM frequency inverters www weg net ly p 8 2 2 3 Regarding the bearings protection According to WEG criteria regarding protection against bearing currents clause 6 9 WEG motors may optionally have protected bearings above including 504 NEMA 315 IEC frames The selected motor frame is 215T NEMA thus needin
28. conducting loop it is enough to isolate the motor bearings only one of them in the case of a single drive end or the both of them in Without bearing protection the case of two drive ends However for the capacitive i DRIVEN MACHINE 1 MOTOR DRIVEN MACHINE 2 components to be withdrawn it would be also necessary to ps SS SS A So Se DRIVEN MACHINE 2 on both shaft ends isolate the bearings of the driven machine in order to avoid TE idducodoninested i n the migration of electric charges from the motor to the rotor E of the driven machine through their shafts which are P i Front f bearing electrically connected in the case of direct coupling Another if way of extinguishing the capacitive discharge current een i E A Rear bearing component consists of short circuiting the rotor and the motor frame by means of a sliding graphite brush This way With bearing protection the inductive current component flowing through the uai P T DRIVEN MACHINE 1 MOTOR DRIVEN MACHINE 2 characteristic conducting loop can be eliminated by A e e rn e q qe da ert MENM i o ee Shaft bus insulating just a single bearing of the motor while the jige induced onthe sha Oj Capacitive current component as well as the transfer of Capacitive charges to the driven machine can be eliminated by use of a short circuiting brush isolated Both bearings i 22 Technic
29. conductors conductance If the shield does not have enough cross section for that then a separate earth conductor is needed and the shield provides EMC and physical protection only The shield high frequency conductance should be at least 10 of that of the phase conductors 6 9 Influence of the inverter on the motor shaft voltage and bearing currents The advent of static inverters aggravated the phenomenon of induced shaft voltage current due to the unbalanced waveform and the high frequency components of the voltage supplied to the motor The causes of shaft induced voltage owing to the PWM supply is thus added to those intrinsic to Technical guide Induction motors fed by PWM frequency inverters the motor for instance electromagnetic unbalance caused by asymmetries which as well provoke current circulation through the bearings The basic reason for bearing currents to occur within an inverter fed motor is the so called common mode voltage The motor capacitive impedances become low in face of the high frequencies produced within the inverter stage of the inverter causing current circulation through the path formed by rotor shaft and bearings back to earth 6 9 1 Common mode voltage The three phase voltage supplied by the PWM inverter differently from a purely sinusoidal voltage is not balanced That is owing to the inverter stage topology the vector sum of the instantaneous voltages of the three phases at the inverte
30. criteria of WEG motors clause 6 6 induction machines rated 500 V are able to bear voltage peaks up to 1780 V and dV dt up to 6500 V us dY 1 04 kWolt In this case it will be possible to analyze the voltage peaks at the motor terminals as requires IEC once the actual installation exists and the factors decisively influencing the occurrence and gravity of overshoots are well defined The next waveforms were obtained by means of measurements accomplished at the inverter terminals upper curves PWM signal before the cable and at the motor 3 terminals lower curves PWM signal after the cable It is E I REEBS 200 Velt Tus important to stand out that the voltage profiles appearing at the motor input would change if other cable were used The cable used herein was not shielded and comprised of 4 conductors 3 phases earth asymmetrically distributed Vea 1040 V peak 7 WEG criterion 1780 V gt 1040 V Ok NEMA criterion gt 3 1 500 1550 V lt 1780 V Ok The inverter was fed by sinusoidal 500 V 50 Hz voltage and aa IEC criterion gt 1300 V lt 1780 V gt Ok scalar control with switching frequency 4 kHz was used Technical guide Induction motors fed by PWM frequency inverters 29 lu gt www weg net Rise time REE 200 Wah Dis t 0 8 0 915 0 25 us At WEG criterion gt 0 1 us minimum at the inverter terminals gt Ok NEMA criterion gt 0 1 us minimum a
31. d for the purpose of making WEG induction motors able to operate at low speeds with constant torque loads still keeping an acceptable temperature rise level without the need of neither oversizing the machine nor blower cooling it It is based on the continuous minimization of the motor 14 losses heat sources by means of the optimization of its magnetic flux parameter controlled by the CFWO9 From the study of the composition of the overall motor losses and their relation with the frequency the magnetic flux and the current as well as the influence of the ventilation system on the motor temperature rise it was found an optimal flux value for each frequency allowing for a continuous minimization of the overall motor losses through the whole speed range The solution obtained was implemented within the CFWO9 in order that the motor magnetic flux optimal condition can be achieved automatically by the drive sufficing for that a simple adjustment of the inverter properly made The motor iron losses strongly depend on the frequency As the operation frequency is varied downwards the iron losses are gradually reduced Therefore it is interesting at low speed operation to increase the magnetic induction flux density of the motor so that the torque can be kept constant with a reduced current which causes reduced Joule losses Thus as the speed falls it is possible to reduce the voltage proportionally less than the frequency result
32. d volt age for that frequency is applied WEG motors when fed by inverters satisfy such criterion up to 90 Hz The maximum torque capability of the motor breakdown torque limits the maximum operating speed in which con stant power operation is possible Attending NEMA recom mendations one can approximately find this limit from the fol lowing equation RPM 2 Toa RPM max base 3 T base 6 5 Influence of the inverter on the insulation system The evolution of the power semiconductors have led to the creation of more efficient but also faster electronic switches The high switching frequencies of the IGBT transistors employed in modern frequency inverters bring about some undesirable effects such as the increase of electromagnetic emission and the possibility of voltage peaks as well as high dV dt ratios time derivative of the voltage that is rate of electrical potential rise occurrence at the inverter fed motor terminals Depending on the control characteristics gate resistors capacitors command voltages etc and the PWM adopted when squirrel cage induction motors are fed by frequency inverters those pulses combined with the impedances of both the cable and the motor may cause repetitive overvoltages on the motor terminals This pulse train may degrade the motor insulation system and may hence reduce the motor lifetime The cable and the motor can be considered a resonant circuit which is excited by the
33. de for the design and performance of cage induction motors specifically designed for inverter supply 2007 3 Induction machines speed variation For an induction motor rotor speed frequency of the voltage source number of poles and slip are interrelated according to the following equation n 120f 1 5 p where n mechanical speed rom f fundamental frequency of the input voltage Hz p number of poles s slip The analysis of the formula above shows that the mechanical speed of an induction motor is a function of three parameters Thus the change of any of those parameters will cause the motor speed to vary as per the table below www weg net p 2 4 Other technical documents of reference GAMBICA REMA Technical Guides for Variable Speed Drives and Motors GAMBICA REMA Technical Reports for Variable Speed Drives and Motors CSA C22 2 No 100 2004 Item 12 Canada Motors and Generators Industrial Products JEM TR 148 1986 Japan Application guide for inverter drive general purpose inverter IEC 60034 18 41 Qualification and design tests for Type electrical insulation systems used in rotating electrical machines fed from voltage inverters Papers and books related to this subject Number of poles Slip Limited frequency range Voltage frequenc a d Utilization of STATIC FREQUENCY Inverters Technical guide Induction motors fed by PWM frequency inverters 5 lu P www
34. ds At higher speeds this control mode is practically as good as the feedback vector control The main difference between the two control types is that the scalar control considers only the magnitudes of the instantaneous electrical quantities magnetic flux current and voltage referred to the stator with equations based on the equivalent electrical circuit of the motor that is steady state equations On the other hand the vector control considers the instantaneous electrical quantities referred to the rotor linkage flux as vectors and its equations are based on the spatial dynamic model of the motor The induction motor is seen by the vector control as a DC motor with torque and flux separately controlled 5 Interaction between inverter and AC power line 5 1 Harmonics For the AC power line the system frequency inverter motor is a non linear load whose current include harmonics frequency components multiples of the power line frequency The characteristic harmonics generally produced by the rectifier are considered to be of order h np 1 on the AC side that is on the power line p is the number of pulses of the inverter and n 1 2 3 Thus in the case of a 6 diode 6 pulses bridge the most pronounced generated harmonics are the 5th and the 7th ones whose magnitudes may vary from 10 to 40 of the fundamental component depending on the power line impedance In the case of rectifying bridges of 12 pulses 12 diodes the m
35. ds on the motor cooling When ventilation does not depend on motor operation for instance TENV motors then kv 1 When ventilation is linked to motor operation for instance TEFC motors then kv 3 It is thus necessary to calculate de derating factor df suitable to each stretch of the duty cycle eee Teepe rs fs pie wl 2 0 oo According to WEG derating criteria for standard motors under constant flux constant V f condition subclause 6 4 1 2 Technical guide Induction motors fed by PWM frequency inverters 31 lu gt www weg net Thus Tso T 100 i T75 a T50 T300 a Ts 2 18 A 2 eq 0 65 0 65 0 77 0 77 0 91 2 18 4 2 18 6 10 T 50 2 T00 a 18 6 AG 0 95 0 95 2 50 2 5 00 8 7512 2 5012 5 00 2 5012 5 00 gt 2 18 4 2 18 6 10 eq a 0 65 0 65 0 77 0 77 0 91 0 95 0 95 2 18 4 2 18 6 10 3 85 2 7 69 18 4 87 44 3 25 7 24 5 49 18 2 63 64 5 26 10 2072 60 5 88 kgfm Tog 2 18 4 2 18 6 10 V 60 The load demands the following horsepower then 9 81 960 a 3600 P Consulting the WEG Electric Motors Manual for 3600 rom and 60 Hz the ideal motor for this application is a three phase 30 kW 40 hp 2 poles 60 Hz frame IEC 200M NEMA 324T TEFC 8 5 Example considering the use of WEG Optimal Flux 8 5 1 Example Considering the same applicati
36. duction motors fed by PWM frequency inverters 19 lu gt www weg net Converter panel MOTOR L1 Line reactor selection criteria according to clause 5 2 L2 Output reactor must be installed next to the inverter 6 8 1 Cable types and installation recommendations The characteristics of the cable connecting motor and frequency inverter as well as its interconnection and physical location are extremely important to avoid electromagnetic interference in other devices 6 8 1 1 Unshielded cables Three core unshielded motor cables can be used when there is no need to fulfill the requirements of the European EMC Directives 89 336 EEC Certain minimum distances between motor cables and other electrical cables must be observed in the final installation These are defined in the table below Emission from cables can be reduced if they are installed together on a metallic cable bridge which is bonded to the earthing system at least at both ends of the cable run The magnetic fields from these cables may induce currents in nearby metalwork leading to heating and increasing losses 6 8 1 2 Shielded cables They help to reduce the radiated emission through the motor cables in the Radio Frequency range RF They are necessary when the installation must comply with the EMC Directive 89 336 EEC as per EN 61800 3 They are also necessary when using Radio Frequency Interference Filter whether built in or external at inv
37. e current through the bearings This current that circulates whenever the grease film is momentarily broken down is often referred to as the capacitive discharge component There is still another current component which is induced by a ring flux in the stator yoke and circulates permanently through the characteristic conducting loop comprising the shaft the end shields and the housing frame that is often called the conduction component www weg net lu gt Stator winding Stator len winding ler Rotor Ic Common mode voltage Cmt Frame Earth C Capacitor formed by the stator winding and the rotor lamination Dielectric airgap slot insulation wire insulation C Capacitor formed by the rotor and the stator cores Dielectric airgap C Capacitor formed by the stator winding and the frame Dielectric slot insulation wire insulation C ng Cm Capacitances of the DE drive end and the NDE non drive end bearings formed by the inner and the outer bearing raceways with the metallic rolling elements in the inside Dielectric gaps between the raceways and the rolling elements bearing grease lou Total common mode current Capacitive discharge current flowing from the stator to the rotor Capacitive discharge current flowing through the bearings These discontinuous electric discharges wear the raceways and erode the rolling elements of the bearings causing small superimp
38. e 1 Fraccionamiento Parque Industrial Huehuetoca Estado de M xico C P 54680 Phone s 52 65 5321 4275 Fax 52 55 5321 4262 info mx weg net www weg net mx Av Prefeito Waldemar Grubba 3000 89256 900 Jaragua do Sul SC Brazil Phone 55 47 3276 4002 Fax 55 47 3276 4060 www weg net NETHERLANDS WEG NETHERLANDS Sales Office of WEG Benelux S A Hanzepoort 23C 7575 DB Oldenzaal Phone s 31 0 541 571080 Fax 31 0 541 571090 info nl weg net www weg net nl PORTUGAL WEG EURO INDUSTRIA ELECTRICA S A Rua Eng Frederico Ulrich Apartado 6074 4476 908 Maia Phone s 351 229 477 705 Fax 351 229 477 792 info pt weg net www weg net pt RUSSIA WEG RUSSIA Pochainskaya Str 17 Nizhny Novgorod 603001 Russia Phone s 7 831 2780425 Fax 7 831 2780424 info ru weg net www weg net ru SPAIN WEG IBERIA S L Avenida de la Industria 25 28823 Coslada Madrid Phone s 34 916 553 008 Fax 34 916 553 058 info esOweg net www weg net es SINGAPORE WEG SINGAPORE PTE LTD 159 Kampong Ampat 06 O02A KA PLACE Singapore 368328 Phone s 65 6858 9081 Fax 65 6858 1081 info sg weg net www weg net sg SWEDEN WEG SCANDINAVIA AB Box 10196 Verkstadgatan 9 434 22 Kungsbacka Phone s 46 300 73400 Fax 46 300 70264 info se weg net www weg net se UK WEG ELECTRIC MOTORS U K LTD 28 29 Walkers Road Manorside Industrial Estate North Moons Moat Redditch Wo
39. e NEMA Application Guide for AC ASD Systems the recommended instrumentation for the measurement of various parameters should be as described in the table below Recommended instrumentation for the measurement of various parameters Analog or digital Very control input Fundamental voltmeter voltage 20 MHz or higher Capture line voltage Transient as storage oscilloscope variation A meter capable of measuring fundamental Very motor input Fundamental MTI of a non sinusoidal voltage wave form i i Compare to the Oscilloscope with a Peak transient and dV dt motor s peak voltage sampling rate of at least A and rise time 1Ms sec withsand capability True ms True ms meter Very feeder size A meter capable of measuring fundamental Fundamental Estimate torque of a non sinusoidal wave form Fundamental Ensure compliance Spectrum analyzer plus harmonics with IEEE 519 Fundamental Ensure compliance l Spectrum analyzer plus harmonics with IEEE 519 Not pratical due to dificulty of NA NA accurately measuring motor output in situ 9 4 Grounding considerations Safe reliable and interference free measurements depend on good grounding practices The manufacturer s recommendations as well as local regulations concerning grounding must always be followed when installing ground wiring 9 4 1 Grounding of control The control must be solidly grounded to the main distribution system ground A ground comm
40. e motor in the processing area on the contrary of hydraulic and mechanical varying speed systems Cost reduction direct on line startings of induction motors cause current peaks that harm the motor as well as other electric equipments linked to the electrical system Static frequency inverters provide softer startings resulting in cost reduction with regard to maintenance Gain of productivity industrial systems are often oversized due to an expectation of future production increase Static inverters allow the proper regulation of the operational speed according to the equipments available and the production needs Energy Efficiency the power system global efficiency depends not only on the motor but also on the control Static inverters are high efficiency apparatuses reaching typically 97 or more Induction motors also present high efficiency levels reaching up to 95 or even more in larger Technical guide Induction motors fed by PWM frequency inverters machines operating at rated conditions When speed variation is required the output changes in an optimized way directly affecting the energy consumption and leading to high efficiency levels performed by the system inverter motor Versatility static frequency inverters suit both variable and constant torque loads With variable torque loads low www weg net ly p High quality the accurate speed control obtained with inverters results in process
41. ed periods of time repeated periodically typically fall into the constant torque classification 1 25 Per Unit Torque and HP of Driven Load 0 00 0 25 0 50 0 75 1 00 1 25 Per Unit of Base Speed Load torque remains constant throughout the speed range Horsepower changes linearly with operation speed Rated load torque and horsepower at base speed Technical guide Induction motors fed by PWM frequency inverters 25 lu gt www weg net 7 1 3 Constant horsepower loads Typical examples Machine tools where heavier cuts are taken at lower speeds and lighter cuts at higher speeds Center driven winders ho an o o Horsepower o Torque ue and HP of Driven Load ay Per Unit Tor Per Unit of Base Speed Load torque drops as speed increases Horsepower results constant throughout the speed range Rated load torque and horsepower at base speed 7 2 Speed duties 7 2 1 Variable speed duty Motors designated for variable speed duty are intended for varied operation over the defined speed range marked on the motor and is not intended for continuous operation at a single or limited number of speeds The motor design takes the advantage of the fact that it will operate at a lower temperature at the load levels for some speeds than at other over the duty cycle 7 2 2 Continuous speed duty Motors designated for continuous speed duty can be operated continuously at any speed within the defined speed range T
42. encies mechanical resonance or magnetic noise may cause a significant increase in sound levels while a change in frequency and or voltage may reduce the sound level Experience has shown that an increase of up to 5 to 15 dB A can occur at rated frequency in the case when motors are used with PWM controls For other frequencies the noise levels may be higher IEC 60034 17 due to harmonics the excitation mechanism for magnetic noise becomes more complex than for operation on a sinusoidal supply In particular resonance may occur at some points in the speed range According to experience the increase at constant flux is likely to be in the range 1 to 15 dB A IEC 60034 25 the inverter and its function creates three variables which directly affect emitted noise changes in rotational speed which influence bearings and lubrication ventilation and any other features that are affected by temperature changes motor power supply frequency and harmonic content which have a large effect on the magnetic noise excited in the stator core and to a lesser extent on the bearing noise and torsional oscillations due to the interaction of waves of different frequencies of the magnetic field in the motor airgap The increment of noise of motors supplied from PWM controlled inverters compared with the same motor supplied from a sinusoidal supply is relatively small a few dB A only when the switching frequency is above about
43. equency The following diagram depicts the three stages of an indirect Diode bridge Rectification of the AC input voltage frequency inverter rn EB BB EB SE SE SK LG AC Rectifier Inverter Motor 50 60 Hz 1 o or 3 q Variable voltage and frequency Input i Vbo 1 35 Vp or 1 41 Vy Output ee Technical guide Induction motors fed by PWM frequency inverters 7 lu P www weg net NOTES Under light load or at no load conditions the DC link voltage tends to stabilize at J2 Viede 141 V rede However when the motor drives heavier loads for instance at full load the DC link voltage tends to the value 3 11 N 1 39 Vade The criteria used to define the insulation system of WEG motors fed by inverters presented further on consider the highest of those values 1 41Vin which is more critical to the motor In this way WEG motors attend both situations satisfactorily 4 2 Control Types There are basically two inverter control types scalar open loop and vector open or closed loop The scalar control is based on the original concept of a frequency inverter a signal of certain voltage frequency ratio is imposed onto the motor terminals and this ratio is kept constant throughout a frequency range in order to keep the magnetizing flux of the motor practically unchanged It is generally applied when there is no need of fast responses to torque and speed comm
44. erminals As the VSD system is still at the dimensioning stage and there is no actual motor at the application it is understood that the final motor environment is still not defined so that measures are made unfeasible and the actual voltage peak and rise time values at the motor terminals are unknown Such values will depend on type and length of the cable employed at the end user www weg net lu gt 8 1 2 3 Regarding the bearings protection According to WEG criteria regarding protection against bearing currents clause 6 9 WEG standard motors have optional protected bearings for frames above including 315 IEC 504 NEMA The selected motor frame is 132 M IEC 215 T NEMA thus not needing shaft earthing system neither special insulated bearings 8 1 2 4 Regarding the noise When fed by inverter the acoustic noise produced by the motor may increase up to 11 dB A considering that scalar control type will be used 8 2 Squared torque application centrifugal pump 8 2 1 Example Please dimension an induction motor WEG NEMA Premium Efficiency TEFC to operate with a CFW 09 on vector control driving a centrifugal pump rated 10 hp 7 5 kW at maximum speed 2700 rom General information Power line 3 phase 460 V 60 Hz Environment maximum temperature 40 C altitude 1000 m normal atmosphere WEG frequency inverter CFW 09 tr 0 1 us fchav 2 5 kHz 8 2 2 Solution 8 2 2 1 Regarding the temperature rise derating
45. ers 33 lu P www weg net 10 Conclusion The fast advance of the power electronics have allowed induction motors the traditional solution for fixed speed rotating power applications to be used successfully also in variable speed drive systems In such cases though the motor must be fed by means of a static frequency inverter rather than directly by the sinusoidal power line The utilization of squirrel cage induction motors with electronic inverters presents great advantages regarding costs and energy efficiency compared with other industrial solutions for varying speed applications Nevertheless the inverter affects the motor performance and might introduce disturbs into the mains power line The increasing number of applications with induction motors fed by PWM inverters operating in variable speed duty thus requires a good understanding of the whole power system as well as the interactions among its parts one another power line frequency inverter induction motor load This Technical Guide aimed to clarify the main aspects related to the application of squirrel cage induction motors together with static frequency inverters presenting theoretical basics and practical criteria for specific topics originated from studies and from the experience of WEG s technical body in this subject The most important and internationally recognized technical references concerned with such matters are mentioned and also discu
46. erter input Minimum distances between motor cables and other electrical cables for instance signal cables sensor cables etc must be observed in the final installation as per table below Cable Length Minimum separation distance 25cm l 6 8 1 3 Installation recommendations IEC 60034 25 presents cable types and construction details 20 The basic given recommendations are summarized in the table below For more details and updated information the current standard version shall be consulted The grounding system must be capable to provide good connections among equipments for example between motor and inverter frame Voltage or impedance differences between earthing points can cause the flow of leakage currents common mode currents and electromagnetic interference Examples of shielded cables recommended by IEC 60034 25 Afe steel or galvanized iron Symmetrical Shielded Cables three core cable with or without conductors for protective earth symmetrically constructed a concentric copper or aluminum protective shield armour PE protective earth conductor SCU concentric copper or aluminum screen Cable shield must be grounded at both ends motor and inverter Good EMC practices such as 360 bonding of the shields are recommended in order for low impedance for high frequency to be provided For the shield to operate also as protective conductor It should have at least 50 of the phase
47. fundamental characteristics of electronic inverters Once the basics of adjustable speed drives are known the behavior of the whole power system Is analyzed Each component of the power system AC power line frequency inverter induction motor load is focused as well as the overall interactions between them resulting from speed variation In this manner the whole drive system can be well understood At last examples of VSD systems designs are presented for a better understanding of the matters exposed throughout the document Always looking out for a technical elucidation as complete as possible along this guide some controversial points are emphasized Divergences existing among distinct standardization organisms are discussed and WEG s point of view Is explained 4 Technical guide Induction motors fed by PWM frequency inverters 2 Normative Aspects 2 1 NEMA MG1 Motors and generators United States Parte 30 Application considerations for constant speed motors used on a sinusoidal bus with harmonic content and general purpose motors used with adjustable frequency controls or both 2006 Parte 31 Definite purpose inverter fed polyphase motor 2006 2 2 NEMA Application Guide for AC Adjustable Speed Drive Systems 2001 2 3 IEC 60034 Rotating Electrical Machines International Parte 17 Cage induction motors when fed from inverters application guide 2006 Parte 25 Gui
48. g neither shaft earthing system nor special insulated bearings 8 3 Special application long cable o Ree AMBP RSL Ems 2 Ref By 500 Volt ms 8 3 1 Example l Evaluation of the voltage peaks at the terminals of a special Upper curve PWM voltage leaving the inverter WEG motor rated 9 kW 2115 rom 500 V 72 Hz Due to Lower curve PWM voltage arriving at the motor matters intrinsic to the application the motor must be fed by a PWM inverter through a 100 m long cable 8 3 2 Solution Supposing that the derating bearing protection and noise criteria have already been verified and that the motor in question fully attends them the insulation system of the motor is still left to be evaluated It must be assured that the motor insulation will bear the application s conditions Owing to the long length of the cable leads there is the A ee a chance of excessive voltage peaks overshoots to occur at the motor terminals and therefore special attention must be Zoom in the voltage pulse shown beside for an analysis addressed to the insulation matter In this case for an oft and V appropriate evaluation of the insulation system the highest speed within the operating range must be considered in order that maximum voltage levels come to the motor terminals causing the voltage peaks to be as high as possible as well Magnitude of the voltage peak appearing at the motor terminals peak According to the insulation
49. ge and a practically sinusoidal current so that the voltage harmonics generally present higher magnitudes than the current harmonics 10 There are basically the following solutions to mitigate the harmonics generated by a PWM frequency inverter Installation costs increase Installation of output passive filters Restrictions for vector control operation L LC sinusoidal dV dt Voltage drop motor horsepower reduction Costs increase Inverter reliability decrease Control complexity increase Use of multi level inverters Space Vector Modulation SVM Voltage control upgrade Higher system inverter motor Pulse Width Modulation quality improvement optimization of pulse patterns efficiency Inverter efficiency decrease higher switching losses Switching frequency increase Common mode leakage current flow Increase All frequency inverters manufactured by WEG employ Space Vector Modulation 6 1 1 Normative considerations about the inverter output harmonics There is no international standardization defining maximum acceptable values for voltage and current harmonic distortion However the international standards do consider the increase of motor losses due to the non sinusoidal supply IEC 60034 17 provides an example of motor losses increase owing to PWM supply Motor info 315 IEC frame rated torque and speed values Time dependence of the impressed quantity 100 Losses 25 3 Eff
50. he line reactor decreases the medium voltage of the intermediate circuit but attenuates more effectively power supply voltage transients Besides that it extends the semiconductors and the DC link capacitor bank lifetimes as Technical guide www weg net IT p a result of the decrease of both the rms current of the rectifying diodes and the current ripple through the middle circuit capacitors Converter input current El 80 a 40 a Corrente Amp Corrente Amp to 8 gt 8 a 0 Converter input voltage Tens o no PCC Tensdo no PCC a b Current and voltage waveforms with b and without a line reactor It can be seen that line reactors soften the peaks thus reducing the harmonic content and the rms value of the input current Additionally diminution of the supply voltage waveform distortion is thereby caused A minimum line impedance that introduces a voltage drop from 1 to 2 depending on the inverter size is recommended in order to ensure the inverter lifetime As rule of thumb it is recommended to add a line reactor to the existing power supply impedance including transformers and cables so that a total voltage drop of 2 to 4 is achieved This practice is considered to result in a good compromise between motor voltage drop power factor improvement and harmonic current distortion reduction The value of the li
51. he equations above The ratio V1 f1 is kept constant up to the motor base rated frequency From this frequency upwards the voltage is kept constant at its base rated value while the frequency applied on the stator windings keeps growing as shown next voltage Frequency Thereby the region above the base frequency is referred to as field weakening in which the flux decreases as a result of frequency increase causing the motor torque to decrease gradually The typical torque versus speed curve of an 6 inverter fed induction motor is illustrated below Torque Field weakening Frequency It comes out that torque is kept constant up to the base frequency and beyond this point it falls down weakening field Since the output is proportional to torque times speed it grows linearly up to the base frequency and from that point upwards it is kept constant This is summarized by the graph beside Power Frequency The number of variable soeed applications controlled by means of a frequency inverter has increased significantly over the recent years This may be explained by the many benefits orovided by such applications Aloof control the control can be installed remotely at a suitable location Keeping just the motor in the processing area on the contrary of hydraulic and mechanical varying speed systems Aloof control the control can be installed remotely at a suitable location Keeping just th
52. he motor is designed on the principle that it may be operated at its load level at the speed which results in the highest temperature rise for an indefinite period of time 8 Dimensioning and analysis of actual drive system applications Practical examples 8 1 Constant torque application compressor 8 1 1 Example Please dimension a WEG standard squirrel cage induction motor TEFC to operate with any WEG frequency inverter from 180 to 1800 rpm driving a compressor demanding 34 Nm of torque Temperature rise of thermal class B 80 K wanted General data Mains 3 phase 400 V 60 Hz Environment maximum temperature 40 C altitude 1000 m normal atmosphere Frequency inverter CFW 09 tr 0 1 us fchav 5 kHz 26 8 1 2 Solution 8 1 2 1 Regarding the temperature rise on the windings derating torque Compressors are loads that feature a constant torque demand along the whole speed range The motor must be dimensioned to cope with the most critical operation condition in this example the lowest speed within the operating range in which the ventilation is reduced to its minimum while the torque demand remains constant Considering that the operation speed may change from 180 to 1800 rom and that the base frequency is 60 Hz then a 4 pole motor must be chosen Neglecting the slip the demanded horsepower at the base point of operation is T kgfm eo s pa nr om 9 81 960 3 1800 _ 65kW Nevertheless f
53. iciency Sinuscidal voltage Technical guide Induction motors fed by PWM frequency inverters Time dependence of the impressed quantity 115 Losses 94 5 Efficiency Voltage source converter with optinized pulse pattem pulse frequency 3 kHz Losses caused by fundamental frequency A Stator winding losses B Rotor winding losses C Iron losses D Additional load losses E Frictional losses Losses caused by harmonics F Stator winding losses G Rotor winding losses H Iron losses Additional load losses J Commutation losses NOTE frame 315 IEC motor operating at rated speed and torque IEC 60034 25 illustrates the motor losses increase due to PWM supply by means of the following curves sine supply PWM supply Losses kW sine supply PWM supply Frequency Hz NEMA MG1 Part 30 considers a derating factor torque reduction to avoid excessive overheating of a general purpose motor fed by converter compensating for the circulation of harmonic currents due to the PWM voltage harmonic content mud No load losses at m No load losses at eo E l load losses at Fullload losses at www weg net li p O O 00 Derating Factor o o o 0 N 0 02 0 04 0 06 0 08 0 10 0 12 Harmonic Voltage Factor HVF O Where n order of the odd harmonic not including those divisible b
54. ies and lower operation speeds often stands out the electromagnetic noise However in variable speed drive systems especially at low operating speeds when ventilation is reduced the electromagnetically excited noise can be the main source of noise whatever the motor polarity owing to the harmonic content of the voltage Higher switching frequencies tend to reduce the magnetically excited noise of the motor 6 13 Criteria regarding the noise emitted by WEG motors on VSD applications Results of laboratory tests 4 point measurements accomplished in semi anechoic acoustic chamber with the inverter out of the room realized with several motors and inverters using different switching frequencies have shown that the three phase induction WEG motors when fed by frequency inverters and operating at base speed typically 50 or 60 Hz present and increment on the sound pressure level of 11 dB A at most Induction motors fed by PWM frequency inverters 23 lu P www weg net 6 14 Normative considerations about the noise of inverter fed motors NEMA MG1 Part 30 the sound level is dependent upon the construction of the motor the number of poles the pulse pattern and pulse frequency and the fundamental frequency and resulting speed of the motor The response frequencies of the driven equipment should also be considered Sound levels produced thus will be higher than published values when operated above rated speed At certain frequ
55. ing in an optimal V Hz ratio greater than the rated value which minimizes the motor losses altogether It is considered thereby that the major motor losses occur due to Joule effect on the windings This solution was especially conceived for low speed applications with constant torque loads and must be used in no way with variable torque loads or above the motor base frequency Besides the Optimal Flux WEG solution is applicable only when the motor is fed by WEG inverter CFWO9 version 2 40 or higher sensorless vector type control is used z 2 Y 3 3 5 Fa y 3 E 3 e 3 0 0 0 1 02 03 0 4 05 06 OF 08 09 1 0 11 1 2 13 14 15 16 1 7 18 19 20 21 ffn Frequency p u for termperature rise of therralc hss F 105 K for temperature nse of thermal c hss B 80 K 6 4 2 Breakaway torque According to NEMA MG1 Parts 30 and 31 the motor should be capable of producing a breakaway torque of at least 140 of rated torque requiring not more than 150 rated current WEG motors when fed by inverters attend such recommendation Technical guide Induction motors fed by PWM frequency inverters 6 4 3 Breakdown torque Above base speed the motor voltage must be kept constant for constant power operation as already shown NEMA MG1 Part 31 prescribes that the breakdown torque at any frequen cy within the defined frequency range shall be not less than 150 of the rated torque at that frequency when rate
56. inverter rectangular pulses When the values of R L and C are such that the peak voltage exceeds the supply voltage Vo 1 41 V the circuit response to this excitation is a so called overshoot The overshoots affect especially the interturn insulation of random windings and depend on several factors rise time of the voltage pulse cable length and type minimum time between successive pulses switching frequency and multimotor operation 6 5 1 Rise Time The PWM voltage takes some time to rise from its minimum to its maximum value This period is often called rise time Due to the great rapidity of switching on the inverter stage the growth of the voltage wavefront takes place too fast and with the power electronics advance these transition times tend to be more and more reduced Technical guide www weg net lu gt Then the inverter fed motor is subjected to extremely high dV dt rates so that the first turn of the first coil of a single phase is submitted to a high voltage level Therefore variable speed drives can considerably increase the voltage stress within a motor coil though owing to the inductive and capacitive Characteristics of the windings the pulses are damped on the subsequent colls S 1st coil Motor terminal turns Voltage pulse at the first turn Voltage between adjacent conductors Delayed voltage pulse through turns of coil Upeak Pulse voltage A Wo AS
57. l reference wave or space phasor modulation Additionally oscillating torque of twice the pulse frequency are generated These however do not exert detrimental effects on the drive system since their frequency is far above the critical mechanical frequencies IEC 60034 25 If the inverter have appropriate output characteristics and if due care is taken with respect to the mechanical characteristics and the mounting of the motor vibration levels similar to those resulting from sinusoidal environment will be produced Therefore there is no need for defining vibration criteria different from those already established in IEC 60034 14 for sinusoidal supply Vibration levels measured with decoupled motors are indicative of the motor quality only but in measurements accomplished at the actual application with the motor finally installed rather different values of vibration levels may be obtained 7 Interaction between motor and driven load 7 1 Load types The correct dimensioning of the variable speed drive system depends on the knowledge of the behavior of the load that is how the load is related with soeed and consequently how much torque is demanded on the motor shaft In most processes the load may be described by one of the following terms variable torque constant torque and constant horsepower 7 1 1 Typical examples Typical examples Centrifugal pumps Centrifugal fans Centrifugal blowers Centrifugal compressors Vari
58. motors If more than one motor is connected to a control there can be additional overshoot due to reflections from each motor The situation is made worse when there is a long length of lead between the control and the common connection of motors This length of lead acts to decouple the motor from the control As a result reflections which would normally be absorbed by the low impedance of the control can be carried to another motor and add to the overshoot at its terminals 18 L y O E i INVERTER When connecting multiple motors to a single inverter L must be as short as possible 6 6 Criteria regarding the insulation system of WEG motors on VSD applications When WEG low voltage induction motors are used with inverters the following criteria must be attended in order to protect the insulation system of the motor If any of the conditions below are not satisfied filters must be used NOTE Applications with motors rated for use in hazardous areas must be particularly evaluated in such case please contact WEG VNOM lt 460 V 1600V lt 5200 V us 460 V lt VNOM lt 575 V lt 1800V lt 6500 V us 0 1 us 575 V lt VNOM lt 690 V lt 2200V lt 7800 V us gt 6us Informed by the inverter manufacturer The maximum recommended switching frequency is 5 kHz Moisture is detrimental to insulating materials and therefore must be avoided for a longer motor life to be guaranteed In order to keep the mot
59. n a coil because while the first turn stands a peak to peak voltage value the voltage on the last turn is very low probably zero In the case of the example shown above the MTBP was below 6 us and there were actually motor failures due to short circuit between turns ae Ref A 500 Volt 10 us pap E pa E S wats i 6 5 4 Switching frequency f Beside the effects caused by the rise time and the MTBP there is also the frequency at which they are generated Differently from eventual impulses caused by line handles it is about a pulse train Supported at a certain frequency Owing to the fast developments on power electronics oresently this frequency reaches easily values such as 20 kHz The higher the switching frequency the faster the degradation of the motor insulation takes place Studies bear out that there is no simple interrelation between the insulation life and the switching frequency in spite of that experiences have shown interesting data If fs lt 5 kHz the probability of insulation failure occurrence is directly proportional to the switching frequency If fs gt 5 kHz the probability of insulation failure occurrence is quadratically proportional to the switching frequency High switching frequencies can cause bearing damages On the other hand switching frequency increase results in the motor voltage FFT improvement and so tends to improve the motor thermal performance besides reducing noise 6 5 5 Multiple
60. n and may be driven by a frequency inverter unknown Application info 50 Nm at full load Speed range from 540 to 3600 rom Temperature rise of thermal class B 80 K wanted on the windings Technical guide Induction motors fed by PWM frequency inverters Direct coupling Duty cycle as shown below No forced ventilation wanted General data Mains 3 phase 380 V 6 OHz Room temperature 40 C altitude 1000 m Load torque Speed rpm time min Period 8 4 2 Solution Considering the operation range from 540 to 3600 rpm and the base frequency 60 HZ a 2 pole motor must be chosen because higher polarities would lead to high operating frequencies and increasing torque reduction above 60 Hz At base speed neglecting the slip the load demands 3800 _ 1872 kW 960 Tr kgfm n rom _ 50 De oe c O CV 716 9 81 According to WEG standard motors torque derating criteria valid for constant flux constant V f condition subclause 6 4 1 2 for operation at 60 Hz 1 per unit the torque must be reduced to 0 95 per unit in order for the temperature rise of the machine to attend the limits of thermal class B However it is not possible to reduce in 5 the load because is demands constant torque Since the use of independent ventilation is also out of question the motor has to be oversized Thus the motor rated horsepower is actually higher than the value firstly reckoned p 1 2 _ 41970
61. ne reactor needed for the desired voltage drop to be obtained can be calculated as follows voltage drop V l g Plo line H V 3 2 T ine ol ast The a line reactor and b DC bus choke electrical installations are shown next e E Cel 7 Fal AC Input comme Fuses Readioe Switch a Input line reactor connection Induction motors fed by PWM frequency inverters 9 m gt www weg net AC Input b DC bus choke connection 6 Interaction between inverter and motor 6 1 Harmonics influencing motor performance The induction motor when under PWM voltage coming from the inverter is subjected to voltage harmonics frequency components above the fundamental frequency Depending on the type of PWM employed the switching frequency and other peculiarities of the control the motor may present efficiency decrease and losses temperature noise and vibration levels increase Furthermore other effects may appear when induction motors are fed by inverters Insulation system dielectric stress and shaft voltages allied with potentially damaging bearing currents are well Known side effects Although not produced specifically by harmonics but by other matters that will soon be approached these are important effects and should not be neglected The motor current and voltage waveforms when under PWM supply are illustrated below Then the motor fed by frequency inverter sees a pulsating PWM volta
62. nfluenced by the following parameters electromagnetic design type of inverter resonance of frame structure and enclosure integrity mass and configuration of the base mounting structure reflection of sound and vibration originating in or at the load and shaft coupling windage It is recognized that it is a goal that motors applied on inverter type supply systems for variable speed service should be designed and applied to optimize the reduction of sound and vibration in accordance with the precepts explained above However since many of these influencing factors are outside of the motor Itself it is not possible to address all sound and vibration concerns through the design of the motor alone IEC 60034 17 The asynchronous time constant torques generated by harmonics have little effect on the operation of the drive However this does not apply to the oscillating torques which produce torsional vibrations in the mechanical system In drives with pulse controlled inverters the frequencies of the dominant oscillating torques are determined by the pulse frequency while their amplitudes depend on the pulse width With higher Technical guide Induction motors fed by PWM frequency inverters pulse frequencies in the order of 21 times the fundamental frequency the oscillating torques of frequencies 6 x f1 and 12 xf1 are practically negligible provided a suitable pulse pattern is applied e g modulation with a sinusoida
63. on of the last example please dimension a self ventilated squirrel cage induction WEG Premium Efficiency motor to be driven by a frequency inverter WEG model CFW 09 software version 2 40 It is desired temperature rise of thermal class F 105 K 8 5 2 Solution Observing the motor line Premium Efficiency and the inverter characteristics CFWO9 version 2 40 or higher it is remarkable that in this case the optimal flux can be beneficially used This example aims to evidence the advantages provided by the employment of the optimal flux solution It will be necessary to reckon again the derating factor df applicable at each stretch of the duty cycle but this time according to the torque derating criteria valid for Premium Efficiency motors at optimal flux condition subclause 6 4 1 2 considering the temperature rise of class F DOE ECON E O CI IT Pa o e e e 0 0 According to WEG torque derating criteria valid for high efficiency motors under optimal flux optimal V f condition subclause 6 4 1 2 32 Thus T 509 a T 100 4 I 2 E ea 1 00 1 00 T75 T50 T 00 i 8 A 2 1 1 00 1 00 1 00 2 18 4 2 18 6 10 T 50 T300 i 8 6 10 1 00 1 00 2 50 2 5 00 18 3 75 4 2 50 24 5 00 18 2 50 64 5 00 10 i 1268 75 4 60 kgfm Tog 2 18 4 2 18 6 10 60 The load demands the following horsepower then 45 98 360
64. on with electrical welding equipment or large current electrical equipment typically 5x rating of the control should not be used If either of these these conditions exist an isolation transformer sized for the installed control with a wye secondary neutral solidly grounded should be used Where more than one control is used each of them should be grounded directly to the system ground terminal they should not be loop grounded nor installed in series www weg net p 9 4 2 Grounding of motor The output ground conductor may be run in the same conduit as the AC motor power leads The grounded metal conduit carrying the output power conductors can provide EMI shielding but it does not provide an adequate ground for the motor a separate ground conductor should be used The motor s ground wire should not be connected to the metallic conduit 9 5 Measurement procedures Actual operation conditions especially concerning motor speed control type and switching frequency should be attended when taking measurements It is worth noting that higher speeds imply higher voltage levels and therefore operation at the highest speed within the operation frequency range will probably result in the highest possible voltage peaks at the motor terminals 9 5 1 Waveform visualization The correct evaluation of a VSD System strongly depends on a proper analysis of the waveforms measured The visualization of one cycle or specific parts of a cycle
65. onnonononon nono nonnnonrn non none nono n nono nonenrnrn rana neennnnnos 8 o A A 8 Normative considerations about the NArMoOniCs ccccesccsscesssssssscsssesseecsseesseessnscseeeseeesenscsueeseeesnsssueeseeesensssuessessenesseessseessneesaeess 9 Line reacio DO DUS CHOK ota 9 Interaction between inverter and MOTOM cccccccssesssscssscsssesscecsescseesseecsnsssseesseecsuessueeseeeceusssueeseessussauesseessensssueeseeesenessueeseessenseaes 10 olaa estate ol o o E E 10 Normative considerations about the inverter output harMonliCS ccccccoccccooonnononcnonnnononnnnnnnononnnonnnconnnnonnnnonnn nono nnonnnncnnancnnnnos 10 Considerations regarding energy SIC yc ici sense ics ecu ste acca d eave dele dee nee 11 The influence of the speed variation on the motor effiCienCy ccccccccnoccnnonnnconononnnnononnononnonn nono nnnonnnnnnnnconnncnnn near nnrnrnncnnnncnnnnos 12 Normative considerations about the efficiency of inverter fed MOTtOFS cococcccnocncononcononcnonnnononnonnnnononcnnonconnnnonnnnonnnnonnnnonnss 12 Influence of the inverter on the temperature rise Of the WINGINGS cccccescsssesssesssecsssesseesseecsseeseessnecsueeseesenessueesseesenseaes 13 Criteria regarding the temperature rise of WEG motors on VSD applicatiONS ccccccccnocnnnonononanonncnnnnnonnnnnnonononanonancnnnnonanono 13 ite ve Wlege cieclil gre PPPoE 13 e e E E eee en eee ees 14 ESOO gara EE E E e A oo il OEA 15 Influence of the inverter on the in
66. or windings dry it is recommended the use of heating resistors The insulation system to be used in each case depends on the motor rated voltage range and on the frame size 6 7 Normative considerations about the insulation system of inverter fed motors NEMA MG1 if the voltage at the inverter input does no exceed the motor rated voltage and if the voltage observed at the motor terminals does not exceed the limits shown below it may be assumed that there will be no voltage stress reducing significantly the life of the insulation system Technical guide Induction motors fed by PWM frequency inverters V lt 600V V 1kV rated peak Rise time 2us lt 600V V _ lt 2 04 Vnom V rated peak Rise time lus Votos gt B00V Viga lt 3 1 V peak rated Rise time 0 1us lt 600V V 204V rated peak Rise time lus V rated WEG motors fully attend NEMA MG1 Parts 30 and 31 IEC 60034 for motors up to 500 V the insulation system must stand voltage peak levels as shown below For motors above 500 V reinforced insulation systems must be applied or filters shall be installed at the inverter output aiming to increase the rise time and to limit voltage peaks IEC 60034 17 General purpose motors Ui V 2 000 1500 1 000 500 fr ps IEC 829 02 Valid for standard motors IEC 60034 25 Definite purpose motors 2 4 Le 2 0 1 8 1 6 1 4 1
67. osing punctures Long term flowing discharge currents result in furrows fluting which reduce bearings life and may cause the machine to fail precociously Technical guide Induction motors fed by PWM frequency inverters 21 li gt www weg net Motor with one drive end Shatt DRIVING MOTOR DRIVEN MACHINE ee en Crater occasioned by electroerosion on the inner raceway of the bearing SHORT CIRCUITING BRUSH ISOLATED REAR BEARING ON THE FRONT SHAFT END MR Pee i 3 Without bearing protection MOTOR DRIVEN MACHINE ee ae ee f Shaft bus induced on the shaft a Bearing raceway damaged by bearing currents flow NDE i cee Be bearing jl bearing i Ls ds Ge e e e o ey gt With protected bearing MOTOR DRIVEN MACHINE ind seat ha i J SUALI Fluting caused by electric discharges within induced on the sha las the bearing Isolated l NDE bearing ree g 1 4 Lh RA A AA Motor with two drive ends 6 9 3 Methods to reduce or mitigate the bearings peers MU 7 5 ftem currents in inverter fed motors For the motor bearing currents to be impeded to circulate gt La gt z gt both the conduction induced on the shaft and the capacitive sake fy easement an a i discharge resultant from common mode voltage components must be taken into account In order to eliminate the current flowing through the characteristic
68. ost harmful harmonics generated are the 11th and the 13th ones The higher the order of the harmonic the lower can be considered its magnitude so higher order harmonics can be filtered more easily As the majority of drives manufacturers WEG produces its low voltage standard inverters with 6 pulse rectifiers 8 The power system harmonic distortion can be quantified by the THD Total Harmonic Distortion which is informed by the inverter manufacturer and is defined as THD where A are the rms values of the non fundamental harmonic components A is the rms value of the fundamental component The waveform above is the inout measured current of a 6 pulse PWM inverter connected to a low impedance power grid Technical guide Induction motors fed by PWM frequency inverters 5 1 1 Normative considerations about the harmonics The NEMA Application Guide for AC ASD Systems refers to IEEE Std 519 1992 which recommends maximum THD levels for power systems lt 69 kV as per the tables presented next This standard defines final installation values so that each case deserves a particular evaluation Data like the power line short circuit impedance points of common connection PCC of inverter and other loads among others influence on the recommended values The maximum harmonic current distortion recommended by IEEE 519 is given in terms of TDD Total Demand Distortion and depends on the ratio lso l where
69. otor is able to operate 1000 1 with variable torque loads dV AV 520 45V _ that is no torque derating is needed throughout the speed di At Olus 7 9200 us range Then the derating factor will be 1 According to WEG insulation criteria clause 6 6 WEG motors rated 460 V are able to bear Ta 25 99 Nm Voy af 1 19 77 Ibft dV dt values up to 5200 V us at the drive terminals thus satisfying the needs of the application of this example if 0 1 t the ter t inals th tt ing thi Consulting the WEG NEMA Motor Catalog the most PO EE Lacie INS le lication appropriate 3 phase IP55 motor is the NEMA Premium e i Owing to the operation in the weakening field region the motor breakdown torque must be also verified According to aia the WEG motors breakdown torque criteria subclause 6 4 3 the motor is able to attend the application needs appropriate 3 phase IP55 NEMA Premium Efficiency motor is Therefore after both technical and economical analyses the most suitable motor for this application turns out to be the 4 pole 7 5 kW 10 hp 60 Hz 460 V frame 215T NEMA According to NEMA criteria the situation is the following Efficiency 15 hp 11 kW 2 poles 60 Hz frame 254 T Vpeak lt 1430 V at the motor terminals If this condition is not attended at the definitive installation filters must be connected to the inverter output 4 pole motor WEG recommends that switching frequencies up to 5 kHz
70. r output does not cancel out but results in a high frequency electric potential relative to a common reference value usually the earth or the negative bus of the DC link hence the denomination common mode Common mode voltage YV common oS o 0 005 0 01 0 015 0 02 0 025 0 03 The sum of the instantaneous voltage values at the three phase inverter output does not equal to zero This high frequency common mode voltage may result in undesirable common mode currents Existing stray capacitances between motor and earth thus may allow current flowing to the earth passing through rotor shaft and bearings and reaching the end shield earthed Practical experience shows that higher switching frequencies tend to increase common mode voltages and currents 6 9 2 Equivalent circuit of the motor for the high frequency capacitive currents The high frequency model of the motor equivalent circuit in which the bearings are represented by capacitances shows the paths through which the common mode currents flow The rotor is Supported by the bearings under a layer of non conductive grease At high speed operation there is no contact between the rotor and the earthed outer bearing raceway due to the plain distribution of the grease The electric potential of the rotor may then rise with respect to the earth until the dielectric strength of the grease film is disrupted occurring voltage sparking and flow of discharg
71. rcestershire B98 9HE Phone s 44 0 1527 596 748 Fax 44 0 1527 591 133 info uk weg net www weg net uk UNITED ARAB EMIRATES WEG MIDDLE EAST FZE JAFZA JEBEL ALI FREE ZONE Tower 18 19th Floor Office LB181905 Dubai United Arab Emirates info ae weg net www weg net ae USA WEG ELECTRIC CORP 1327 Northbrook Parkway Suite 490 Suwanee 30024 Phone s 1 770 338 5656 Fax 1 770 338 1632 info uS weg net www weg net us VENEZUELA WEG INDUSTRIAS VENEZUELA G A Avenida 138 A Edificio Torre Banco Occidental de Descuento Piso 6 Oficina 6 12 Urbanizacion San Jose de Tarbes Zona Postal 2001 Valencia Edo Carabobo Phone s 58 241 8210582 58 241 8210799 58 241 8211457 Fax 58 241 8210966 info ve weg net www weg net ve 28 00 122009 Sujeito a altera es sem aviso pr vio As informa es contidas sao valores de refer ncia
72. rom the thermal point of view the worst working point of this self ventilated motor is 180 rom 6 Hz which means the lowest speed and therefore the lowest effectiveness of the cooling system of the motor within the defined speed range For this reason the torque derating must be calculated for this very condition According to the WEG derating criteria subclause 6 4 1 2 when operating at 6 Hz a torque reduction of 40 results in a temperature rise of 80 oC on the motor windings Furthermore it must be assumed constant V f condition because it is asked that the motor be able to operate with any WEG drive for the optimal flux solution to be applicable a WEG high efficiency motor must be driven by a WEG inverter model CFW 09 version 2 40 or higher f 6 Hz gt f fn 6 60 0 10 per unit f fn 0 10 p u gt Tr 0 6 per unit That is at 180 rom the motor will be able to supply only 60 of its rated torque Once the load demands constant torque equal to the torque demanded at base speed throughout the operating range the motor must be oversized in accordance with the derating calculated J T us 94 56 7 Nm de 0 6 Thus the motor rated horsepower will be P 567 1899 _ 10 83 kW 9 81 960 Consulting the WEG motors catalog the ideal motor for this application is the 11 kW 15 hp 4 pole 60 Hz frame IEC 132M NEMA 215 T The use of forced cooling system would be an alternative option In this case mo
73. s According to the document the solution adopted to avoid bearing currents depends on which current component is to be avoided lt may be made either by means of insulated bearings or shaft grounding system though CSA 22 2 N 100 Item 12 shaft earthing brushes must be used in motors of frame above IEC 280 NEMA 440 Gambica REMA Technical Guide for motors of frames below IEC 280 the effects of bearing currents are seldom appreciable and therefore no extra protection is needed In such cases adhering strictly to the motor and drive manufacturers recommendations regarding the installation cabling and grounding is enough For frames above IEC 280 the effects of bearing currents may be significant and for security special protection is advisable This may be obtained by means of insulated NDE bearing and shaft grounding system use In such case care must be taken not to bypass the bearing insulation 6 12 Influence of the inverter on the motor acoustic noise The rotating electrical machines have basically three noise sources The ventilation system The rolling bearings Electromagnetic excitation Bearings in perfect conditions produce practically despicable noise in comparison with other sources of the noise emitted by the motor In motors fed by sinusoidal supply especially those with reduced pole numbers higher speeds the main source of noise is the ventilation system On the other hand in motors of higher polarit
74. s of the R amp D Department WEG Equipamentos Eletricos S A Motores WEG Technological Reports TT 2000 002 TT 2003 011 Technical Notes of the Development of Products Department WEG Equipamentos El tricos S A Automa o Minimization of Losses in Inverter Fed Induction Motors Optimal Flux Solution Waldiberto L Pires and Hugo G G Mello PCIC BR 2006 Low Voltage PWM Inverter Fed Motor Insulation Issues Michael J Melfi IEEE Transactions on Industry Applications vol 42 Technical guide Induction motors fed by PWM frequency inverters 35 WEG Worldwide Operations ARGENTINA WEG EQUIPAMIENTOS ELECTRICOS S A Headquarters San Francisco Cordoba Sgo Pampiglione 4849 Parque Industrial San Francisco 2400 San Francisco Phone s 54 3564 421484 Fax 54 3564 421459 info ar weg net www weg net ar AUSTRALIA WEG AUSTRALIA PTY LTD 3 Dalmore Drive Carribean Park Industrial Estate Scoresby VIC 3179 Melbourne Phone s 61 3 9765 4600 Fax 61 8 9753 2088 info au weg net www weg net au BELGIUM WEG BENELUX S A Rue de l Industrie 30 D 1400 Nivelles Phone s 32 67 88 8420 Fax 32 67 84 1748 info be weg net www weg net be CHILE WEG CHILE S A Los Canteros 8600 La Reina Santiago Phone s 56 2 784 8900 Fax 56 2 784 8950 info cl weg net www weg net cl CHINA WEG NANTONG ELECTRIC MOTOR MANUFACTURING CO LTD No 128 Xinkai South Road
75. scielvad cesetrasscedca R oro FEN OU NG FCM ON ates cette ote secs A A A an 33 SEA a o OOO ETC TER Centr ee en 33 995 Measurement proce US rra n 39 Sal Wav omn VAO EEEE EEE ERE EEEREN E 33 9 5 2 Oscilloscope scale Nc 6 roots aero eres io oeste 29 oo O A e E EE e A an 33 OA E 34 11 O O A 3D Technical guide Induction motors fed by PWM frequency inverters 3 y gt 1 www weg net 1 Introduction The number of industry applications in which induction motors are fed by static frequency inverters is growing fast and although much has already been done within this field there is still a lot to be studied understood regarding such applications The advance of variable speed drives systems engineering increasingly leads to the need of specific technical guidance provision by electrical machines and drives manufacturers so that such applications can be suitably designed in order to present actual advantages in terms of both energy efficiency and costs This technical guide aims to clarify the main aspects concerning applications of low voltage lt 690 V induction motors with static frequency inverters supply for frames lt IEC 355 NEMA 587 in a didactic and concise approach First of all the principal and most broadly followed international standards about the subject are mentioned Then the theoretical basis of soeed variation on induction machines by means of indirect static inverters is presented as well as the
76. sesesuesesaesssaeessaeesceseseesesessesesueeseuesssaeessaeesessesesseseseessatsesaeeseaees 25 ARAS ergo ao AA e o O E O e Oe E E E E E 25 A cs o a ANAR 26 Te gt ee 0 B os COPE UOC o no ea er eee eae ee 26 lll VAART 26 Re COMMAUGUS SEC 26 8 Dimensioning and analysis of actual drive system applications Practical exaMples oooocccccnocccoocncnonncoonanonnnnonannononncnnos 26 81 ConstantiorqueabblIcalon COMI SS SO amaste 26 lt P iy Pr e 26 A o SE Ue 26 82 squarea torque application CSU GA DUI evitando idad 2r oe Me MON T EAEE A AEEA A PEL EI o e e 27 lt A o Aner anne E AA TE IE A 21 83 Special appicavon long caba 29 Sn aaa 29 o A e eE o e 29 8 4 Variable torque variable speed application textile InQUSTFY occccncconocanocnnonnnonnnonanonnononononnononcnnnnonn nono n non nnonn nono n rana nonenonnnos 30 cual E asii 30 Sa OIM taa 31 8 5 Example considering the use Of WEG Optimal FIUX cccccscsscssscssesssescssecsseseseseseeessesesescssecssusaesessecssesestscsueseaeseateesuessseseateenaes 32 ol ta OZ O UL PP DEI 32 9 Recommendations for the measurement Of PWM wavefOrMS cocccccccccccnoncnoncnonnononononnnno nono nnnnnnnnon cnn n nn nn nar n nro n nana nnnen rar nanrnrnnos 32 e ene ee eee eee eee eee eee eee eee a de A A e 32 go Parameter Mmea Weme O sesir iate trir asen sesu seextamekerstateaestavshanstansheashavstecstenitausieuekenstenseere 33 A POU COA WONG e E E eacosecssecescedstetena suds deaesctoseas
77. ssed It must be finally considered that the criteria presented here are not permanent Like every technology they may change as new materials are developed and new experiences are accomplished So the application criteria established so far may be altered without previous advice and therefore It is important that this document be periodically revised and updated 34 Technical guide Induction motors fed by PWM frequency inverters www weg net lu gt 11 Bibliography NEMA MG1 Part 30 Application considerations for constant speed motors used on a sinusoidal bus with harmonic content and general purpose motors used with adjustable frequency controls or both 2006 NEMA MG1 Part 31 Definite purpose inverter fed polyphase motor 2006 NEMA Application Guide for AC Adjustable Speed Drive Systems 2001 IEC 60034 17 Cage induction motors when fed from inverters application guide 2006 IEC 60034 25 Guide for the design and performance of cage Induction motors specifically designed for inverter supply 2007 GAMBICA REMA Technical Guides for Variable Soeed Drives and Motors GAMBICA REMA Technical Reports for Variable Soeed Drives and Motors Brochure of the mini course Squirrel Cage Induction Motors Fed by PWM Frequency Inverters R amp D of the Product Department WEG Equipamentos Eletricos S A Motores WEG Stock Products Catalog WEG NEMA Motors Catalog User s Guide of the CFW 09 Technical Report
78. sulation SYSTEM cccccscccssscsssseessecsseesseesseessseesseeeseeesseeesssesseaeeseasesenesesseseeesesaeessneeseaees 5 E T a E E E E E E cee eases eeeeceetaee 15 Cable VEIN OA noe Po o E O E RO OOO 16 Minimum time between SUCCESSIVE pulses MT BP c cccsscsscsecssescsrscssecssesesescseecssesesescssecssesestecseessaeseatecssesaesenteeeuesatseateseaes 17 AO Th SM rra lio 18 PIONS MOS anio ooo 18 Criteria regarding the insulation system of WEG motors on VSD applicatiONS ccocccccnocnnoonononononanonnnonnnonononanononcrnnnonanononos 18 Normative considerations about the insulation system of inverter fed MOTOFS cccccccccccononnononnononnnnonnnnnnnononnconnnconnnninannons 18 Recommendations for the cables connecting WEG motors to INVEITELS cccecesecsscsesssscesscsesssessseseessesceeseseseesseseaeeness 19 Cable types a a iistallation recommendation Siria ardid a 20 Influence of the inverter on the motor shaft voltage and bearing CurrentS occcccoccnoccnioncnoncnonnnnnnononcnnnnononononnnnnnnrncnonnnnnnns 20 e qeu oo AMA e Ro A 2 Equivalent circuit of the motor for the high frequency capacitive CULONIS cccccccsscssssessssesssresssessnsessneessseessseeessseessatessaees 21 Methods to reduce or mitigate the bearings Currents in inverter fed MOTOFS ooocccccccnoncnononnnoonnonnonononononnnnnnona nono nonnnnonons 22 Criteria regarding protection against bearing currents Shaft voltage of WEG motors on VSD applic
79. t the inverter terminals gt Ok 1 Ref B 200 Volt 2 us t 0 8 1 24 0 99 us At IEC criterion do not establish minimum value for t at motor terminals dV dt At inverter terminals AV 0 8 Vink pc 0 8 500 1 414 565 6 V At 0 25 us dV dt AV At 2262 7 V us At motor terminals AV 0 8 Ves 0 6 1040 832 V At 0 99 us dV dt AV At 840 4 V us WEG criterion gt 6500 V us gt 2262 7 V us gt Ok NEMA criterion gt 6500 V us gt Ok IEC criterion gt 840 4 V us lt 6500 V us gt Ok 30 MTBP minimum time between successive pulses 15 Fef E 500 Volt AG us MTBP 8 6 us the waveform shown beside is the very waveform shown in the other figures throughout this example but a convenient zoom was given to It in order to benefit the evaluation of the minimum time between successive pulses WEG criterion 6 us minimo gt Ok So all the insulation criteria of WEG motors are attended and therefore the use of filters is not necessary However these conclusions are valid strictly for the ensemble inverter motor cable leads investigated As mentioned before the utilization of other cable or inverter would cause the voltage peaks at the motor terminals to change 8 4 Variable torque variable speed application textile industry 8 4 1 Example A standard IP55 squirrel cage induction WEG motor must be dimensioned for a textile industry applicatio
80. tial Discharges PD a complex phenomenon resulting from Corona Between adjacent charged conductors there is relative voltage which gives rise to an electric field If the established electric field is high enough but below the breakdown voltage of the insulating material the dielectric strength of the air is disrupted that is if there is sufficient energy oxygen O is ionized in ozone O The ozone is highly aggressive and attacks the organic components of the insulation system damaging it For this to happen though the voltage on the conductors must exceed a threshold value the so called Corona Inception Voltage that is the local breakdown strength in air within the void The CIV depends on the windings design insulation type temperature Superficial characteristics and moisture www weg net gt ueg conductor O insulation Q Insulation degradation eee due to partial C discharges Partial discharge effect on the motor insulation system Damaged insulation due to PD activity PD is thus a low energy discharge which after long term activity prematurely degrades the motor insulation The erosion reduces the thickness of the insulating material resulting in a progressive reduction of its dielectric properties until its breakdown voltage capability falls below the level of the applied voltage peak then the insulation breakdown occurs 6 5 3 Minimum time between successive pulses MTBP The volt
81. tions to avoid excessive overheating of the inverter ted motor Torque derating oversizing of the self ventilated motor frame Utilization of independent cooling system separate ventilation Utilization of the Optimal Flux Solution exclusive to applications using WEG drives and motors 6 4 Criteria regarding the temperature rise of WEG motors on VSD applications 6 4 1 Torque derating In order to keep the temperature rise of WEG motors when under PWM supply within acceptable levels the following loadability limits must be attended observe the motor line and the flux condition NOTE Applications with motors rated for use in hazardous areas must be particularly evaluated in such case please contact WEG 6 4 1 1 NEMA market 143 1000 1 587 ao Constant 60 120 Hz flux 60 120 Hz Optimal flux Constant 60 120 Hz flux W EG e 120 Hz Optimal flux E 1000 1 587 a ee 1000 1 Constant 60 120 Hz flux 587 ia ea 60 120 Hz Optimal flux Constant 1000 1 6 1 1000 1 flux SS 60 120 Hz Optimal flux Satisfactory motor performance depends on proper drive setup please contact WEG WEG drive CFW 09 version 2 40 or higher operating in sensorless open loop vector mode Motors with rated power lt 250 hp Criteria also valid for motors of the frame sizes 447 and 449 Motors with rated power gt 250 hp Criteria also valid for motors
82. to ground Interruption of this current therefore requires insulating both bearings Alternately shaft grounding brushes may be used o divert the current around the bearing It should be noted that insulating the motor bearings will not prevent the damage of other shaft connected equipment NEMA Application Guide for AC ASD Systems the circulating currents caused by common mode voltage may cause bearing problems in frame sizes smaller than 500 most likely in the 400 and larger frames IEC 60034 17 for machines with frame numbers above 315 it is recommended either to use an inverter with a filter designed to reduce the zero sequence component of the phase voltages so called common mode voltages or to reduce the dV dt of the voltage or to insulate the motor bearing s The need to insulate both motor bearings is seldom necessary In such a case the examination of the whole drive system by an expert is highly recommended and should include the driven machine insulation of the coupling and the grounding system possibly use of an earthing brush Technical guide www weg net lu gt IEC 60034 25 do not specify a minimum frame size on which bearing protection must be applied Within the clause broaching the effects of magnetic asymmetries as shaft voltages bearing currents cause it is mentioned that bearing currents commonly occur in motors above 440 kW For other causes no mention is made concerning frame size
83. tor oversizing is not needed and a motor rated 7 5 kW 10 hp 4 pole frame IEC 1325 NEMA 213T would satisfactorily attend the application needs Technical guide Induction motors fed by PWM frequency inverters This way it is assured that the temperature rise of the motor will be equal to or less than 80 K at any operation condition 8 1 2 2 Regarding the insulation system According to NEMA criteria the situation is the following Voltage at the motor terminals Voltage V V peak at motor terminals 400 1 414 565 6 Y 0 90 565 6 509 04 Y 0 10 565 6 56 56 Y bpe wb ee wm ene nem m enn ene wenn eee time s According to WEG insulation criteria clause 6 6 WEG motors rated 400 V are able to stand dV dt values up to 5200 V us at the drive terminals thus satisfying the needs of this example tr gt 0 1 us at the inverter terminals thus attending this example s application Vpeak lt 1430 V at the motor terminals If this condition is not attended at the definitive installation filters must be connected to the inverter output The switching frequency defined for this example 5 kHz is in agreement with WEG recommendations too Therefore the motor designed fully attend this application s demands with regard to the insulation system However it will not be possible to evaluate the matter on the point of view of IEC because it requires the measurement of the voltage at the motor t
84. weg net The utilization of static frequency inverters comprehends currently the most efficient method to control the speed of induction motors Inverters transform a constant frequency constant amplitude voltage into a variable controllable frequency variable controllable amplitude voltage The variation of the power frequency supplied to the motor leads to the variation of the rotating field speed which modifies the mechanical speed of the machine The torque developed by the induction motor follows the equation below T K Om lo Despising the voltage drop caused by the stator impedance the magnetizing flux is found to be V P n Ko 1 where T torque available on the shaft N m Pm magnetizing flux Wb I rotor current A gt depends on the load V stator voltage V k e k constants gt depend on the material and on the machine design Considering a constant torque load and admitting that the current depends on load therefore practically constant current then varying proportionally amplitude and frequency of the voltage supplied to the motor results in constant flux and therefore constant torque while the current remains unchanged So the motor provides continuous adjustments of speed and torque with regard to the mechanical load Losses can be thus minimized in accordance with the load conditions by keeping the slip constant at any speed for a given load The curves below are obtained from t
85. y three V per unit magnitude of the voltage at the nth harmonic frequency 6 2 Considerations regarding energy efficiency The lack of international standards that specify test procedures to evaluate the system motor inverter efficiency allows such tests to be carried out in many different and non contestable ways Therefore the results obtained should not influence the acceptance or not of the motor except under mutual accordance between customer and manufacturer Experience shows the effectiveness of the considerations below An induction motor fed by PWM voltage presents a lower efficiency level than when fed by purely sinusoidal voltage due to the losses increase caused by harmonics Anyway when induction motors are fed by static inverters the efficiency of the overall system rather than the motor efficiency only should be evaluated Each case must be properly analyzed taking into account Characteristics of both the motor and the inverter such as operating frequency switching frequency speed range load conditions and motor power THD etc The measuring instrumentation is extremely important for the correct evaluation of electrical quantities on systems under PWM duty True RMS meters must be used in order to permit reliable measurements of power Higher switching frequencies increase the motor efficiency and decrease the inverter efficiency due to the increase of commutation losses High efficiency motors

Induction motors fed by PWM frequency converters

Contents

Download Pdf Manuals

Related Search

Related Contents