5. ELECTRIC DRIVES
Contents
1. Frequency reference Signal converter Figure 5 20 PID control of a pump drive 952. The mechanical characteristics of the induction motor are characterized by maximum or breakdown torque When the breakdown point is crossed the velocity decreases and motor stops when the load stays This is a short circuit operation of the motor Because of this when the load changes in a large area induction motor are difficult to use or not recommended Commonly the braking of the induction motor by plugging with automatic stop on zero speed or by dynamic DC braking can be done The main drawback of plugging is the high current slightly greater current than the starting current and a need of zero speed sensors to stop the motor In the case of dynamic DC braking the additional DC voltage source or semiconductor diodes must be used A disadvantage of dynamic braking is also a non linear dependence between the torque and speed It is relatively complicated to protect the induction motor When the armature current of the DC motor is proportional to the load and resulting motor load in an induction motor the dependence between the stator current and the load is much more complicated Mathematical description of physical processes in the induction motor is difficult To obtain objective 79 information about the physical processes by measuring electrical signals is complicated because the measured stator current and voltage characterized the state of motor only particularly Soft starters are intended for smooth starting and stopping with
3. stopping For this reason acceleration and deceleration times are selected by using formulas in Table 5 1 87 Table 5 1 Kiirus Dynamical braking power 1 Tus Tma Motor power 0 8 230 V system 148 2 Dynamical braking power _ Braking resist 400 V system 593 Dynamical braking power Braking resist For motor torque compensation voltage frequency function U f f settings are is used Figure 5 8 Before setting of parameters the character of the motor load must be determined e g torque is independent of the speed T const or torque is proportional to the speed fans pumps Nonlinear voltage frequency function A02 1 is selected on the fan load Linear voltage frequency function is sufficient at constant load In either case the decrease in torque at low frequency must be compensated with a voltage increase A02 0 The value of basic frequency foase is 50 Hz When the basic frequency which determines the regulation area of the frequency differs from the default value then its value must be set before motor start with parameter B00 3 Voltage Voltage A021 A02 2 Frequency A02 26 ie Frequency fooni 2 fo hi fmax B00 3 B00 2 Figure 5 8 Settings of torque compensation of voltage frequency function Automatic torque compensation decreases the voltage automatically when the motor load decreases Figure 5 9 Compensation parameter A02 2
4. commutator today rarely are used Instead brush less DC motors DC motor with a semiconductor commutator are used In terms of construction a brush less DC motor is similar to AC synchronous motor fed from the frequency converter Third the AC network feeds the electric drives and using of a supply converter rectifies the voltage of the motor Supply converters of DC and AC drives consist of rectifiers Armature windings of a DC motor are commutated depending on the revolutionary angle of the armature Such kind of a principle can be used to control the synchronous machines too This is just one difference between DC and the types of motors The principal scheme of a DC drive is given in Figure 5 2 The power circuit of the drive consists of a non controlled rectifier a filter a braking chopper and a 4 quadrant pulse width converter GTO thyristor switches used in drives with large power are shown in the figure In drives with smaller power IGBT transistor switches are used Whereas non controlled rectifier is not able to convert energy between the drive and the network the energy of the generator must direct to filter capacitor Cr voltage increased or dissipate in the brake resistor Rg For that reason the brake chopper PLpg is switched on Lr IX ZX ZX wf A O CF E AAA Z 2 PLg Rectifier Filter Brake chopper Figure 5 2 Principal scheme of a DC drive A 4 quadrant pulse width converter enables operating of a DC machine o
5. dynamical braking is used the regenerative torque limit is determined by the formula 93 V2 T Motor power kW 100 Pae E resist F i where coefficient V2 is 148 2 when power supply voltage is 230V and 539 when the power supply voltage is 400 V Motor heating time depends on energy losses current of windings thermal time constant and cooling conditions Cooling conditions depend on the speed of motor rotation Because of this motor protection is set on in relation to the time current characteristic the form of which depends on real the operation mode Overcurrent limit settings are selected by the time current characteristics parameter B30 0 Depending on the setting of the parameter B30 0 the form of the time current characteristic varies Fig 5 18 The form of the characteristics depends also on the motor load e g CT constant torque or VT variable torque Relative current is determined on the base of the rated current 100 Operating time B30 0 50 B30 0 100 3 nalia cece Tere a poo VT load i B00 7 2 2 os H CT load 2 7 B00 7 1 a 1 50 100 150 Relative output current Figure 5 18 Second ampere characteristics of over current limit There are additional settings for forward F RUN and reverse R RUN commands The lower limit of the starting and braking frequency determines the frequency level when the motor will stop At frequency setting 0 the real minimu
6. of electrical drives Traditionally the frequency converter was intended for smooth regulation of voltage and frequency of a motor But modern frequency converters have a number of functions In principle a frequency converter with an induction motor is a complex electrical drive A frequency converter consists of a feeding converter sensors and a control system The control system is able to control the actuator or complicated automatic circuits of the technological process By help of network interface a frequency converter is usable on automatic systems with a number of levels Frequency converter has a user interface for the drive operation mode control Use of frequency converters in electrical drives was intended already after the invention of the induction motor First machine converters were used for this purpose Machine converters consist of a variable speed DC motor connected mechanically to a synchronous generator This converter a feeding source of variable speed drive was very expensive and economically inefficient After invention of the thyristor 1956 the idea of developing static without rotating machines frequency converters became realistic At the beginning of the 1960s the first thyristor frequency converters were built At the same time the first pilot thyristor frequency converter was built at the Department of Electrical Drives and Power Electronics of Tallinn Technical University However frequency converters use in in
7. on the rated current is selected in the range 0 20 of rated voltage conventionally 3 5 Too high value of parameter A02 2 makes the drive unstable and overcurrent protection turns on 88 Voltage High load ea Small load A022 Nh B00 3 BO0 2 foase fmax Frequency Figure 5 9 Automatic torque compensation Slip compensation is intended to stabilize motor speed when the load increases Figure 5 10 The value of setting is equal to the motor slip on the rated load and is selectable in the range of 0 5 Frequency Load 0 Time Figure 5 10 Motor slip compensation The character of the output voltage depends on if the DC voltage automatic regulation is operating or not BOO 4 0 and if the output voltage on the basic frequency is equal to the input voltage DC voltage automatic regulation decreases the output voltage fluctuations caused by the variation of input voltage If setting of the maximum frequency fmax parameter BOO 2 must be observed that its value does not exceed the value on the motor nameplate Basic frequency of the converter output voltage and current must be stated in relation to the motor nameplate values Output voltage must be not higher than rated voltage of the motor Motor rated current J parameter BOO 5 must not be greater than rated current of the converter and smaller than 30 of the rated current of converter It means that Lim gt gt 0 3 Im where I is the rated current of the co
8. to externalize operation of a drive on the mechanical resonance frequency For this some prohibition places are determined in the frequency regulation area of the motor with width 0 10 Hz where the continuous regulation of frequency is impossible To set the frequency skip mechanical qualities of the drive and the actuator must be studied Ratio interlock settings and upper lower limits make it possible to fit analogue frequency setting signals with the regulation qualities of the system To select the module and the polarity of the amplifier coefficient A the amplitude and polarity of the displacement signal B and limits of control signal upper and lower limits must be established A scheme of signal fittings and ratio interlock settings is given in Fig 5 16 The output signal is calculated by using formula Y AX B C Help input is used as displacement signal C Ratio interlock settings are principally is used in drives with closed loop control systems e g to control a pump by help of pressure feedback Signal level detections are intended to check output signals of the drive frequency current and speed and to obtain logic signals in the output which shows the logic state of the analogue signal below or over the threshold level The logic output signal can be used to control the output relay Detection of output frequency current and speed is shown in Fig 5 17 Detection level is selected by parameter B26 0 lower Fmax 1 20 Contr
9. 5 ELECTRIC DRIVES 5 1 General description Electric drive is an electromechanical system mechatronic system intended to set into motion technological equipment They consist of an electric motor motors a transfer mechanism an electrical energy converter and a control system The control system consists of a microcontroller with data connection interfaces data channels data network sensors and actuators A generalized structure of the electric drive is shown in Fig 5 1 In general the main task of the electric drive is the motion control of mechanisms An electric drive is an automatic control system with a number of feedbacks where different automatic control principles such as error driven feedback control model based control logical binary control or fuzzy logic control methods are used Depending on a particular technical solution and selected control principle different sensors for measuring of currents voltages velocity acceleration torque etc in an electric drive are used Other information like pressure signal for controlling pumps and compressors air humidity and or temperature signal for controlling of fans etc is also necessary For that reason the controller of the drive must process different information Data network 3 x U Data communication Mechanical with PLCs and PCs transmission Microcontroller Power L hi with I O converter oad machine interfaces f Optical link between signal and powe
10. 95 U 3 Duration of acceleration ramp t 0 5 60 s 3a Rapid increase of motor voltage after reaching the rated velocity 4 Current limit of motor 2 5 I where Z is the rated current 5 Rated voltage U 5a_ Rated current Z 6 Energy saving operation motor operating at reduced voltage 7 Stopping with the motor switch off coasting 8 Deceleration ramp Duration of deceleration ramp depends on the moment of inertia and duration of acceleration ramp e g td ramp 2 ta ramp Deceleration ramp may be linear or non linear 8a Variation of voltage on deceleration 9 Dynamical braking e g duration of braking is 5 s Braking begins after the end of deceleration ramp Current of dynamical braking can be selected e g 2 I Energy saving operation is possible only when the motor operates at constant speed Motor voltage is regulated depending on the power factor Motor efficiency power factor and energy consumption depend on supply voltage and the optimal value of feeding voltage can be determined Fig 5 6 As the result of the regulation of motor voltage the active energy depending on motor load will be saved and the power factor will be improved reactive energy decreases Energy consumption 100 load 50 load 0 Motor supply voltage Figure 5 6 Energy consumption depending on the load and supply voltage Low speed or jogging makes it possible to position the shaft of motor or the driven machine This is a short t
11. dustrial drives was very slow for a long period The main reason was the high price and low reliability of thyristor frequency converters Appearance of high power transistors as insulated gate bipolar transistors IGBT changed the situation profoundly Today frequency controlled AC drives are applied almost everywhere where only DC drives were used before Frequency control is traditionally intended to adjust motor torque speed or motor shaft position rotating angle by help of closed or open loop control circuits provided output voltage and frequency change relatively slowly and motor operates in a continuous duty Closed loop control systems make it possible to increase the measuring accuracy of output signals and make dynamical qualities better In converters with traditional control duration of acceleration and deceleration ramps is strongly limited Overloading level of an induction motor at conventional frequency control is relatively small 2 2 5 times Sharp changing of the motor load may cause breakdown of the induction motor Then motor is stopped or rotates at a very low speed jogging Motor losses are increased and the temperature of windings grows sharply For these reasons conventional frequency 84 converters for the control of continuous duty operating devices such as pumps cooling fans and compressors are used Today s fast operating electric drives are controlled by using the vector control The vector control is the c
12. e middle value of the voltage When durations of positive and negative half wave are equal output voltage is equal to zero PL is continuously in open state LEU GE Pl PL is continuously in open state Pree Figure 5 3 Regulation of voltage and variation of polarity 4 quadrant pulse width converter by using a one or two switches or b four switches 78 5 3 Soft start of the induction motors Today most commonly used electrical motor is an induction motor As compared to other electrical machines induction motor is reliable durable in heavy conditions and need a little current maintenance Because of this induction motors are economically efficient For a long time induction motors were known as electric machines difficult to control and were used only in electrical drives with constant speed Induction motors are mainly used to set into motion general purpose machines like fans compressors pumps and other machines like disc saws wood planes These machines operate in a continuous duty regime but to optimize the processes and of save energy regulation of productivity and speed is often necessary Because ten years ago flexible speed regulation of induction motors was very difficult had to be put up with lower efficiency of a machine or lower quality of production Problems of starting braking and protection of induction machines may be solved by help of power electronics and microprocessor tech
13. he motor is started by using start with pick up flying start then the converter synchronizes frequency with the motor speed automatically When the rotating motor starts without pick up the protection may operate Setting parameters of the converter determine the auto start conditions When the corresponding parameter is on state 1 OFF the auto start is switched off The drive will be started by command RUN ON after the recharging of DC circuit capacitor When the corresponding parameter is on state 2 after switching on the power supply voltage the motor starts automatically without the pick up start provided that command RUN and converter recharging are finished When the corresponding parameter is on state 2 then 3 the motor starts with the auto pick up start The latter method is used to continue operation after the motor stops by means of disturbances of the power supply voltage The drive starts automatically after the power supply voltage is recognized 90 Output frequency The speed when motor free wheeling Deceleration ramp The stopping by free wheeling Time FRUN Figure 5 13 Methods of stopping the drive Recurrence switching and pick up is used to restart of the drive after unexpected switching off This function is very important if the drive is intended for continuous duty operation Fig 5 14 This function is useful if the supply voltage is occasionally switched off due to operation of an
14. ime operation e g up to 120 s and the speed may be selected as 7 or 14 of the rated speed Jogging speed is achieved by the selected switching of thyristors Thyristors are switched to the stator winding every 7 or 14 half wave of the alternating current Thus variable polarity current pulses at the frequency of the first harmonic 1 7 14 or 1 14 7 are generated 82 Linear acceleration and deceleration ramp is achieved by using the feedback signal of tachogenerator see Fig 5 4 Help of switches connected with the soft starter may select the operating mode of the motor e g acceleration deceleration dynamic breaking jogging speed etc There is a possibility of motor overload protection in a soft starter To determine the operating state of the induction motor the rotor current and resistance or the magnetic flux of the air gap must be measured However it is very difficult Because of this the induction motor which has reliable and simple construction fails in reason of overload when its protection device is faulty Because of this microprocessor technique software based protection systems are used Operating time of the protection device depends on value of over current The higher the over current the shorter the operating time of protection device is The time current characteristics of protection are shown in Fig 5 7 The protection may be set on the standard duty or heavy duty operation mode of the motor On the heavy du
15. l cycles amount of production etc can be used But unfortunately this software may be more expensive than the converter hardware New sensors and optical data connection Today the numeric control of electric drives is widely used and the traditional analogue sensors will be replaced with the new digital sensors for measuring of speed and position These sensors may be the pulse sensors as well the code sensors to measure the position On ones own problem is measuring of voltages and currents with high harmonic components specially then when the control information is necessary very quickly and then the filtration of the signals is very difficult Current transformers defaced the signals and they are not acting so fast Bypasses in the current circuits gives the accurate current signal but galvanic connected sensor signals with power circuit are not usable in microcontroller By this reason Hall sensors based magnet electric current converters which are guaranteed galvanic separation of control and power circuits in electric drives are used To forward the output signals of the controller to converter fibber optic data channels are used 76 5 2 Electric Drive with DC Machines For some reasons today the term DC drive is not clear First the operating principle of the electric machine assumes periodic commutation of the current of the winding because of this the current of the winding is actually AC current Second DC machines with a mechanical
16. m frequency is 0 1 Hz and motor does not stop Motor stopping is possible by using settings B32 0 Upper limit of the starting frequency is used when the motor must start to the low speed The motor does not start when the frequency setting is higher than the limit f 0 20 Hz but the command RUN is at ON Hysteresis with width 0 20 Hz of the starting and braking frequency externalizes the instability of the state of the drive Starting time delay is used to synchronize the operation with other devices such as mechanical brake A time diagram of the starting timer is shown in Figure 5 19 parameter B32 3 The starting timer form a programmable time delay fgeiay after the given start command F RUN 94 Figure 5 19 Starting time delay The direction rotation frequency of operation and operating cycles can be determined by the macro program of the frequency converter and can be changed automatically The programmable settings of a programmable operation mode enable a cyclic operating of the electric drive on principles of sequential logic On PID control a feedback signal to the additional input AUX is used The block scheme of the PID control is shown in Fig 5 20 Setting parameters of the described frequency converter provide only an overview of the possibilities of using contemporary converters The full information about programmable parameters is given in user s manual of every type of a converter ees converter E gt
17. n 4 quadrants of torque velocity surface It means that directions of torque and velocity are changeable and the machine may operate as a motor or and a generator There are two ways to control the pulse width converter First using couples of semiconductor switches PL1 and PL2 or PL3 and PL4 may choose direction of the motor velocity When one couple is chosen the other couple is continuously switched off and the direction of motor velocity is not changed The voltage of the converter and the velocity of the motor are regulated by semiconductor switches for example PL and PL2 using the principle of the pulse width modulation 77 To regulate voltage and velocity it is sufficient to have pulse width control of one semiconductor switch e g PL To change the direction of the motor velocity the direction of the armature current will be changed The direction of the excitation current is not changeable Therefore the couple of semiconductor switches e g PL and PL2 are switched off and the other couple e g PL and PL4 are switched on To regulate the motor velocity semiconductor switch PL is used Output voltage diagrams of the 4 quadrant pulse width converter are shown in Figure 5 3 A 4 quadrant converter may be used such that semiconductor switches PL and PL2 and PL3 and PL4 are switched in a couple Then the output voltage of the converter has a quadrangle form Changing the relative switching time of semiconductors will regulat
18. ncy setting for jogging acceleration deceleration time torque boost DC brake setting and sometimes settings of input signals Frequency setting Frequency setting entered to the control panel with the local control regime influence directly the operating speed of the drive Frequency is selected by help of keys increment and 4 decrement Frequency setting on the local control may be given by using a potentiometer connected to the converter DC voltage or current source and on the remote control by help of a control computer 85 Figure 5 5 Power circuits of a frequency converter Frequency converter Analogue inputs Setting by voltage R 2kQ 2W Voltage output Setting by current PO qe i i OV Ono 0 10 V Auxiliary input 10 V Load current for feedback max 1mA Analogue inputs 5 OO ee On common terminal Max 1A 250V AC or 30VDC Max 1A 125V AC Digital inputs o i or 1A30V DC Sequential logic inputs 5 mA Open collector Max 30V DC 30mA Figure 5 6 Connection scheme of control circuits 86 Digital frequency settings conventionally consist of robust and exact settings By using robust setting frequency is changed with step 1 Hz and help of exact setting changes frequency changed with step 0 01 Hz Acceleration and deceleration time settings are changeable in a large area 0 1 6000 s Acceleration and deceleration time settings are given in Figure 5 7 Frequency at the beginning of
19. niques by using soft starters The progress of power semiconductor technique microelectronics calculation techniques and other technologies make it possible to develop an operating regime optimizing soft starters to control the induction motors Earlier induction motors caused a number of problems For example high power induction motor start was a problem With direct on line start of the induction motor the stator current increases up to seven times over the rated value causing high current peaks in the electrical lines Starting current of an induction motor does not depend on the motor load and is a constant value for each motor Relative starting current Istar Iratea is given on the motor nameplate and commonly it is 4 7 times higher than Z atea In long electrical lines and networks with high resistance starting of a high power induction motor causes an essential voltage drop on other consumers Thereby the starting torque of the induction motor is relatively small as compared to the DC motor Because of this the duration of the starting process is so long when the load and moment of inertia are large heavy start This case causes essential heating of motor windings and it is dangerous to the insulation when protection devices are not used As different firm the DC motor the torque of which increases proportionally when the load increases the mechanical characteristic of the induction motor relation between torque and velocity is non linear
20. nverter Increasing the carrier frequency of the pulse width modulation can reduce the electromagnetic noise Sometimes the carrier frequency is decreased automatically when the load increases Fig 5 11 Because of this when the carrier frequency is lower loading capability of the motor increases Relation between the maximum relative current of the motor and the carrier frequency is shown in Figure 5 12 For example on the variable torque load when the temperature of the heat sink of the converter rises up to 70 C the carrier frequency decreases automatically to 4 kHz 89 Period of carrier frequency BOO B00 1 f1 t B00 5 Supply voltage Figure 5 11 Settings of the carrier frequency and the output voltage and current Output current 100 Varying torque 8 per 1 kHz 100 Constant torque Carrier frequency 3 4 5 6 7 8 9 10 11 12kHz Figure 5 12 Relation between the maximum relative current of the motor and the carrier frequency The selection of starting and stopping methods helps to determine the type and purpose of the switches that will be used for drive control The drive may be stopped by coasting or by the deceleration ramp With the deceleration ramp motor speed is reduced when the frequency decreases until the braking frequency and then dynamical braking is used To start the motor after coasting it should be observed that the motor stops Switched off but until rotating t
21. o the stator winding A separated switch makes it possible to select a low speed operation or jogging A soft starter has a number of output terminals They intended to control the relays connected series with the voltage source or contactor The possible use of output terminals is shown in Figure 5 4 The voltage and current time diagrams of starting stopping and the energy saving operation Figure 5 5 can characterize the operation of the soft starter The dynamic braking after the end of deceleration ramp on low speed is possible 80 Line voltage L1 L2 L3 Supply of control circuits 220 240 V FA Output terminals A1 A2 B1 B2 SOFT STARTER Programmable End oframp DC breaking Start Alarm Input terminals H FTI 7 j TAV S Ho Poi l Ta Start Stop Speed up Small speed Pet a r operation ermistor generator Figure 5 5 Voltage and current time diagrams of starting stopping and energy saving operation down 81 The voltage and current time diagrams consist of the following stages 1 Acceleration ramp duration of ramp some periods for example 0 1 s la Voltage of acceleration ramp regulation area 30 95 U Shock start voltage selected until
22. ol signal ATN attains the output switched ON when the output frequency approaches the frequency setting For output current detection a detection level by setting B26 1 5 300 rated current is selected Control signal IDET current detection level is switched ON or OFF with hysteresis 5 For speed detection the detection level SD1 or SD2 is selected by setting B26 2 or B26 3 depending on the maximum frequency fmax in the range of 1 105 Control signal SPD depends on speed detect level and is switched ON or OFF with hysteresis 1 92 Auxiliary input B24 0 f1 2 Upper and lower boundary Gain Voltage or current input Frequency control Y Polaatgus Polarity B23 0 f0 1 B23 0 f1 1 ain 2 B23 1 B23 2 Gain Bias Figure 5 16 Signal fittings and ratio interlock settings Frequency setting Output current Output frequency B26 0 CON ON Time N Time s ON Tig ATN IDET Ai EEN a ee ON Figure 5 17 Detection of output frequency current and speed Motor current can be limited by the frequency speed decreasing like that does not overcome the value set by the parameter B29 0 Motor rated current is considered as 100 current The setting of overcurrent limit must be higher than the current of the unloaded motor At drive braking or at constant speed operation the regenerative torque is limited The limit of regenerative torque is e g 10 parameter B29 1 when dynamical braking is not used When
23. ontrol method by help of the rotating direction vector of the magnetic field Because of this vector control is called field orientation control Conventional frequency control may be called scalar control because it is based on measuring of the effective values of frequency and voltage These values are average Vector control is based on the dynamical model of the induction motor model based control Vector control is similar to DC drive control and may be used to control mechanisms with the dynamical load elevators robots etc Power circuits of a frequency converter are shown in Figure 5 5 and the connection scheme of control circuits in Figure 5 6 To control a frequency converter by local and remote control methods the program control is used Standard software of a frequency converter consists of a number of typical macro program and the selection and settings of parameters determine the operation mode of the drive Setting parameters of the frequency converter may be classified to some groups Traditionally three groups of parameters are distinguished e frequently varied parameters e rarely varied parameters e parameters varied by the producer First two groups of parameters may be changed by a user As a rule a user has no information about the parameters of the third group and the producer may change these only Parameters of a frequency converter are changed digitally Frequently varied parameters are frequency setting freque
24. out current peak and with adjusted acceleration of an induction motor and reducing of energy consumption on the variable load specially if the load is smaller than the rated load By using of soft starters is also possible to control the braking of the induction motor Some of soft starters allow the DC braking short time low speed operation or jogging and controllable linear braking ramps by help of tachogenerator Also a kick start is possible when starting conditions are hard Soft starter is a thyristor based AC voltage regulator where thyristors are controlled by help of a microcontroller Due to a microcontroller soft starter performs relatively complicated functions For example with voltage regulation is possible to decrease the starting current peak to choose duration of acceleration and deceleration ramps and to optimize energy consumption power factor on a variable load A microprocessor system makes it possible to realize sufficient second ampere characteristics of the overload protection By commutating of thyristors it is possible to switch on and off half waves of 50 Hz alternating voltage In this way a short time low speed operation can be used A microprocessor system of a soft starter is Operating as starting and protecting system The protection functions are as follows adjusted over current limit protection against input or output phase interruption circuit break short circuit of thyristor heat sink overheating wrong freq
25. r circuits Currents voltages Speed position torque Signal converters K Sensors Figure 5 1 Generalized structure of an electric drive Microprocessor technique enables us to apply different control methods in today s control systems including control of electric drives In the drive controller the signal processors with inbuilt analogue digital converters timers pulse width modulators and other devices which simplify drive control are commonly used Integration of devices and functions Microprocessor techniques make it possible to solve all control tasks like energy conversion between the drive and the converter protection and tasks connected with the sequence logic of a technological process Because of this a 75 microcontroller is a complex control device which is realizes the computer control programmable controller functions and supervision of the process A drive controller is programmed and controlled by help of commands and subprograms of special functions Software of a drive is complemented with supplement functions Since modern industrial systems are based on data communication networks recent controllers are intended for action with field buses A controller guaranteed the flexible speed regulation and motor protection Software solutions Today s theories published will be realized as software of devices You can tell already today that drive controllers are able to solve all problems and tasks desc
26. ribed in course books of electric drives Controller software includes a number of contemporary control methods like adaptive control model based control etc Vector control of AC drives was a technical novelty ten years ago but today this method is largely used in frequency converters Economic considerations A situation may arise when functional qualities of a drive controller and completeness of the interface are not harmonized To reduce the price market compelled producers often simplify the interface of converters Therefore the number of control buttons and decimal places of sign number display must be reduced to 3 To display the large variety of drive controller states different faults signals etc every segment of the 7 segment digit indicator could be used to display coded information It will be very inconvenient for the operator of the drive equipment to acquire this very special coding Naturally it is a different question if the user needs programming or reprogramming the converter or not or if the programming and service must be done by specialized firms Power converters with the most intelligent user interface can communicate in common national languages understandable for everyone Maximum convenience will be achieved by special HMI PC software In this case all the useful information about the drive and technological machines can be displayed on the PC screen Different functional diagrams about energy consumption technologica
27. the start called the start frequency Braking frequency is the frequency when the dynamical braking is turned on The value of DC voltage used for dynamical braking depends on the power of the converter and is changeable within range 0 1 20 default 5 whereas on higher power lower voltage is selected The duration of dynamical breaking is determined by corresponding parameter in the range of 0 20 s Jogging frequency setting is selected before the start Jogging frequency setting is intended for motor short time operation at low speed and for positioning of the mechanism A motor start to the forward or reverse direction of jogging takes place by help of commands F JOG or R JOG forward jogging or reverse jogging Acceleration and deceleration times will be set by help of parameters B21 9 and B21 3 Fig 5 7 Frequency Maximum frequency Fmax Jogging freq DC breaking Starting freq Braking time Braking voltage J ta tb i i ta tp A01 0 A01 1 B21 2 B21 3 Forward or reverse start Jogging Figure 5 7 Programmable acceleration and deceleration times Acceleration and deceleration times must be fitted with real parameters of electric drive For drives with high inertia selection of very short acceleration and deceleration times is not possible because then high torque and power of the drive are necessary In real drives selection of so short acceleration and deceleration times causes protection setting and motor
28. ty operation mode of the motor the permitted overload and temperature of windings are higher Characteristics show that the operation time of protection is 30 seconds and over current is 3 on standard duty operation mode and 4 5 on heavy operation mode 10000 s 1000 100 2 3 4 5 10 lp Figure 5 7 Time current characteristics of overload protection After the operation of overload protection the cooling period will follow During the cooling period of the motor the restart is impossible 83 5 4 Frequency controlled electric drives Many problems arise with velocity regulation of an induction and a synchronous motor It is well known that the best way of velocity regulation of a squirrel cage induction motor and a synchronous motor is variation of frequency of the supply voltage For a long time there was no simple and cheap way for frequency regulation For that reason the variation of the pole number of a motor is used The velocity regulation of an induction motor makes it possible to use the induction motor with the phase winding with slip rings and brushes because the slip of the rotor and the velocity of this motor are inversely proportional to the resistance of the rotor circuit But in this case there are high losses in the rotor rheostat Today s the frequency control of induction machines is the main method of velocity regulation and frequency converters are the main components
29. uency of feeding voltage and troubleshooting of microprocessor Other protection functions like thermal overload protection stopped rotor protection unexpected load disappearing protection troubleshooting of memory and long lasting start or long lasting low speed operation protection in some soft starters can be realized It is possible to connect a soft starter to the industrial data communication network by help of the data communication interface Soft starters with analogue and digital control panels are manufactured A soft starter can protect also the others technological devices For example the smooth starting of pumps externalizes hydraulic shock in the piping Construction and operating principle of soft starter The scheme of power circuits and external control circuits of a soft starter is shown in Fig 5 4 Push buttons Start and Stop are connected to the input terminal of the control circuit The two push buttons with impulse contacts or one tumbler with two fixed position may be used Thermal relay FT is intended for motor protection and will be selected depending on the motor rated current The fuses F1 protect feeding circuits of a soft starter The thermistor is foreseen for the motor protection against the overheating To regulate speed automatically and obtain the linear starting ramp the signal of a tachogenerator is connected to the input terminal When starting conditions are hard heavy start rated voltage can be provided t
30. y protection device but the fault occurred is probably temporary By corresponding parameters the number of recurrence switching and retry wait time will be set When the flying start is impossible the converter will send the output signal that the retry time 1s out Output frequency The speed detection 1 2 s The restoring of output frequency Motor speed 1 waiting time after the overcurrent ie PWS 2 flying restart B28 1 3 flying restart i n 1 n 2 nes 4 successful flying restart Internal logic jn andendofprobes signal Figure 5 14 Recurrence switching and flying start The frequency converter has the emergency stop function which makes possible to select the emergency stop method e g stopping by coasting or by deceleration ramp For smooth starting and braking S shape acceleration and deceleration ramps are used Figure 5 15 Help of parameter B21 4 which determines the time interval ts of smooth change of acceleration whereas acceleration and deceleration times ta sets shape of the ramp and t are not changing Different converters enable ramp setting by selection of typical ramps Smooth control of acceleration is very important in control systems of electric drives of trams trains trolleys etc because passengers are very sensitive to variation of acceleration push 91 Output frequency Figure 5 15 S shape acceleration and deceleration ramps Frequency skip settings are intended
Download Pdf Manuals
Related Search