AC Motor Drives
Contents
1. eee 4 3 1 Protective Grounding amp Functional Grounding 4 3 2 Ground Mo 0 cT 5 3 3 Earlhitig SYSTEMS dera ceeds taastatuna taas 5 Chapter 4 Solution to EMI Shielding eea 9 ASV AV hiatis Shilelding odo uei ten pon a eut od pote d et ttis 9 4 2 How to Reduce EMI by Shielding sssssssseeee 10 Chapter 5 Solution to EMI Filter rrrrrnnnnnenenrree 12 Ni M ida ta mda tada 12 5 2 Harmonic 181 2111548 1170 TM ieget eee Rc pet REPE ERE b oebdeues ebccenrepbters 14 Chapter 1 Introduction 1 1 What is EMC Electromagnetic Compatibility EMC is the ability of an electrical device to function properly in electromagnetic environments It does not emit electromagnetic noise to surrounding equipment and is immune to interference from surrounding equipment The goal is to achieve high immunity and low emission these two properties define the quality of EMC In general electrical devices react to high and low frequency phenomena High frequency phenomena are electrostatic discharge ESD pulse interference radiated electromagnetic field and conducted high frequency electrical surge Low frequency phenomena refer to mains power harmonics and imbalance The standard emission and immunity levels for compliance depend on the installation location of the drive A Power Drive System PDS is installed in an industrial or domestic env2. 5 When excessive pulse modulated currents pass through an un grounded AC drive cable it acts as an antenna and creates radiated interference Chapter 3 Solution to EMI Grounding The leakage current of an electronic equipment is conducted to ground via the grounding wire and the ground electrode According to Ohm s law potential differences may arise when the electrode s ground and the ground s ground resistance are different According to Ohm s law the earth resistance for electrode and the ground are different in this case potential differences may arise 3 1 Protective Grounding amp Functional Grounding Please carefully read the following instruction if two types of grounding are applied at the same time Protective grounding is applied outside buildings and must have low resistance On the other hand functional grounding can be applied inside buildings and must have low impedance The goal of EMC is to avoid any interference effects Grounding for EMC can be distinguished by frequency For frequencies lower than 10kHz a single point ground system should be used and for frequencies higher than 10 kHz a multiple point ground system should be used e Single Point Grounding all signal grounds of all IT equipment are connected in series to form a single reference point This point can be grounded directly to earth to the designated grounding point or to the safety point that is already grounded e Multiple Point Grounding al
3. in the power cable is transmitted through the stray capacitance and coupled into the adjacent signal cable as shown in Figure 2 Several methods can be applied to reduce the effect of this common mode noise for example shield the power cable and or the signal cables separate the power and signal cables take the input and output side of the signal cable and twist them together to balance out the stray capacitance let power cables and signal cables cross at 90 etc Unshielded cable Y Cstray Power Susi supply Cable ystem Ground 3 Common mode noise is coupled via the power cable to other power systems then the cable of such a power system is coupled to the transmission system as shown in Figure 3 Unshielded cable M Noise Cstray T Send ER Receive 1 gt t 6 Load v Ground 4 The common mode noise of an unshielded power cable is transmitted to the ground via the stray capacitance Since both shielded wire and unshielded wire are connected to a common ground other systems can be interfered with by the common mode noise that is transmitted from the ground back to the system via the shield See Figure 4 IM Noise Unshielded cable A od Send Receive NN Cstray ES v ES C Load ed J J Cstray J gt Ground
4. 50kHz 300MHz and low frequency 100Hz 3000Hz High frequency noise fades more over distance and has a shorter wave length while low frequency noise fades less over distance and has a longer wave length Both types of interference are transmitted through power cables and power leads affecting the power supply side High frequency interference at the power side can be eliminated or attenuated by mounting a filter The filter consists of coils and capacitors Some drives do not have a built in filter in which case the installation of an external option filter is required The drawing below shows a standard filter diagram 1 Differential Mode Section 2 Common Mode Section Ground Lx Source of Interferences The Unqualified a Electrical Device N Ly A filter is composed of a Differential Mode section to eliminate noise below 150kHz and a Common Mode section to eliminate noise above 150kHz For high freguency noise the inductor acts as a high impedance to form an open circuit and the capacitor acts as a low impedance to form a short circuit Proper design and dimensioning of inductors and capacitors give a resonant circuit to absorb harmonic currents Capacitor Cy is earthed to lead the harmonic currents to the ground External Filter The filter and the AC drive should be installed in the control cabinet or on the mounting plate that is earthed to ground The motor cable must be shielded and as short as possible Please u
5. Jin d have to travel 80r d 18 g d 6 601 g 2 d 12 40 2 6 d 4 s d g 2 sg Lae er P cB om 0 pers oq o f i 005 Ol 02 05 1 2 5 F lt 0 5Fcutoff SE is approximately 27d g 4 2 How to reduce EMI by Shielding Iron and other metals are high conductivity materials that provide effective shielding at extremely low frequencies But conductivity will decrease as 1 High frequency signals are applied to the conductor 2 Equipment is located in a strong magnetic field 3 The shielding frame is forced into a specific form by machines It is difficult to select a suitable high conductivity material for shielding without the help from a shielding material supplier or a related EMI institution Metallic Shielding Effectiveness Shielding Effectiveness SE is used to assess the applicability of the shielding shell The formula is SEdB A R B Measures in dB where A Absorption loss dB R Reflection loss dB B Correction factor dB for multiple reflections in thin shields The absorption loss refers to the amount of energy loss as the electromagnetic wave travels through the shield The formula is AdB 1 314 foy 1 2t where f frequency MHz p permeability relative to copper o conductivity relative to copper t thickness of the shield in centimetres The reflection loss depends on the source of the electromagnetic wave and the distance from that source For a rod or straight wire antenna the wa
6. a zero phase reactor at the power supply side and or the AC Motor Drive s output depending on where the interference is Since currents are large at the power input and the AC Motor Drive s output please carefully select the magnetic core with suitable current handling capability An ideal magnetic material for large currents is compound magnetic powder It has a higher current handling capability and higher impedance compared to pure metallic magnetic cores It is therefore suitable to implement in a high frequency environment The impedance can also be enhanced by increasing the turn ratio Zero Phase Reactor Installation There are two installation methods depending on the size of the zero phase reactor and the motor cable length 1 Wind the motor cable through the middle of a zero phase reactor 4 times Place the reactor and the AC Motor Drive as close to each other as possible Zero Phase Reactor C RIET U T1 OS L2 VIT2 OT L3 W T3O Power Supply Q 2 Place all wires through the middle of four zero phase reactors without winding Zero Phase Reactor Power Supply 13 Analog Input Signals If the analog input signals are affected by noise from the AC motor drive please connect a capacitor and a ferrite core as indicated in the following diagram Wind the wires around the core in same direction for 3 times or more ft AVI ACI AUI C ACM Ferrite core 5 2 Harmonic Int
7. ansformer is achieved by wrapping a metal sheet between the primary and secondary windings or by adding an enamel wire to the winding wire which is then earthed e a shielding coating which is made of metal mesh or conductive fibres to provide effective protection for the workers who work in a high voltage environment In the picture below the radio appears to be not fully covered by metal but if the conductivity of the metal is high radio waves are completely blocked and the radio will not receive any signal co AN A S oa ki b NS Mobile phone connections are also established through the transmission of radio waves This is why the mobile phone reception is often cut off when we walk into an elevator The metal walls of the elevator create the same shielding effect just as if we had entered a metal cage Another example is a microwave oven The microwave door may seem transparent in visible light but the density of the metal mesh in the microwave door blocks the electromagnetic waves A higher density of the metal mesh offers better shielding Electromagnetic fields ee LLL Wall of t d shielded g depth enclosure Greater leakage gap form bigger apertures A Waveguide below cut off G gap doesn t leak very much diri c A does not have to be a tube Shield mng effecti VETIGSS distance that fields v
8. erference The AC motor drive s input current is non linear the input rectifier generates harmonics Harmonics must be limited to within a certain range to avoid impact the mains power and to avoid current distortion to ensure surrounding devices are not influenced An AC Motor Drive with built in DC reactor suppresses harmonic currents Total Harmonic Current Distortion THID effectively and therefore reduces the harmonic voltage peaks Total Harmonic Voltage Distortion Harmonic Current at the Power Supply Side A Fundamental Sine Wave A Fundamental Sine Wave 60Hz 60Hz 7 B 3 Harmonic th 180Hz B 5 Harmonic 800Hz C Harmonically 3 C Harmonically Distorted Wave s ae Distorted Wave Suppression of Harmonic Currents When a large portion of lower order harmonic currents 5 rip ul etc occur at the power input surrounding devices will be disturbed and the power factor will be low as a result of reactive power Installing a reactor at the AC Motor Drive s input effectively suppresses lower order harmonic currents AN ZX D e inverter E a MM Bus Line Chokes AC Reactor Installed in series with the power supply and is effective in reducing low order current harmonics Features of an AC reactor include 1 Reduces the harmonic currents to the AC Motor Drive and increases the impedance of the power supply 2 Absorbs interferences generated by surr
9. http www delta com tw industrialautomation AC Motor Drives EMC Standard Installation Guide EMC Compliance Practice Preface When an AC motor drive is installed in a noisy environment radiated and or conducted noise via signal and power cables can interfere with the correct functioning cause errors or even damage to the drive To prevent this some AC motor drives have an enhanced noise resistance but the results are limited and it is not economical Therefore an effective method would be finding the cause of the noise and use the right solution to achieve no emission no transmission and no reception of noise All three solutions should be applied Finding the Noise e Ascertain whether the error is caused by noise e Find the source of the noise and its transmission path e Confirm the signal and the source of noise Solutions e Grounding e Shielding e Filtering Table of Contents it 5 E FERE OCDE ESPERTI i 121011 Xs kes ccc E E ii Chapter T Introduction a enres ene natote e sholatclaialobessialats a ar e e 1 Tot Mat is EMG coss cto oe ett eb tote Gol Leto 1 1 2 EMC for AC Motor DTrIVO a osito aka aaa Sac dao fao dae Pac caeno k nd kald 1 Chapter 2 How to prevent EMI itta saagas 2 2 1 Types of EMI common mode and differential mode noise 2 2 2 How does EMI transmit Noise transmission 2 Chapter 3 Solution to EMI Grounding
10. ilding or the control cabinet and form mutual inductance and stray capacitance As shown in the following figure a long grounding wire could become a vertical antenna and turn into a source of noise 4 Painted sheet metal Long PE Ta O 3 2 Ground Loops A ground loop occurs when the pieces of eguipment are connected to more than one grounding path In this case the ground current may return to the grounding electrode via more than one path There are three methods to prevent ground loops 1 Use a common power circuit 2 Single point grounding 3 Isolate signals e g by photocouplers e o A Accompanying cable Equipment Equipment Equipment B A B Very good a Cable Earth plane Earth plane In order to avoid Common Mode Noise please use parallel wires or twisted pair wiring Follow this rule and also avoid long wires it is suggested to place the two wires as close to each other as possible 3 3 Earthing Systems The international standard IEC60364 distinguishes three different earthing system categories using the two letter codes TN TT IT Equipment A e The first letter indicates the type of earthing for the power supply equipment generator or transformer T One or more points of the power supply equipment are connected directly to
11. inet lowers the shielding effectiveness and increases the chance of electric waves leaking through the openings and emitting interference We recommend that the drilled holes are as narrow as possible When the wiring holes are not used properly cover the holes with metal plates or metal covers The paint or the coating of the metal plate and metal cover should be thoroughly removed to ensure a metal to metal contact or a conductive gasket should be installed 3 Install industrial conductive gaskets to completely seal the electrical cabinet and the cabinet door without gaps If conductive gaskets are too costly please screw the cabinet door to the electrical cabinet with a short distance between the screws 4 Reserve a grounding terminal on the electrical cabinet door This grounding terminal shall not be painted If the paint already exists please remove the paint before grounding Electrical wires and cables Shielded Twisted Pair STP is a type of cable where two insulated copper wires are twisted together with a metal mesh surrounding the twisted pair that forms the electromagnetic shielding and can also be used for grounding The individual electrical wires and complete cable are surrounded by synthetic rubber that provides insulation and also protects against damage There are two types of electrical cables high voltage and low voltage The high voltage cable differs from the low voltage cable in that it has an additional insulation la
12. ironment A PDS in a domestic environment must have lower emission levels and is allowed to have lower immunity levels A PDS in an industrial environment is allowed to have higher emission levels but must have more severe immunity levels 1 2 EMC for AC Motor Drive When an AC motor drive is put into operation harmonic signal will occur at the AC drive s power input and output side It creates a certain level of electromagnetic interference to the surrounding electrical devices and the mains power network An AC motor dive is usually applied in industrial environments with a strong electromagnetic interference Under such conditions an AC drive could disturb or be disturbed Delta s AC motor drives are designed for EMC and comply with EMC standard EN61800 3 2004 Installing the AC motor drive accurately will decrease EMI influences and ensure long term stability of the electricity system It is strongly suggested to follow Delta s user manual for wiring and grounding If any difficulties or problems arise please follow the instructions and measures as indicated in this EMC Standard Installation Guide Chapter 2 How to prevent EMI 2 1 Types of EMI Common mode and differential mode noise The electromagnetic noise of an AC motor drive can be distinguished into common mode and differential mode noise Differential mode noise is caused by the stray capacitance between the conducting wires and common mode noise is caused by the common mode coup
13. l signals of all IT equipment are grounded independently e Hybrid Grounding this type of grounding behaves differently for low and high frequencies When two pieces of IT equipment A and B are connected via a shielded cable one end is connected directly to ground while the other end is connected to ground via a capacitor This type of grounding system fulfils the criteria for high and low frequency grounding e Floating grounding the signals of all IT equipment are isolated from each other and are not grounded DC current flows evenly throughout the conductor section But AC current flows towards the conductor s surface as frequency increases this is called the skin effect It causes the effective cross section area to be reduced with increasing frequency Therefore it is suggested to increase the effective ground cross section area for high frequencies by replacing pigtail grounding by braided conductors or strip conductors Refer to the following figure emp esl AA sl BER 3 gt This is why a thick short ground wire must be implemented for connecting to the common grounding path or the ground busbar Especially when a controller e g PLC is connected to an AC motor drive it must be grounded by a short and thick conducting wire It is suggested to use a flat braided conductor ex metal mesh with a lower impedance at high frequencies If the grounding wire is too long its inductance may interfere structure of the bu
14. ling current path created by the stray capacitance between the conducting wires and ground Basically differential mode noise has a greater impact to the AC motor drive and common mode noise has a greater impact to high sensitivity electronic devices An excessive amount of differential mode noise may trigger the circuit protection system of the AC motor drive Common mode noise affects peripheral electronic devices via the common ground connection EMC problems can be more serious when the following conditions apply e When a large horsepower AC motor drive is connected to a large horsepower motor e The AC motor drive s operation voltage increases e Fast switching of the IGBTs e When a long cable is used to connect the motor to the AC motor drive 2 2 How does EMI transmit Noise transmission path Noise disturbs peripheral high sensitivity electrical devices systems via conduction and radiation their transmission paths are shown hereafter 1 Noise current in the unshielded power cable is conducted to ground via stray capacitances into a common mode voltage Whether or not other modules are capable to resist this common mode noise depends on their Common Mode Rejection Ratio CMRR as shown in the following figure li jp Noise Unshielded cable Send Receive i A nr MEE Puce 3L Load oe d il M 1 NEUEN a gt gt Ground 2 Common mode noise
15. m Neutral is either not connected to earth or is earthed via a high impedance In such a system an insulated monitoring device is used for impedance monitoring A built in filter should be disconnected by the RFl jumper and an external filter should not be installed when the AC motor drive or the AC servo motor drive is connected to an IT system Gererabor or transformer Criteria for earthing system and EMC TN S TN C TT IT Safety of Good Good Good Good Personnel Continuity of the PE Continuity of the PE RCD is mandatory Continuity of the PE conductor must be conductor must be conductor must be ensured throughout ensured throughout ensured throughout the installation the installation the installation Safety of property Poor Poor Good Good High fault current High fault current Medium fault current Low current at the around 1kA around 1kA lt a few dozen first fault lt a few amperes dozen mA but high current at the second fault Availability of Good Good Good Excellent energy EMC behavior Excellent Poor Good Poor prohibited should be avoided Few eguipotential Neutral and PE are the same Over voltage risk Eguipotential Over voltage risk Problems Common mode Need to handle Circulation of Problems filters and surge the high leaking disturbance Need to handle the arrestors must currents problem of currents in exposed high leaking handle the phase the device conductive parts curren
16. o a three phase 5wire system but near the power side PE and N are combined into a PEN conductor similar to a three phase 4 wire system Generator or transformer s Ll 4 L2 4 L3 e PA PEN A E De un Earth Consumer S TN C S system TN C S A combined earth and neutral system PEN conductor is used in certain systems but eventually split up into two separate conductors PE and N A typical application of combined PEN conductor is from the substation to the building but within the building PEN is separated into the PE and N conductors Direct connection of PE and N conductors to many earthing points at different locations in the field will reduce the risk of broken neutrals Therefore this application is also known as protective multiple earthing PME in the UK or as multiple earthed neutral MEN Jin Australia Generator or transformer petab die E 4 1 Consumer DC te E ms gt TT system TT The neutral point N of the low voltage transformer and the equipment frames PE are connected to a separate earthing point The Neutral N of the transformer and electrical equipment are connected Generabor or transformer IT system IT The neutral point of the transformer and electrical equipment are not earthed only the equipment frames PE are earthed In the IT network the power distribution syste
17. ounding devices such as surge voltages currents and mains surge voltages and reduce their effect on the AC Motor Drive 3 Increases the power factor 14 DC Reactor A DC Reactor is installed between the rectifier and the DC bus capacitor to suppress harmonic currents and to achieve a higher power factor Current Wave Diagrams Without Reactor With Reactor 150 fi VORR i f l i l h h h j i i i i i i i i 1 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 15
18. se the filters recommended by Delta to ensure compliance with EMC standards Ps The Shielded Chassis cog A IP20 NEMA 1 UL Type 1 N Option Kit ci a 1 Metal Conduits TL EMI Filter T ry qa K UTI E 12 I2 LS L2 vn j 13 LBL3 W T3 D E m oe 2 lt Chassis Grounding Shielded Motor Cable Connect to the steel structure of the building 12 AC Motor Drives with Built in Filter 1 Since interferences are suppressed by installing an earthed capacitor in the filter the amount of current to ground leakage current could result in electric shocks to personnel or the power system Please be aware of this problem 2 Since the leakage current to ground can be high it is crucial to implement protective earthing to prevent electrical shocks Filter Installation With and Without v O mer 550 00 kme 593 5 of obuv 9 mr 355 91 nne 552 7 ofuv 330 m 0 18 1 16 30 0 15 1 10 a wre lt 15m 60Hz with EMI Filter lt 15m 60Hz without EMI Filter gt Zero Phase Reactor Choke Interferences can also be suppressed by installing
19. the same earthing point I Either no point is connected to earth isolated or it is connected to earth via a high impedance e The second letter indicates the connection between earth and the power supply equipment T Connected directly to earth This earthing point is separate from other earthing points in the power supply system N Connected to earth via the conductor that is provided by the power supply system e The third and forth letter indicate the location of the earth conductor S Neutral and earth conductors are separate C Neutral and earth are combined into a single conductor TN system TN The neutral point of the low voltage transformer or generator is earthed usually the star point in a three phase system The body of the electrical device is connected to earth via this earth connection at the transformer protective earth PE The conductor that connects the exposed metallic parts of the consumer neutral N The conductor that connects to the start point in a 3 phase system or that carries the return current in a single phase system Ll L2 TOO L3 N s PE TN S system TN S PE and N are two separate conductors that are combined together only near the power source transformer or generator It is the same as a three phase 5 wire system Generabor or transformer TN C system TN C PE and N are two separate conductors in an electrical installation similar t
20. ts problem of to phase voltage high magnetic field the device High fault current transient disturbances radiation High fault currents transient disturbances RCD Residual current device RCDs subject to nuisance tripping when common mode capacitors are present Eguivalent to TN system for second fault Chapter 4 Solution to EMI Shielding 4 1 What is Shielding Electrostatic shielding is used to isolate equipment so that it will not create electromagnetic field interference or be influenced by an external electromagnetic field A conductive material is used for electrostatic shielding to achieve this isolation A Faraday cage can be made from a mesh of metal or a conductive material One characteristic of metal is that it is highly conductive and not electrostatic which offers shielding and prevents interference by external electrical fields Metal with its high conductivity protects the internal devices from high voltages no voltage will enter the cage even when the cage is experiencing a high current In addition electromagnetic fields can also pass through the Faraday cage without causing any disturbance Electromagnetic shielding is applied to some electrical devices and measurement equipment for the purpose of blocking interference Examples of shielding include e earth high voltage indoor equipment using a metal frame or a high density metal mesh e shielding a power tr
21. ve impedance increases as it moves closer to the source and decreases as it moves away from the source until it reaches the plane wave impedance 377 and shows no change If the wave source is a small wire loop the magnetic field is dominant and the wave impedance decreases as it moves closer to the source and increases as it moves away from the source but it levels out at 377 when the distance exceeds one sixth of the wavelength 10 Electrical Cabinet Design In a high frequency electric field shielding can be achieved by painting a thin layer of conductive metal on the enclosure or on the internal lining material However the coating must be thorough and all parts should be properly covered without any seams or gaps just like a Faraday cage That is only the ideal Making a seamless shielding shell is practically impossible since the cage is composed of metal parts In some conditions it is necessary to drill holes in the shielding enclosure for installation of accessories like optional cards and other devices 1 Ifthe metallic components are properly welded using sophisticated welding technology to form an electrical cabinet deformation during usage is unlikely to occur But if the electrical cabinet is assembled with screws the protective insulating layer under the screw must be properly removed before assembly to achieve the greatest conductivity and best shielding 2 Drilling holes for the installation of wires in the electrical cab
22. yer called the dielectric insulator within the plastic sleeve The dielectric insulator is the most important component in insulation The low voltage cable is usually only filled with a soft polymer material for keeping the internal copper wire in place The shield has two functions 1 Toshield the electrical wire and cable A Electric currents increase as power flows through the power cable and generate an electrical field Such interference can be suppressed inside the cable by shielding the power cables or the electrical wires B To form a protective earthing When the cable core is damaged the leakage current will flow via the shield to ground 2 To protect the cable A power cable used for the computer control purpose generates only relatively low amount of current inside the cable Such power cable will not become the source of interferences but has great possibility to be interfered by the surrounding electrical devices Green Yellow p Plastic jacket x Black Dielectric insulator Brown Ce x Blue Metallic shield 4 Centre core 11 Chapter 5 Solution to EMI Filter 5 1 Filter Electromagnetic interference is transmitted in two ways by radiation and by conduction The most effective and economical method of reducing radiated interference is to use shielding and of reducing conducted interference is to use an electromagnetic filter Noise interference can be divided into two categories high frequency 1
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