EMI in Modern AC Motor Drive Systems

Transcription

1 EMI i Moder A Motor Drive Systems Firuz Zare, Queeslad Uiversity of Techology Brisbae, QLD, Australia f.zare@qut.edu.au Abstract I this paper, several aspects of high frequecy related issues of moder A motor drive systems, such as commo mode voltage, shaft voltage ad resultat bearig curret ad leakage currets, have bee discussed. oducted emissio is a major problem i moder motor drives that produce udesirable effects o electroic devices. I moder power electroic systems, icreasig power desity ad decreasig cost ad size of system are market requiremets. Switchig losses, harmoics ad EMI are the key factors which should be cosidered at the begiig stage of a desig to optimise a drive system. Itroductio Nowadays, more tha 6 percet of the world s eergy is used to drive electric motors. Due to growig requiremets of speed cotrol, pulse width modulated iverters are used i adjustable speed drives. Rapid developmets i semicoductor techology have icreased the switchig speed ad frequecy of power switches dramatically. I a motor drive system, a voltage source coverter with hard switches geerates high dv/dt, which causes leakage currets due to stray capacitaces i a electric motor. As show i Fig.1, a moder power electroic drive cosists of a filter, a rectifier, a D lik capacitor, a iverter ad a A motor. May small capacitive coupligs exist i the motor drive systems which may be eglected at low frequecy aalysis but the coditios are completely differet at high frequecies. Electromagetic Iterferece (EMI) is a major problem i recet motor drives that produces udesirable effects o electroic devices. I moder power electroic systems, icreasig power desity ad decreasig cost ad size of a system are market requiremets. Switchig losses, harmoics ad EMI are the key factors which should be cosidered at the begiig stage of a desig to optimise a drive system. ommo mode voltage creates shaft voltage through electrostatic coupligs betwee a rotor ad stator widigs ad the rotor ad a frame which ca cause bearig currets whe the shaft voltage exceeds a breakdow voltage level of the bearig grease. A icrease i the carrier frequecy of voltage-source Pulse Width Modulated (PWM) iverters based o high-speed switchig devices has improved operatig characteristics of the iverters. High speed switchig ca geerate the followig serious problems due to high dv/dt: Groud curret escapig to earth through stray capacitors iside a motor oducted ad radiated oises Shaft voltage ad bearig curret Models of parasitic coupligs ad high frequecy compoets for a iverter fed iductio motor drive system are ivestigated to determie suitable models to predict 29 IEEE 53

2 Voltage Supply PE Fig. 1. A power electroic motor drive system with capacitive coupligs. (s) Ier Race Widig (w) Rotor (r) Shaft Filter Filter Outer Race Ball Diode Rectifier ww ww dc-lik apacitor Iverter ables Motor Fig. 2. A view of stator slot ad capacitive couplig i a iductio motor. d BO rs Outer Race Ball BO d BI BI Ier Race ws Shaft Rotor wr Ed Widig ball ball wr Ed Widig Motor Frame Ballbearig Ballbearig S Fig.2 shows the capacitive coupligs of a iductio motor ad a view of stator slot, where: wr is the capacitive couplig betwee the stator widig ad rotor ws is the capacitive couplig betwee the stator widig ad stator frame rs is the capacitive couplig betwee the rotor ad stator frame ww is the capacitive couplig betwee turs of stator widig ball is the capacitive couplig of ball bearig Fig.3 shows a geeral structure of a ball bearig ad shaft i a A machie. As show i this figure there are some balls betwee outer ad ier races with lubricated grease betwee the balls ad the races. There are capacitive coupligs betwee the outer ad ier races. Durig operatio, the distaces betwee the balls ad races may be chaged ad will vary the capacitace values ad resultat electric field betwee the races ad balls. Due to this fact, this capacitace has a oliear value. Lubricated grease i the ball bearig caot withstad a high voltage ad a short circuit through the lubricated grease may happe, thus this pheomeo ca be modelled as a switch. PWM iverters have bee foud to be a major cause of motor bearig failures i iverter motor drive systems. All iverters geerate commo mode voltages relative to the groud, which make bearig curret through motor parasitic capacitaces. Accordig to Fig.4.a, phase voltages ad a commo mode voltage (V ) ca be derived based o the power coverter voltages (V a, V b,v c ). Each leg voltage of a three phase iverter is give by: (c) Fig. 3. geeral structure of a ball bearig a view of ball, outer ad ier races ad capacitive coupligs (c) simple model of a ball bearig. bearig currets ad shaft voltage over a wide frequecy rage. A high frequecy model of a electric motor is a importat issue for power electroic egieers, which helps them to aalyse leakage ad bearig currets ad to desig EMI filters. At low frequecy, a equivalet circuit of a electric motor cosists of iductaces ad resistaces without cosiderig stray capacitaces ad ski effect. These issues become more importat at high speed switchig applicatios due to high dv/dt IEEE Ad the: V a 5 V a 1 V V b 5 V b 1 V (1) V c 5 V c 1 V V a 1 V b 1 V c 5 1V a 1 V b 1 V c 2 1 3V (2) It is clear that i a three-phase system: V a 1 V b 1 V c 5 (3) So, the commo mode voltage ca be calculated as: V 5 1V a 1 V b 1 V c 2 /3 (4)

3 I a three phase power iverter, a D voltage is coverted to three phase voltages with 12 phase shift. Fig.4.b shows three phase leg voltages of a iverter with commo mode voltage. The tred i icreasig switchig frequecy improves the quality of curret waveforms i motor drive systems but due to short switchig time, a high dv/dt is produced across the motor termial. The leakage curret is created by a high voltage stress durig switchig time ad capacitive couplig i a A motor. Fig.5 shows a simple equivalet model of a iductio motor which cotais mai capacitaces betwee the widigs, rotor ad the stator frame. 6 4 V a V b V c V a V b V c V 2 Aalysis of a Electric Motor at High Frequecy The stray capacitace betwee the widigs ad the stator frame is the most sigificat parasitic compoet compared to the other stray capacitaces which geerate sigificat coducted emissio oise. At high frequecy, a electric motor ca be modeled as distributed capacitors, iductors ad resistors as show i Fig.6 ad the maximum Fig. 4. three phase voltage source iverter PWM voltage. frequecy ca be determied usig the stadig wave s equatio. We cosider oe v sectio to model the motor at high frequecy as show i Fig.6.b ad oly the stray capacitace betwee the widigs ad the stator Widig Rotor has bee cosidered. Each motor has differet high frequecy parameters due to its structure, size ad materials. wr ws rs ball alculatio of ws The first step is to measure the magitude ad the phase values of the impedace i terms of frequecy based o two differet coectios of the widigs. The first measuremet is based o Fig.7.a where three phases are coected to each other at the termial sides. Thus, the impedace betwee the phases ad the stator (phase ad magitude values) ca be measured i terms of the frequecy usig a impedace aalyzer. Based o this cofiguratio, the impedace of the stray capacitace, ws, is much higher tha the impedace of ad R loss at low frequecies ad the model ca be simplified as two parallel stray capacitaces. Fig.8 shows measuremet results of the impedace betwee the phases ad the stator frame of a 5.5 KW motor. Accordig to Fig.8, the phase value is aroud 9 at,4 KHz which addresses the fact that the two capacitors are coected to each other i parallel. The stray capacitace of the electric motor has bee calculated usig the magitude ad the phase values from Frame Fig. 5. A high frequecy model of a iductio motor. 1 khz up to 4 khz ad the result shows that ws is almost aroud.9 F. The capacitace value is calculated based o the measuremet results ad Z cws 5 1/v ws. alculatio of ad R loss The secod measuremet is based o Fig.7.b where three phases are coected to each other ad the impedace betwee the phases ad star poit is measured. Based o this cofiguratio, the impedace of the stray capacitace is much higher tha the impedace of ad R loss (Fig.9.a) at low frequecies 29 IEEE 55

4 R-tator R-tator R-tator 1 2 Phase R loss ws ws Fig. 6. a distributed model of a electric motor at high frequecy a simple model of a electric motor. ad it ca be simplified as a iductive ad a resistive load as show i Fig.9.b. Fig.1 shows measuremet results of the impedace betwee the phases ad the star poit. The iductace ad resistace values ca be calculated usig the measuremet results ad based o the circuit diagram show i Fig.9.b. The iductace value is decreased from 5.77 mh dow to 5.56 mh whe Phase Phase c ws R Loss c ws R Loss Fig. 7. Two differet coectios to measure the impedace betwee the phases ad the stator the phases ad the star poit. Fig. 9. ircuit diagram for measuremet betwee phases ad star poit a model of motor simplified model for low frequecy rage Z Z Z2 P Z P P2 Fig. 8. Measuremet results magitude ad phase values betwee the phases ad the stator of a 5.5 KW motor P Fig. 1. Measuremet results magitude ad phase values betwee the phases-star poit of a 5.5 KW motor IEEE

5 the frequecy is icreased from 1 khz up to 4 khz. At low frequecy (below khz), the iput impedace of the widigs (accordig to Fig.9.b) is Z RL 5 R loss 3jv /jv 1R loss. The magitude ad phase values of this cofiguratio, which are used to extract the resistace ad iductace values, are give i Fig.1. The simulatios have bee carried out based o the followig equatios drive from the above figures: Z stator_widig 5 Z ws 1Z RL 1 Z ws Z ws 1 Z RL Z widig_widig 5 Z ws 2 1Z RL 2 Z ws 2 1 Z RL Where Z RL 5 R loss 3 jv /jv 1R loss ad Z cws 5 1/v ws There is a short cable from the motor termials to the impedace aalyzer, which ca be modeled as a R_L i series with the motor as show i Fig.11. The secod resoace frequecy show i Fig.1 is associated with this iductace ad the first stray capacitace, ws. Thus the iductace value of the cable ca be calculated usig L cable 5 1/ 12pf2 2 ws H. The resistace value of the cable ca be calculated at the secod resoace frequecy. Based o this aalysis, the parameters of the 5.5 kw motor have bee extracted as follow: R cable L cable ws R loss ws Fig. 11. A high frequecy model of a motor with a short cable mh; ws 5.9 F; R loss V; L cable H; R cable 5 3 V The 5.5 kw motor is modeled based o this aalysis ad the simulatio ad experimetal results are show i Fig.12. The cable impedace is egligible at frequecies below 4-5 MHz but it becomes sigificat above that frequecy rage. Active EMI Filters The use of Active EMI filters based o curret ijectio is a proper solutio to cacel commo mode high frequecy currets. Fig.13 shows a block diagram of a active EMI filter with a commo mode trasducer. The filter composes of a emitter follower usig complemetary trasistors ad a commo mode trasformer to measure the Phase_exp 1 arg(zsimul(f)) 18 π Z_exp 1 Zsimul(f) Z_exp, f Phase_exp, f Phase_exp 1 arg(zsimul(f)) 18 π Z_exp 1 Zsimul(f) Z_exp, f (c) Phase_exp, f (d) Fig. 12. Experimetal ad simulatio results of a 5.5 kw motor; magitude phase values betwee phases-stator (c) magitude (d) phase values betwee phases-star poit. 29 IEEE 57

6 Vs Active EMI Filter I leakage Motor duce udesirable effects o electroic devices. I moder power electroic systems, icreasig power desity ad decreasig cost ad size of system are market requiremets. Switchig losses, harmoics ad EMI are the key factors which should be cosidered at the begiig stage of a desig to optimise a drive system. I most power electroic desigs, EMI issues have ot bee take ito accout as oe of the mai factors; ad mitigatio techiques for EMI are cosidered at the last stage of desig! Fig. 13. A active EMI filter. leakage curret. A high frequecy amplifier geerates the same curret ad ijects it ito the motor drive circuit i order to bypass the curret as show i Fig.13. I this case, the leakage curret geerated by dv/dt is circulated through the active filter. There are few issues to desig a active EMI filter which should be cosidered: A high bad width power amplifier to geerate the leakage curret A three-phase or a sigle-phase curret trasducer with symmetrical cofiguratio to measure curret accurately A separate power supply for the active EMI filter. oclusios A high frequecy model of a electric motor has bee aalyzed i this paper. Oe of the advatages of the high frequecy model of the electric motor is to estimate ad aalyze the leakage ad bearig currets to desig EMI filters usig simulatios. This paper addresses a simple method to extract high frequecy parameters of a electric motor based o the measuremet results. It is recommeded to estimate the behavior of the motor i a frequecy rage, whe the motor acts as a capacitive or a iductive load accordig to the phase values of the impedace. The, the parameters ca be calculated directly from the magitude ad the phase values of the impedace. I this paper several aspects of high frequecy related issues of moder A motor drive systems, such as commo mode voltage, shaft voltage ad resultat bearig curret ad leakage currets, have bee discussed. oducted ad radiated emissios are major problems i moder motor drives that pro- Refereces [1] Firuz Zare, Power Electroics E-Book, from this website, you ca virtually atted lectures ad tutorials ad also access to lecture otes ad computer Labs. [2] Firuz Zare, High frequecy model of a electric motor based o measuremet results, Australia Joural of Electrical & Electroics Egieerig (AJEEE), Vol 4, No 1, 28, page Biography Dr. Firuz Zare received his B.Eg degree i Electroic Egieerig from Gila Uiversity, his MSc degree i Power Egieerig from K.N Toosi Uiversity ad his Ph.D. degree i Power Electroics from Queeslad Uiversity of Techology i 1989, 1995 ad 21, respectively. He spet several years i idustry as a team leader ad developmet egieer where he was ivolved i electroics ad power electroics projects. Dr. Zare wo a studet paper prize at the Australia Uiversities Power Egieerig oferece (AUPE) coferece i 21 ad was awarded a Symposium Fellowship by the Australia Academy of Techological Sciece ad Egieerig i 21. He received the Vice hacellor s Award i research i 29 ad faculty excellece award i research as a early career academic from Queeslad Uiversity of Techology i 27. Dr. Zare has published over 6 joural ad coferece papers ad techical reports i the area of Power Electroics. He has bee ivited as a reviewer ad techical chair of atioal ad iteratioal cofereces ad preseted several semiars ad workshops. He preseted a half-day tutorial at the 27 IEEE Iteratioal Symposium o EM i Hawaii, at the EM Asia Pacific Workshop i Sigapore i May 28 ad at the 28 IEEE Iteratioal Symposium o EM i Michiga. Dr. Zare is a seior lecturer at the Queeslad Uiversity of Techology, Australia ad a seior member of the IEEE. EM IEEE