Analysis of the Overvoltages in PWM-Inverterfed Induction Motors

Dr.B.Basavaraja,Member,IEEE, Dr.D.V.S.S.Siva Sarma,Member,IEEE

Abstract- An increasing part of low voltage standard inductionmotors operates with PWM voltage source inverters using IGBTsand they are applied to a wide range of power 0.1 kW up to someMW. The PWM (Pulse Width Modulation) inverter mode ofcontrol generates a large frequency spectrum of overvoltages. Theresulting undesired overvoltages at the motor terminals stress themotor insulation. Usual standards applied to the induction motorsrecommend to limit the value of rate of rise of voltage to be lessthan (dv/dt) < 500-600V/ps for the motor insulation to be healthycondition. This value is suitable for a motor fed directly by theAC power supply, but when the motor is fed by an inverterwithout any filtering device, the dv/dt may reach up to 6000­7000VIps or even more depending on the installation.

This paper presents the simulation and experimentalinvestigations of transient overvoltages on ac induction motorswhen connected through a cable to IGBT-PWM inverters.Considering classical transmission line and travelling-waveanalysis with zero initial electric charge on cable, these transientover voltages are about twice the dc link voltage with a short risetime of about 200-300ns. Nevertheless for long cables, thesetransient overvoltages may reach 3 or 4 pu. In this paper, byusing the system representation consisting of inverter-cable­motor, simulation is carried out to assess the motor terminalovervoltages. Experimental investigations on a low voltageinduction motor in laboratory are carried out. The simulationresults are compared with experimental results to validate themodelling of the system. The validated model of the system is usedto study the influence of the cable length, rise time and motorrating on the overvoltages produced at motor terminal.

Key words--eoil, mismatch impedance, induction motor,over voltages, PWM Inverter, surge propagation, windingfailure.

I. INTRODUCTION

The growing use of induction motors for high poweradjustable speed applications is essentially due to the quicktechnological evolution of fast switching electronic devices,such as the insulated gate bipolar transistors (IGBT), which arenowadays widely adopted in medium voltage, medium powerconverters, for their performances in terms of driving,switching behavior, etc.

While the high switching speeds and advanced PWM

Dr.B.Basavaraja, Principal, Padmasri Dr. B V Raju Institute of TechnologyVishnupur, Narasapur, Medak, AP, India ( banakara_36@rediffmail.com)

D.V.S.S.Sivasarma, Professor, EED, NIT WarangalAndhra Pradesh, India (Sivasarma@gmail.com )

schemes significantly improve the performance of the PWM­inverter-fed induction motors, the high rate of voltage rise(dv/dt) of 0-650 V in less than 0.1 J.1s has adverse effects on themotor insulation. These steep rising and falling pulses lead toan uneven distribution of voltages within the motor, especiallyduring switching transitions. This contributes to insulationdeterioration and subsequent failure of the motor. In addition,the dvIdt contributes to damaging bearing currents andelectromagnetic interference (EMI). If a long cable isemployed between the inverter and the motor, damped highfrequency ringing at the motor terminals occurs resulting inexcessive over voltage, which further stresses the motorinsulation. Also, the motor impedance, which is dominated bythe winding inductance, presents an effective open circuit athigh frequencies at the end of the long cable. This produces areflected voltage at the end of the cable approximately equal inmagnitude and with the same sign, resulting in twice themagnitude of the incident voltage at the motor terminals asshown in fig 2(b).Hence the problems associated with PWMfed A.C Drives are

1. Terminal over voltages due to PWM Inverter and cables2. Surge propagation within winding3. Bearing currents4. EMI

This paper describes the modeling of the system consistofPWM Inverter, Cable, induction motor and their interaction.The system is simulated in MATLAB and experimental studyis carried out for validating the modeling and simulation of thedrive system. At the end Sensitivity analysis is made to studythe influence of system parameters Viz cable length, motorrating and rise time on motor terminal overvoltages. Finallysimulated are compared with experimental results and found tobe in good agreement.

II. SYSTEM REPRESENTATION

Overvoltages in inverter-cable-motor system are simulatedusing MATLAB software. MATLAB is powerful tool to solve

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-->~Time

Fig. 2(c) Simulated zoomed overvoltage waveform at 3HP induction motorterminal

oI ,-2.0 p.u.+ ----- --- - --- --- - T-- --- __ --- --- - --, _ --- -------------,- --- __ -- _ --- _ -- --I

OS Sms lOms ISms ~Oms

~Time

Fig.2(b) Simulated overvoltage waveform at 3HP induction motor terminal

2.0 p. u. T----------------------------------------------------- ---------------iI II I, I

I

Using the PWM Inverter with high frequency models of cableand motor, a simulation program has been developed inMATLAB and analysis of the terminal over voltages and risetimes are carried out by varying cable length and varying HPRating.

different problems in various applications. In MATLAB,power system simulink tool is used for analyzing theovervoltages at motor and also to perform the parametricstudy. Also studies have been carried out to record thewaveforms experimentally. Fig 1 shows the MATLAB modelfor inverter-cable -motor system. First PWM inverter ismodelled by using sinusoidal and saw tooth parameter blocks.The output pulse from the PWM inverter model is given to themotor through cable. For modelling of cable and motor inMATLAB simulink, series RLC parameters blocks are used. Incable modelling blocks represents the resistance, inductanceand capacitance values per unit length of cable. Motormodelling parameters are represented by using RLC parameterblocks from simulink library. Fig 1 shows the MATLABschematic of inverter cable motor system for 3 different cablelengths 4m, 10m and 14m with the inverter and motor beingsame.

Fig I MATLAB model for inverter-cable-motor system

The Fig. 2(a) to 2(d) shows the overvoltage waveforms atmotor terminals and inverter terminals for different levels ofzooming for cable length of20m and for a 3HP motor.

1.0 p.u. i-------------------------- --------------.--.--------------------- ---iut:1I}

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-1.0 p.u +--------------- -T----------------,---------- ------,.-----------------1as 5ms lOms ISms 2Oms

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Fig.2(a) Simulated output voltage waveform of an inverter

----tim.e-sec

Fig. 2(d) Simulated single pulse zoomed overvoltage waveform at motorterminal

Fig. 2{d) shows a typical transient voltage waveform atmotor terminals for pulse input inverter terminal.

ill SIMULATION RESULTS FOROVERVOLTAGESAT MOTOR TERMINALS

Due to usage of PWM Inverter to induction motor and the longcable connected between inverter and motor, the overvoltagesare produced at motor terminals. These overvoltages aremainly due to the steep fronted surges produced by PWMinverter and impedance mismatch between the motor andcable. In this section simulation of overvoltages is carried outby using MATLAB software.

In simulation, PWM inverter with different cable lengthsand different rating induction motors are used. The supplyvoltage pulse of 420 Volts Amplitude with 20ns rise time isapplied to the motor through cable. The detailed data for cable,3HP and 15 HP motor are shown in Appendix-I

By using data from Appendix-I, simulation study has beencarried out. Figs 3(a) and 3(b) show the simulated waveformsfor terminal overvoltages and inverter output voltage fordifferent cable lengths.

A. Overvoltages due to different cable length

voltage vs cablelength(3 hp)

In simulation, PWM inverter with different cable lengthskeeping motor rating fixed. Here 3HP induction motor andcable lengths of 4mt, 10mt, 14m, 20m, 24m and 50m areconsidered for simulation. The supply voltage pulse of 420Volts Amplitude with 20ns rise time is applied to the motorthrough cable.

voltage vs time(4m cable, 3HP motor)

I

From the above waveforms and the Table 1, it is observed thatthe voltage at the motor terminals becoming higher as thelength of the cable increases for the same rise time and as therise time increases the over voltage reduces for the same lengthof cable. Another observation is as cable length increasesdelay time increases.

Fig.5 shows the waveforms consisting of different cablelength connected to 3 HP inductions motor and simulatedsimultaneously.

VOLTAGE vs TIME fOf" 4m, 10m, 14m,2Om,24m cable length with 3HP motOf"

cable length (m)

Fig.4 Peak voltage Vs Cable length at motor terminal

100time(e-B)sec

I

II

voltage vs time(10m cable, 3HP motor)

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tlme(e-8)sec

Fig. 3(a) Line to Line peak voltage Vs Time at the inverter and motorterminals for 3HP motor and 4m cable

Fig. 3(b) Line to Line peak voltage Vs Time at inverter and motor terminalfor 3HP motor connected with 10m cable

150TIME (e-8) secTABLE-l

EFFECT OF CABLE LENGTH ON PEAK VOLTAGE AND RISE TIME FOR 3HP

INDUCTION MOTOR; APPLIED PULSE AT INVERTER WITH RISE TIME = 20NS,

APPLIED VOLTAGE OF 420vOLTS

4 917.00 2.183 33 x 10 -847 x 10 8

.1 956.68 2.277 37 X 10-8150 x 10 8

14 969.00 2.307 35 xl0-8

180 X 10 8

24 986.00 2.34742 x 10 -8

254 x 10 8

50 996.46 2.373 40 x 10-8

380 X 10 8

-200 '---__-'---__------'--__--J. ....I--__--"----__----"----l

o

Fig. 5 Peak voltage Vs time for 3HP motor with different cable lengths

Waveforms of Fig.5, it is observed that the voltage at themotor terminals becoming higher as the length of the cableincreases for the same rise time.

Similarly simulation is carried out for 7.5HP 10 HP and15HP rating induction motor by varying cable length whichgives the almost proportional results as obtained for 3HPinduction motor.

B Output voltage for different HP ratings

With the developed models, a comparative study has beenmade to study the effect of HP rating of the motor and lengthof the cable, on the resulting overvoltage magnitudes and risetimes. The rating of the machines varied by keeping cablelength fixed, overvoltages are analysed as shown below. HereHP ratings are varied from 2 HP to 15HP by fixing the cablelength.

Fig. 6(a) Peak voltage Vs time for 10m cable length with 2HP motor

Simulation of ASD system is performed in MATLAB fordifferent HP ratings of the motor connected to PWM inverterthrough a 10m cable. Analysis results are shown in Fig. 6(a)and 6(b) for 2HP and 3HP, motors respectively

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verified that a 3HP motor is affected with higher magnitude ofovervoltage than the 15HP motor.

Voltage -Time curve for output of inverter.at 3HP and 15HP induction motor1 :;: 0 0 r-----.,....-------r------r----'T-~__.__--~--_

Voltage at 3HP motor terminal Yolatge at 15H~ motor terminal--r----------~---------~- _. -----~1---------

-~~

-40 0 z~-~~---:---__:--__:--___,!_--..i.---____!

Time (microsec)

Fig.7 Line to line voltage at motor and inverter terminals for 3HP, 15HP

motors for acable length 60m

~ 800

.:!::::

~ 400

j> 200

BO 100 120tirne(e-B)sec

voltage vs time(10m cable. 2HP motor)

-200o

voltage vs time( 1 Om cable, 3HP motor )

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TABLE 2EFFECT OF MOTOR RATING FOR ACABLE LENGTH=60M; RISE

TIME=20NS, ApPLIED VOLTAGE OF 420vOLTS

tlme(e-B)sec

Fig.6(b) Peak voltage Vs time for 10m cable length with 3HP motorSimilarly simulation is carried out for 14m, 24m and 50mcable length by varying HP rating between 7.5HP to 15HPrating induction motor, which gives almost proportional resultsas obtained for 10m cable above.

IV.EXPERIMENTAL SETUP OF INVERTER-CABLE-MOTOR SYSTEM

The Fig 8 shows experimental setup used in the laboratory forinvestigation. It consists of an induction motor fed by a PWMinverter through a cable. The components used in the setup areIGBT based PWM Inverter of rating of 3-<1>, 420V line voltagewith IGBT switches of fast commutating period 200-300ns.The different lengths of 4m, 10m, and 14m are used betweenthe inverter and motor. The induction motors of 0.5HP, IHPand 3HP motors are used for analysing the overvoltages.Tektronix make, TD S3000B Series Digital PhosphorOscilloscope of 300 MHz bandwidth is used for analysing thevoltage waveforms during experimentation and IGBT PWMinverter rating of 2.2KW is used.

Fig. 8 Experimental setup of ASD

165 10HP I-.ling

HP rating vs Peak voltage956.75

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C. Simulation analysis combining 3HP and 15HP motor

Fig. 6(c) shows the magnitudes of peak voltage at the motorterminals for a 10m cable with the variation of motor ratings.From the plot it is verified that the higher rated motorsexperience less peak voltage magnitude as compared withlower rated motor.

With the developed models, a comparative study has beenmade to study the effect of HP rating of the motor on theresulting overvoltage magnitudes. 3 HP and 15 HP ratingmachines were considered for this study fixed cable length.The inverter output and motor terminal voltages for thesemotors is shown in Fig. 7. The peak overvoltage magnitude for3HP and 15HP motors is given in Table 2. From the table it is

Fig. 6(c) 2-D plot for HP rating Vs peak voltage for fixed Cable length of10m

V. EXPERIMENTAL ANALYSIS OF TERMINALOVERVOLTAGES

The above experimental setup is used for the analysis of peakvoltage magnitudes that occur at the inverter output and at themotor terminal. These voltage magnitudes are measured for3HP rating induction motor connected to inverter with a cableof different lengths (4m, 10m and 14m). This analysis helps tocompare the effect of cable length on the terminalovervoltages.

A. Experimental ana(vsis ofterminal voltage for 3HPinduction motor

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Fig. IO(b) Enlarged PWM inverter output voltage and motor terminalvoltage waveforms for 3HP with 10m cable••_p...=;;~~~.......-,--.........................-...---......-.-............,..------~........-........-.--~...........--.---y

Inverter outpu~Sing1ep~sePWMoutput

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Inverter output voltage

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, Ove:rvoltages at motor terminal

.--~- _.':JI!"~_~-==._: t;:;~:0:-;: ,::.0_~~:: ...7,0_0nw [

The Figs. 9 to II, the voltage waveforms obtained duringthe experimental analysis with 3HP induction motor, verifythat overvoltages occur at the motor terminals.

VI. COMPARISONS OF SIMULATION ANDEXPERIMENTAL ANALYSIS

The complete PWM drive system is simulated by combiningthe above models of PWM inverter, cable and motor. Thesimulation is performed in MATLAB for 3HP induction motorconnected to inverter with 14m cable. The inverter outputvoltage and the motor terminal voltage waveforms are shownin Fig 12.

Fig. ll(b) Enlarged PWM inverter output voltage and motor terminalvoltage waveforms for 3HP with 14m cable

Fig.II (a) PWM inverter output voltage and motor terminal voltagefor 3HP with 14m cable

3-phase, 420V, 3HP, induction motor is considered forexperimental analysis. This induction motor is connected to 3­phase inverter with different cable lengths of 4m, 10m and14m. The inverter output voltage and the motor terminalvoltage waveforms for these three lengths of the cable areobtained from the storage oscilloscope. Fig. 9(a) shows thevariation of line-to-line voltage at inverter output and motorterminals with time for 3HP motor with 4m cable. Fig. 9(b)shows the enlarged waveforms of Fig. 9(a). From the figure itcan be seen that overvoltages occur at the motor terminals.Figs.IO (a) and (b) show the captured voltage waveforms andenlarged waveforms respectively with 10m cable, and Figs.I I(a) and (b) are the respective voltage waveforms with 14mcable connected.

The experimental results of line to line voltage peak vstime for 3HP motor with 4m, 10m and 14m cable are shown inFigures below.

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E..~!L~._._ ...._ ...__

Fig. 9(a) PWM inverter output voltage and motor terminal voltagefor 3HP with 4m cable

PVVM output voltage

.... ..._.---~t·,· ., >'..~,.~,+"_'. ~w.~._~~.·

" Ove:rvoltages at motor terminal

.;:.; ";.~=:~.";"~,; ..? ..~~~~-,-_.-:= .. oorY"W"" I....~-~----_ _--_ +

Fig. 9(b) Enlarged PWM inverter output voltage and motor terminal voltagewaveforms for 3HP with 4m cable

Time (nsec)~

Fig. 12 Simulation results for 3HP induction motor with 14m cable length.

Experimental results of the inverter output voltage and themotor terminal voltage waveforms are shown in Fig 13.

Fig. 1O(a) PWM inverter output voltage and motor terminal voltagefor 3HP with 10m cable

TUne (ns e c) -----=--Experimental results for 3HP induction motor with 14m cableFig. 13

length.

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By observing the simulation and experimental waveformsofFig 12 and 13, it is seen that voltage at the motor terminal ismore compared to the inverter output voltage. Henceovervoltage occurs at motor terminal. This is due to impedancemismatch between the cable and motor winding. It is observedthat this overvoltage in some cases may go up to 4pu, whichdepends on the instant of inverter switches in different phases.

By comparing the simulation waveforms and experimentalwaveforms, it is observed that, as cable length increasespropagation time as well as peak voltage increases and peakvoltage will occur at different instants of time. This willhappen upto critical cable length, for which voltage doublingoccurs. Hence by increasing cable length upto some extent,rise time increases and insulation stress decrease, so that motorlife increases. Thus simulation results are in good agreement ascompared with experimental results. Another observationmade in this section, as cable length increases propagationtime is also increases. The propagation times observed insimulation and experiments are compared. Though they thereis difference between simulation and experimental results,considering the complexities involved in modeling andsimulation, the deviation is considered to be withinengineering accuracy.

VIII. CONCLUSIONS

This paper discusses the analysis of the over-voltagephenomena in long cable PWM drives due to voltagereflection theory. Experimental and simulation results arecompared. Effect of cable length, rise time and HP rating onthe over voltages are analysed. As the cable length is increasedover voltage magnitude increases. Also the effect of rise timeon dv/dt is presented. Shorter rise time results in high dv/dtwhich stresses the motor insulation. For large HP ratings overvoltage is less. Also simulation results of mitigating high dv/dtis explained by adopting filter.

VIII. REFERENCES

[1] Melhorn,Christopher J.,Le Tang, "Effects of PWM ASDs on StandardSquirrel Cage Induction Motors" 1994 PCIM ConferenceProceedings, Dallas Texas,September 1994, pp.356- 364.

[2] J.A Pomilio,C R de Souza, L,Matias, P.L.D.Peres and I.Sbonatti"Driving AC Motors through a Long Cable: The Inverter SwitchingStrategy", IEEE Transactions on Energy Conversion, Vol.I4,NoA,December 1999.

[3] T. Takahashi, M. Termeyer, T. Lowery, and H. Tsai, "Motor lead lengthissues for IGBT drives," in Proc. IEEE Pulp and Paper Conf., 1995,pp.21-27.

[4] R. Kerkman, D. Leggate, and G. Skibinski, "Interaction ofdrivemodulation and cable parameters on ac motor transients," IEEETrans./nd. Applicat., vol. 33, pp. 722-731, May/June 1997.

[5] G. Skibinski, R. Kerkman, D. Leggate, J. Pankau, and D.Schlegel,"Reflected wave modeling techniques for PWM ac motorrives," in Proc. IEEE APEC'98, Anaheim, CA, Feb. 15-19, 1998, pp.1021-1029.

[6] G.Suresh, Hamid A Tollyat,A Rendusara, and Prasad N.Enjeti,"Predicting the transient Effects of PWM Voltage wave form on thestator windings of random wound Induction motors" IEEETransaction on power electronics Vol 14 No.1 ,January 1999.

[7] A. F. Moreira, T. A. Lipo, G. Venkataramanan, and S. Bernet, "HighFrequency Modeling for Cable and Induction Motor Over-Voltage

Studies in Long Cable Drives" IEEE Industrial Application Society 36thAnnual Meeting Chicago, illinois, USA, September 30-October 5,2001.

[8] G. Grandi, D. Casadei, and A. Massarini, "High frequency lumpedparameter model for ac motor windings," in Proc. EPE'97, 1997,pp.2.578-2.583.

[9] K. A. Corzine, 1. T. Tichenor, J. L. Drewniak, and S. D. Sudhoff, "Highfrequency characterization ofa 3-phase induction motor," in Proc.1997Naval Symp. Electric Machines, Newport, RI, July 28-31,1997,pp.251­256.

[10] A.von Jouanne and P.N. Enjeti, "Design Considerations for an InverterOutput Filter to Mitigate the Effects of Long Motor Leads in ASDApplications," IEEE Transactions on Industry Applications, vol. 33, no.5, pp. 1138-1145, Sep/Oct 1997.

[II] S. Evon, D. Kempke, L. Saunders, and G. Skibinski, "Riding thereflected wave-IGBT drive technology demands new motor and cableconsiderations," in Proc. IEEE Petroleum and Chemical Industry Con!,Philadelphia, PA, Sept. 23-26, 1996, pp. 75-84.

[12] E. Persson, "Transient effects in application of PWM inverters toinduction motors," IEEE Trans. Ind. Applicat., vol. 28, pp. 1095­1101,Sept.lOct. 1992.

[13] P. Van Paucke, R. Belmans, W. Geysen, and E. Ternier, "Overvoltagesin inverter fed induction machines using high frequency powerelectronic components," in Proc. IEEE APEC'94, Mar. 1994, pp. 536­541.

IX. BIBLIOGRAPHIES

Dr.Basavaraja Banakara was born in1970.He is IEEE Member since2005. Heobtained his B.Tech (EEE) degree fromVijayanagar Engineering College Bellary,Gulbarga University and M.Tech from BVBEngineering College, Karantaka Unversity,India. He completed his Doctoral degree fromNational Institute of Technology, Warangal,

India. He worked as a Lecturer in VEC Bellary & AssociateProfessor at SSJ Engineering College Mahaboobnagar and worked asProfessor at SR Engineering College Ananthasagar, Waranagal India.Presently he is working as a Principal for Padmasri Dr. B V RajuInstitute of Technology Vishnupur, Narasapur, Medak, AP, India. Heis also ISTE Managing committee member for Andhra Pradeshsection. His areas of interest include power electronics, drives, ASD'sand High voltage Engineering and EMTP applications.

Dr.D.V.S.S.Siva Sarma was born in 1964. Heis IEEE Member since2004. He obtained hisB.Tech (EEE) and M.Tech (Power Systems)from JNTU College of Engineering, Anantapurin 1986 and 1988 respectively. He obtained hisDoctorate degree from Indian Institute ofTechnology, Chennai in 1993. Since 1992, he

is working as Professor in Department of Electrical Engineering atNational Institute of Technology, Warangal, Andhra Pradesh(Formerly Regional Engineering College, Waranga!). His areas ofinterest include Power System Transients, Fault diagnostics,Protection and Condition Monitoring of Power Apparatus, HighVoltage Engineering and EMTP applications. Presently he is achairman of Indian EMTP User Group and counselor for IEEEstudent branch of NIT Waranga!.