I:~EETransactions ~n Encr~y Conversion. Vol. 7. No.1, March 1992 I ~245

STARTING TRANSIENTS IN SLIP ENERGY RECOVERY INDUCTION MOTOR

DRIVES- PART 2: FLOWCHART AND PERFORMANCE

by

E. Akpinar, P. Pillay. MIEEE. A. Ersak

Department of Electrical and Electronic EngineeringMiddle East Technical University

Ankara,Turkey

. Department of Electrical EngineeringUniversityof New Orleans'Lakefront, LA 70148,USA

Kevwords :Induction Motor, Slip Energy, Starting Transients,Modeling.

ABSTRACT

The first part of this ~wo-part paper described the startingtransientsof a slip energy recovery 1Mdrive using the rotor rectifierand a single resistor instead of three resistors and an ac Circuitbreaker. The starting transient wasdivided into three stages; the firstwas the resistance starting, the second was the connection of the

'inverter and the third was,the disconnection of the starting resistor,The diode bridge rectifier imposes constraints onto the machineequations; the relevant equations for all three stages were developedin detail. This part presents a fiowchar!for the solution of the systemequations and presents detailed simulation results, backed up withextensiveexperimentalverification.

1. iNTRODUCTION

Slip-energy recovery induction motor (IM) drives have been usedfor many years for the speed control of large (excess of 100 kW)wound rotor IMs. A more energy consCious industry has injected arenewed interest [1-9] in the drive and have prompted researchers anddrive designers into understanding the performance in greater detail.The improved understanding allows semiconductor components to berated properly,speed and current controllers to be designed optimallyand the overall performance to be predicted accurately. The driveshould be protected against faults both in the assoCiated power systemand within the drive itself to minimiie the damage to healthyequipment and to reduce downtime. All of the above point to the useof accurate models which form the heart of computer programs to

predict the performance of. these drive systems. Although they aremostly used in applications that operate over a resiricted speed range,this is not an inherent defect of the drive. Slip~energy drives are quitecapable of operating from a few r.p.m up to rated speed; therectifier and inverter ratings are considerably reduced however by ,

limitingthe speed range. '

91 SM 388-9 EC A paper recommendedand approved

by the IEEE Electric Machih~ry Commi~tee of the IEEEPower Engineering Society for presentation at the'IEEE/PES 1991 summer Meeting, San Diego, California

July 28 - August I, 1991. Manuscript submittedJanuary 28, 1991; made available for printing,May 2, 1991.

Most of the previous work done on slip energy recovery 1Msystemshave concentrated on the steadystate. Simplifiedmodelshavebeen usetl which neglect the overlap in ihe rotor rectifier and theharmonicsgenerated by the inverter. Errors [7,8]in the applicationofreference frame theory to the slip energy recovery drives have alsobeen discovered in [6]. Only recently has the proper mathematicalmodel [1] for the steady-state operation been presented. Themathematical model for the transient analysis [2] and its computerprogram [3] have also been developed to examine the transientperformance in detailwhen starting witha resistancestarter connectedthrough an ac Circuitbreaker. This is the systemthat was examinedin[1-3,5-9].Great care must ,be exerCisedin the determination of theinitial conditions of the drive after the ac Circuitbreaker has beendisconnected [3].

Recently, an alternative starting technique has been proposed [4],a detailed examination of whieh is the purpose of this two-part paper.The 1M is connected to the rotor rectifier before starting, the outputof which is ,connected to a resistllnce starter. Thus the ac circuitbreaker is removed entirely. When the motor reaches a predeterminedspeed, the inverter is connected so that the order and theconfiguration of the drive system changes. Finally only the inverter isleft connected to the 1M but with a snubber capaCitor still connected.Hence even the steady state system is different from that consideredpreviously since now an extra state exists. This part presents aflowchart for the solution of the system equations developed in Part1 and presents detailed simulation results, backed up with extensiveexperimental verification. The paper is organized as follows: Section1 has the introduction. Section 2 presents the flowchart while section3 has the results. The conclusions are presented in section 4.

4. FLOWCHART OF COMPUTER PROGRAM

. Figure 1 shows the flowchart'of the computer program used topredict the starting transientsof slipenergyrecovery1Mdrive systems.The followingdrive and control parameters are read hi:

(a) machine parameters,(b) transformer turns ratio,(c) load specifications,(d) external rotor resistance,(e) magnetizingreactance of the transformer,(f) the magnitude of the time-step DELTA,(g) the time to stop DELTA 1,(h) the end of the time required for the resistance starting DELTA3,(i) the end of time for the transition period DELTA 4.

Time is set to zero at the initialization.The initialvalues of all statevariables are assumed zero; however the rotor terminal vollages are.not zero because GTOI and GT02 are initiallyoff so that rotor open

0885-8969/9 2$03.00@ 1992 IEEE

---

246

Read Machine parameters, Transformer turn ratioDELTA, DELT.\ 1. DEI.TA 3, DELTA ", DELTA 5Load Specificutions, ExterlllllResistancE>Magnetizing reactance of transformer

Initial values of stale variahles

Calclllllte

Write the state variables

the rotor phnse andvoltages

Iin", to line

Decide the conduction state hy comparingthe c:ompllted value of state variables with theconstraints 011 eac:h conduction st.ate and diode

(V-T) characteristic; find tht:' rotor state, .J

YES

Startingwith aid of an external resistance

II

,,-

I

YES

NO('

YES

Transition period including ~xtp.rnllland slip ellergy circuit

reHistllnc£'

c*0

NO

YES YES

snuhber

F

inclllriingSlip energy recovery systemc i rCI!i t.

--~J=12

4 th orderRunga-Kutta subroutine (R-K)

NO

NO

G

END

Figure 10. Flowchart of the computer program

circuit voltages exist by transformer action through the airgap. Theinstantaneous value of the rotor terminal voltages, when the rotorphase currents are zero, can be calculatedby using (1), (2) and (3)from [10]. It Is assumed that OT011s turned on at time = zero so thatthe programstarts to predict the transientsof the 1Mwith the startingresistor.

During both the steady state and the transient, the conductionsequence of the diode bridge must be decided. The particular diodesthat conduct dictate the particular rotor phases that are effective andhence the corresponding equations from (7) to (12) that must bechosen for integration. The insiantaneous rotor voltages are calculatedat each time step and the conduction sequence logic of the diodebridge rectifier [3] is used to specify the rotor conduction sequences.Once the diode conduction sequence is known, the appropriate stateequations to be integrated are chosen and integrated using a4th-order Runga-Kutta routine. When the machine is started with thehelp of the resistor, twelve conduction sequences define the systemperformance. The inverse of the' inductance matrix for each rotorconduction sequence is calculated once in subroutine (H) using theGauss-Jordan method and stored for further use.

The time is then incremented and the procedure iterated until thetime reaches DELTA 3. The average value of the rectifier voltage iscalculated to predict the appropriate recovery inverter firing angle.Simpson's Rule is used for the numerical integration in subroutine (I)to calculate the average value just before entering the transitionperiod. In the estimation of the tiring angle, the voltage drop on thesIl!.°othing reactor is ignored and the mean back emf of the recoveryinverter is assumed to be equal to the average value of the rectifieroutput voltage.

The final value of the state variables during the period of resistorstarting is taken as the initial conditions in the transition periodexcept that the initial value of the inverter input current, Id is zero.There are twelve rotor conduction sequences in the transition period;they are chosen on the basis of their instantaneous rotor terminalvoltages and rotor phase currents and the (v-I) characteristic of ~hediodes. Time is then incremented .and procedure is iterated until theend of transition period, DELTA 4.

The fin!!1values of the state variables in the transition periodwhen time reachesDELTA 4 are taken as the initialvaluesduring thefinal stage. The snubber capacitor voltage is assumed to be zeroinitially. If the effect of the snubber capacitor is neglected, thenumber of state variablescan be reduced by one and the slip energy .recoverysystemcan be modeled.as wasdone in [2].The equations aresolveduntil the time to stop is reached.

s. RESULTS

The estimation of the starting transients uses a 4-th orderRunge-Kutta integration routine operating on (7) - (24). Thefollowingresults were obtained on a slip energy recovery systeminvolvinga 3-phase,380V, 3.5 kW,8.1A, 50 Hz,4 pole, slip ringinductionmachinehavingthe followingparametersreferredto therotor side:

The effective rotor/stator turn ratio is O.4.Themoment of inertia

constant, J including load is 0.2 kg.ml, while the damping-coefficient,B including load is 0.008 Nm/rad/s. The secondary/primary turn ratio,a, of the recovery transformer is 0.36, and the external resistance, R.is 15 ohms. The snubber capacitor, C is 8 up. The load torque, TL is4 Nm. The value of the snubber capacitor was chosen with the helpof this program.

The inductionmotor starts with the external resistor,R. connectedto the output of the rectifier. The machine runs up to a steady-stateoperating speed of 100 rad/s under the load specified above. TIlerectifier mean output voltage at this speed is 55 V and the inputvoltage, VI of the line-commutated, 3-phase bridge- inverter istherefore adjusted to a value equal to or less than this value. Mterthese conditions have been reached, GT02 'is turned on.Measurements have been taken for the first four seconds of theexternal resistor starting period.

The transition period, in which both R. and the recovery inverterare present in the circuit lasts for 0.4 s followingthe external resistorstarting period. The disconnection of the external resistor from thecircuit is carried out at the end of this transition period, leaving theinverter to extract the rotor power alone.

In the measuredresults that follow, all resultsshould be multipliedby 100.The firststage of starting,with the resistanceconnected to the

output of the rectifier took 4.6s of which 4s are shown in this paper.(The final 0.6s showing essentially steady-state behavior). The inverterwas then introduced at 4.6s and this transition was allowed to run for

O.4s up to 5s. Note that in this transition period, the rectifier outputcurrent flows through both the starting resistor and the inverter.Finally, the starting resistor is disconnected at 5s, leaving the entire dclink current to flow through the inverter. Although severalmeasurements were made, space limitations dictate that only a fewselected results be shown here. For the initial starting stage, the rotorand stator currents and the rotor line-line voltages are shown.

Figures 2a and 3a show the predicted and measured rotor phasecurrent results respectively, during the first two seconds of startingwith a resistor connected to the output of the rectifier. The presentedresults are divided into two-second intervals because of di(ficultiesexperienced in capturing the entire transient in one oscillogram.Figures 2b and 3b show the next two second.s.A surge of current isdrawnon starting,characteristicof the 1M startup transients; the surgebeing limited by the starting resistor. The initial magnitude and thegeneral shap~of the envelope for the entire four-secondduration arepredicted accurately using the model developed in [10] and thecomputer program described earlier. The initial starting currentmagnitude is 12A peak, decreasing to 5A peak after 4s.

Figures 2c and 3c as well as 2d and 3d show the correspondingresults for the stator current. Again the predictionmatches themeasured accuratelyboth in the magnitudeof the initial surge as wellas the magnitude and' shape of the envelope. The stator currentmagnitudehas an initialpeak of 7A, which after 4s is reduced to 3.5A.This reduction is not as large as that of the rotor phase current. Thereason is the addition of the magnetizing current component to thereflected rotor phase current to yield the stator current.

Figures 2e and 3e aswell as 2£ and 3f show the results for therotor line-line voltage. The voltage reduces from 190V peak whenstarting under load to 73V peak after 4s. These results are predictedaccurately. The stator input voltage is 380V and the stator/rotor turnsratio is 0.4. Hence the open circuit rotor voltage should be380*0.4*1.414= 215V peak. The measured open circuit voltage of190V peak is a result of the voltdrop due to the starting stator androtor currents flowing through their respective leakagereactances andresistances. .

The next set of figures show the transition period when theinverter is connected' and the final stage when t~e resistor isdisconnected. The transition period is shown from 4.6s to 5s while thefinal stage is shown from 5s to 6.5s. Figures 4a and 5a.show thepredicted andmeasuredrotor phasecurrents respectively.The overlapin the rotor current is evident as are the superimposed harmonics thatare causedby the inverter. When the resistor is removed, there is asudden reduction in the rotor phase current. This is expected sinceone of the current paths has been removed. The GTOI snubbervoltage shownin figures4b and 5b whichwas initially zero, risesto an

R.= 0.144ohms, . = 0.065H,

Rr = 0.2ohms, Lr = 0.065 H, M = 0.0522H

Rc = 1.49ohms, Lc = 0.04 H.

248

-2-4-.~.-'0-12-14

-II Time(s)

II

14

'2

0 0.2 0.' 0.' 0.'. , 1.2 '.'Figure 2a. Predicted rotor phase current

I.'I.'

A'0

-2-..-.-8

-10-12--14 -j-11 Time(s)

108.71 -,

It -I.3210

-1-2-3-4 '-5 '-I'-7.-8 .-8 .

-10.

2.2 2.4 2.1 2.8 3 3.2 3.' .1.1

Figure 2b. Predicted rotor phase current3.8

A

Time(s)0.2

,0.8

1.80.' 0.8 1.2 I.' 1.8

'088785432,0

-I-2-3-.-5-I-7-8-8

-10

Figure 2c. Predicted stator phase current

A

Time(s)2.2 2.' 2.1 2.8 3 3.2 3.' 3.1 3.8

Figure 2d. Predicted stator phase current

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Figure 3b, Measured rotor phase currentX-axis: O.2s/div,Y-axis: 5Ndiv

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11/11! ,,"Ii' ,I',:",','!! ';""':,1 ,:' " ", :', ',' . I

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Figure 3c. Measured stator phase currentX-axis: O.2s/div,Y-axis: 5Ndiv

I'./ji

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n

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Figure 3d. Measured stator phase currentX-axis: O.2s/div,Y-axis: 5Ndiv

JOO

v200

100

, Time(s)-,,-""" r

0.2 0.4 0.' 0.8 I . 1.2 . 1.4 1.' 1.'

Figure 2e. Predicted rotor line-line voltage

" , '.. . "

-100

-200

Time(s)-300 +.- r""-'--,.'-,--r-,-,--, ,--r-r-'",- -,--,'-,--,-,.-,-

2 U ~ U U 3 U U U u

Figure 2f. Predicted rotor line-line voltage

4 .

,-

-I-2-]-4-I-I -,

-7-I

4.1

Time(s)

U . U ~ U U I U

Figure 4a. Predicted rotor phase current

Time(s)

',2 ... '.1 ... I '.2 ...

Figure 4b. Predicted snubber capacitor voltage

Figure 3e. Measured rotor line-line voltageX-axis: O.2s/div,Y-axis: lOOV/div

l ., " " .' :, .

tmm~~~~\!Mmnmmmm~m~m~AI. !iwmmmmmmmmm~m~mvm. !

Figure 3f. Measured rotor line-line voltageX-axis: O.2s/div.Y-axis: lOOV/div

~~n~ ~. . . ... .. .j"i"IIJ I fl 1t j\ It j jfl

1.4 Figure Sa. Measured rotor phase currentX-axis: O.2s/div.Y-axis: 5Ndiv

Figure 5b. Measured snubber capacitor voltageX-axis: O.2s/div,Y-axis: 20V/div

-'00

-200

-JOO0

3001v

200

100

'0

IV70

60

.0

40

30

20

10

0

U 4.'

250 --J .

Time(s)

'.. ...,-05 5.2 5.' 5.. 5.. 6 ..2

Figure 4c. Predicted dc link current

6.'

averagevalue of 50V as soon as GTOI is turned ofC.The inverter delink current shown in figures 4c and 5c rises to take on the totalcurrent from the rotor rectifier.The dc link inductor prevcnts the dclink current from rising instantaneously and some time must beallowed to let the dc link current reach its desired value. Thisreduction in the dc link current implies a reduction in the electrictorque of the motor and hence a reduction in speed. The predictedvariablesare accuratelyverified by measurements.

4. CONCLUSIONS

The first part of this two-part paper formulated and developedthe models to study the starting transients of slip energy recovery

. induction motor drives. This second part prcsentcd a computerprogram and a detailed set of experimental and theoretical results togive an insight into the expected behavior of the drive.

The starting resistor allows the peak motor torque to bedeveloped at starting or for the starting currents to be limited to aprespecified value. The transients associated with the initialconnectionof the inverter can be limited by the appropriate choice ofthe inverterfiringangle.A transient occurswhen the resistor is finallydisconnectedbecause of the removalof a parallel current path. Boththe overlap in the rotor rectifier and the harmonics reflected by theinverter have been included in the modeling and the computerprogram.This removesthe need for "correction factors" to be used inthedesignof a practicaldrive. .

The modelspresented and the computer program describedcanbe used for the performance prediction of prototype drives evenbefore they are designed and built to examine the expectedperformance.The program can also be used for CAD of these drivesystems, with the designer running the program several times toexaminethe effectsof different motor and drivesystemparameters onthe performance.For example,one drive manufacturer does not usea link inductor at all and obtains the desired link current smoothingby increasing the motor's leaaakage inductance. The driveperformance of such a system can be studied with the aid of thisprogram.Faults, both in the drive and on the associatedpower systemcan be studied to develop appropriate protection algorithms. Inaddition the performance of speed and current controllers, designedusing linear techniques,can be examined during transients when thedrive model becomes nonlinear. . .

Acknowledl!ements

E. Akpinar would like to thank the British Council for funding hisresearch at the University of Newcastle upon Tyne and the MiddleEast Technical University (METU) for funding his research at METUunder contract no AFP-9Q-O:i-Ol-02.

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Figure 5c. Measured dc link currentX-axis: O.2s/div,Y-axis: lA!div'

REFERENCESI.

I ::

[1] I.E. Brown, W. Drury, B.L. Jones and P. Vas, "Analysisof thePeriodic Transient State of a Static KramerDrive",ProclEE, Vol.133,Pt a, No.1, Jan. 1986,pp 21-30. .

i,

[2] E. Akplnar and P. PiI~ay,"Modeling and Performance of SlipEnergy Recovery Induction Motor Drives."1989 IEEE Trans. onEnemv Conversion,Vol. 5, No.1, March 1990,pp 203-210.

[3] E. Akplnar and P. Pillay, "A Computer Program to Predict thePerformance of SlipEnergy RecoveryInduction Motor Drives",1990IEEE PES Winter Meeting, Atlanta, Georgia, USA

,.

[4]S.R. Doradla, S. Chakravorty Rnd K.E. Hole, "A New Slip PowerRecovery Scheme with Improved Supply Prime Power Factor", IEEETrans on Power Electronics. Vol 3. No.2, April 1988, pp 200-207.

[5]R.J. tee, P. Pillay and R.G. Harley, "D.Q. Reference Frames forthe Simulationof Induction Motors",EPSR Journal. Vol 8. Oct 1984,pp 15-25. .

[6]P.C. Krause, O. Wasynczuk and M.S. Hildebrandt, "ReferenceFrame Analysisof A Slip Energy Recovery System"IEEE Trans. onEnerl!V Conversion. Vol 3. No 2, June 1988, pp 404-408.

[7]V.N. Mittle, K. Venkatesan and S.C. Gupta, "Switching Transientsin Static Slip-Energy Recovery Drive", IEEE Tlans, Vol. PAS-98, No .4, July/Aug 1979, pp 1315-1320.

[8]V.N. Mittle, K.Venkatesan and S.C. Gupta, "Stability Analysis of aConstant Torque Static Slip Power Recovery Drive", IEEE Trans, Vol.IA- 16, No.1, Jan/Feb 1980, pp 119-J26.

[9] W. Shepherd and LN. Hulley, "Power Electronics and MotorControl", 1987.

[10] E. Akplnar, P. Pillay and AErsak, "Starting Transients in SlipEnergy Recovery iM drives-Part 1: Formulation and Modeling",companion paper.

25\

EyuD Akpinar received the B.S. and M.Sc degrees in electricalengineering from the Middle East Technical University (METU),Ankara, Turkey in 1981 and 1984 respectively.His M.Sc thesis wasconcerned with the optimum design of multipole synchronousgenerators. He was in the Ph.D program at the University ofNewcastleupon Tyne in England, which he is completing at METU.His research interests include the control, modeling and design ofelectricmachinery.He is n student member of the Power EngineeringSociety of the IEEE.

Pra!!asen Pillav (S'84-M'87) received the Bachelor of EngineeringDegree from the Universityof Durban-Westville in South Africa in1981. the Masters degree in Electrical Engineering from theUniversityof Natal. South Africa In 1983 and the Ph.D degree fromthe Virginia Polytechnic Institute and State University, Va in 1987.From January 1988 to August 1990 he was with the University ofNewcastle upon Tyne in England. He is currently with the Universityof New Orleans, Louisiana. He is a past recipient of an IEEE prizepaper award. He is a member of the Power Engineering, IndustryApplications and Industrial Electronics Societies of the IEEE and amember of the Industrial Drives, Electric Machines and EducationCommittees of the Industry Application Society. He is also a memberof the lEE, England and a chartered ciectrical engineer. His researchinterests are in modeling, design and control of electric motors anddrives and the interaction between drives and the power system.

Aydin Ersak has the Bachelor's, Master's and Ph.D degrees, all inelectrical engineering. All degrees were obtained from the Middle

. East Technical University. Ankara, Turkey. His research interestsinclude the control and design of electric drives and power converters.