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Performance Analysis and Simulation of Inverter-fedSlip-power Recovery Drive

A K Mishra, Non-memberProf A K Wahi, Non-member

In this paper, performance analysis and PSPICE simulation of inverter-fed slip-power recovery drive is presented. Thisdrive is different than other slip-power recovery drives in the sense that the slip-power from rotor terminals is recoveredto the dc-link of feeding inverter to the motor while in other schemes slip-power is recovered to the ac source. Recovery of

slip-power from rotor terminals to ac supply is blocked by diode bridge rectifier, which is used to feed the motor inverter.Steady-state performance analysis of the drive is done with the help of dc and ac mathematical models. PSPICE simulationof this drive is done using equivalent circuit model of the wound rotor induction motor. This drive is developed in thelaboratory for a 2-kVA wound-rotor induction motor. Theoretical and experimental performances and simulationresults are found as expected.

Keywords: Inverted Fed; Induction motor; Slip-power; PSPICE

A K Mishra is Scientist D, RCMA, Korwa C-39, HAL Township, HAL,Korwa Sultanpur 227 412 (UP) and Prof A K Wahi is with Department. ofElectrical Engineering, Institute of Technology, Banaras Hindu University,Varanasi 221 005 (UP).

This paper (redrafted) was received on September 9, 2003. Written discussionon this paper will be received until November 30, 2004.

Vol 85, September 2004 89

NOTATION

C : capacitor in the dc link,mF

D : drain terminal of the MOSFET

d : duty-ratio of the chopper

E : rms voltage of the input supply, V

E1 : line to line rms input voltage to the statorof WRIM, V

E2 : rotor terminal voltage at standstill, V

f : supply frequency, Hz

Id : rotor rectified dc current flowing throughthe chopper inductor,A

Ir : rotor current in equivalent ac model,A

Irf : fundamental component of the rotorcurrent,A

L1 : dc link inductor in transient analysis,H

Lc : self-inductance of the chopper inductor of

the proposed scheme, HLf : self-inductance of the chopper inductor in

rotor resistance control,H

LR 1', LR 2', : rotor leakage inductances transferred toLR 3' stator for PSPICE simulation, H

LRM1', LRM2', : rotor magnetizing inductances transferredLRM3' to stator for PSPICE simulation, H

LS1,LS2,LS3 : stator leakage inductances for PSPICEsimulation,H

LSM1,LSM2, : stator magnetizing inductances forLSM3 PSPICE simulation,H

ma : amplitude modulation-ratio

n : stator to rotor turns-ratio

: efficiency of the machine

R1 : stator resistance transferred to rotor side,

R2 : stator resistance transferred to the rotor

side,

R2 : rotor resistance,

Rc : resistance of the chopper inductor,

RL1 : resistance of the dc link inductor forPSPICE simulation,

RR 1',RR 2', : rotor resistances transferred to stator for

RR 3' PSPICE simulation,

RS1,RS2,RS3 : stator resistances for PSPICE simulation,

s : motor slip

Vc : chopper input voltage, V

Vo : output voltage of the step-up chopper anddc link voltage, V

Vr : rotor rectified voltage, V

Vrs : rotor rectified voltage at standstill, V

Xm

: magnetizing reactance of the motor

transferred to the rotorside,

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X1 : stator leakage reactance transferred to

rotor,

X2 : rotor leakage reactance transferred to

stator side,

X2 : rotor leakage reactance,

INTRODUCTION

Slip-power recovery scheme is an efficient method of speedcontrol of wound rotor induction motor. A Lavi and R J Polgepresented the basic concept of this scheme1 in 1966. Theanalysis of the scheme using thyristor was first time reported byShepherd and Stanway2. The main draw-back of this scheme isthe poor supply power factor due to reactive power drawn fromthe source both by motor as well as line commutated inverter.Many improvements and new schemes are presented in past toimprove the line power factor. W Shepherd and Khalil

presented capacitive compensation3

, S K Pillai and Desai usedstep-up chopper4, Taniguchi and Mori used Power Chopper toThyristor Scerbius5 and Dordala, et al, used PWM inverter6 toimprove the supply power factor. L Refoufi, et al, presentedanalysis of a step-down chopper controlled slip energy recoveryinduction motor drive claiming an improvement in powerfactor7. Fan Lio, et al, tried to recover the slip energy to a part ofthe stator winding to avoid the use of a recovery transformer8. GD Marques and P Verdelho presented a different circuitconfiguration in which a boost up chopper connects the rotorrectifier to a dc link of voltage controlled line commutatedinverter connected in parallel with a capacitor9. YoshikaKawabata, et al, proposed a vector controlled inverter fedwound rotor induction motor for high power applications10. Anew scheme was presented in which the motor was fed througha rectifier-inverter set and slip-power from rotor terminals wasrecovered to the dc link of the inverter11. Study of these papersrevealed that these schemes have some limitations12 as ( i) speedcontrol range is small to reduce the cost of the drive; (ii) anumber of starting equipments are required, eg, starting liquidresistors, isolators, circuit breakers, fuses, transfer-over switch,speed sensing devices; (iii) shoot through fault oftenly damagethe drive; (iv) supply line voltages increase whenever a largeamount of power is recovered to the mains; and (v) Large

harmonics are injected in the supply line due to switching of atleast six switches of the recovery inverter.

In the present paper, theoretical analysis of the dc link slip-power recovery drive is done with the help of the mathematicaldc and ac models. The results are experimentally verified and acomparative study of the theoretical and experimental results ispresented. The equivalent circuit of the motor is used forPSPICE simulation, which gives switching simulation of thedrive at particular speed.

DESCRIPTION OF THE DRIVE

The schematic diagram of the proposed circuit is shown in theFigure 1. In the proposed circuit three-phase ac supply is inputto the stator of the wound rotor induction motor by using arectifier-inverter set. Using diode bridge rectifier ac supply isconverted into dc. Depending on the supply available powerrating of the motor, the diode bridge rectifier may be single-phase or three-phase. The ripples from the rectified dc voltage

are reduced by connecting a large filter capacitor across theoutput of the rectifier. The same capacitor is used in the outputof the step-up chopper, which holds the voltage variationminimum. The rectified dc voltage is input to a PWMcontrolled self-commutated three-phase bridge inverter. Asimple firing circuit is used to control the inverter so that aconstant three-phase voltage at 50 Hz is input to the stator of theWRIM.

To recover the slip-power from the rotor terminals, the rotorthree-phase voltage is rectified using a three-phase diode bridgerectifier. This rectified voltage is stepped-up, using a step-upchopper, to the level of the dc voltage of the dc link of the

rectifier-inverter set, feeding the stator winding of the WRIM.So long as the chopper output voltage remains less than the dclink voltage, no slip-power is recovered from rotor terminals.As soon as chopper output voltage becomes equal to the dc linkvoltage, the motor current is partly drawn from the chopper andthe rest is supplied by the mains. Hence, the slip-power is fedback to the motor itself. The operating principle of theproposed scheme is different from other schemes, in the sensethat the slip-power recovered is being fed back to the motorwithout coupling any other machine with the Wound RotorInduction Motor.

Figure 1 Schematic diagram of the drive

1 f

Filter Inductor

VDC

1-Phase Diode Bridge Rectifier 3-Phase Bridge Inverter3-Phase Diode Bridge Rectifier

Chopper Inductor D1

CSCHVR

sE2

I M

E1S1 S3 S5

S4 S6 S2

dc Link Slip-Power Recovery Drive

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PERFORMANCE ANALYSIS

For performance analysis11 of the scheme, followingassumptions are made:

(1) No commutation overlap in the diode bridges.

(2) There are ripple free dc currents.

(3) Under the above assumptions, the supply phasecurrents and the rotor phase currents will have six-stepwaveforms.

(4) No losses in diode-bridge and power- switches.

With the above assumptions, rectified rotor current of thedrive11 is given by

IV s d n

n Rdo

m

= 1 4315 1 1 4315. [ {( ) / . }]

(1)

whereR R X X s R Rm c= + + + +{ ( )/ }2 1 3 1 2 2 2 (2)

This gives the no load slip as

sd n

0

1

14315=

( )

.(3)

Per phase fundamental air-gap powerPgcan be calculated as

Pg I R R srf x A= +2 /b g (4)

where

I Irf d= 6 / (5)

R R Rx c= +{( / ) }( . )2

29 1 0 5 (6)

and

R R Rd V

IA c

o

rf

= + +

( . )( )

2 0 51

3 6

(7)

The average torque developed by the motor at the shaft is

calculated as

Ta =3 2

2I R sr A

s

/

(8)

Efficiency of the machine is given by

=+

P

Pm

m Losses(9)

wherePm = mechanical power output = ( )1 s Pg.

Active power fed to the load is given by

P E I 1 1 2 2= cos ( ) (10)

where I2 is rotor current and 2 is the angle between stator

current and input voltage.

Is E

n sR R

R

s s X X xA

21

1

2

1 2

2

=

+ +F

HG

I

KJ+ +

b gn s

(11)

and

cos( )/

21

1

2

1 2

2

= + +

+ +FHG

IKJ + +

sR R R s

sR RR

ss X X

x A

xA b gn s

(12)

Total active power drawn from the dc link

PA =Pg+ constant losses SPr (13)

where, Spris slip-power recovered.

Reactive power drawn by the motor

QE

Xm

m

= 12

(14)

where,Xm is the magnetising reactance of the motor.

Reactive power required by the load

Q E I1 1 2 2= sin ( ) (15)

Total reactive power supplied by the source

Q Q QA m l= + (16)Total apparent power input to the motor

P P Q P AP A A H = + +2 2 2e j (17)

where,PHis the total harmonic power loss, given by

P V R X H H= +2 2 2/ e j

where, VH= E n N N 1 6 1/ [ ( )], being natural number.

R R R R s nx A= + +1 2( / )

and

X X X n= +1 22

The stator input power factor is given by

PF = total active power input to the

stator/total apparent power

= P PA AP / (18)

PSPICE SIMULATION

PSPICE simulation of the scheme is done by using theequivalent circuit shown in Figure 2. Here, inverter is operated

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in 120 mode of operation. The inverter is made up of six self-

commutating switching devices. Six pulses each of width of 1/3rd(6.66 ms) of the time period (20 ms for 50 Hz) of the output of theinverter are input to the GATE terminals of the switching

devices. The Induction Motor cannot be directly simulated

using PSPICE, so equivalent transformer model of the

induction motor with variable rotor reactance is used for itssimulation in PSPICE. Because the rotor circuit parameters aretransferred to the stator side and the equivalent magnetizingreactances of the stator and rotor are coupled with each other

(LSM1 coupled withLRM1',LSM2 coupled withLRM2' and

LSM3 coupled with LRM3'), the variation of rotor currentfrequency is not observed in the simulation results of the rotorcurrent. The effect of the rotor current frequency is on thereactances of the rotor circuit and its effect on the rotorresistance is negligible. Thus PSPICE simulation results giveinstantaneous voltages and currents at various points of the

equivalent circuit model of the drive for particular set of slipand chopper duty-ratio.

The equivalent circuit of wound rotor induction motor consistsof connected stator and rotor as per the specifications of theoutput of the experimental inverter and ac motor (universal

3

N1

1 D1 D3

D4 D2

0

10 L1, RL1 11

1 35

12

4 6

2 13

LSM1

LS1

RS LSM3

LS3

RS3

RS2 LS2 LSM2

RR'1

LR'1

LRM'1

LRM'3

LR'3

RR'3

RR'2 LR'2 LRM'217

15 18L2, RFC

20 10

CSCH

01614

Figure 2 Equivalent circuit used for PSPICE simulation

Solid linesanalytical and dashed linesexperimental12.00

10.00

8.00

6.00

4.00

2.00

0.00

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10

Torque,

N

m

Speed, pu

D=0.8

D=1.0

D=0.9

D=0.9

D=1

.0

D=0.8

D=0.6

D=0.6D

=0.4

D=0.4

Figure 3(a) Torque against speed (pu) characteristics

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Solid linesanalytical and dashed linesexperimental

D=0.9

D=0.9D

=0.8D=

0.8

D=0.7

D=0.7

D=0.6

D=0.4

D=0.6

D=0.4

Efficiency

Speed, pu

Figure 3(b) Efficiency against speed characteristics

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10

PowerFa

ctor

D=0.9


Figure 3(c) Stator power factor against speed characteristicsSpeed, pu

D=1.0D=1.0

D=0.9

D=0.7

D=0.7

D=0.5D=0.4

D=0.5

D=0.4

7.00

6.00

5.00

4.00

3.00

2.00

1.00

0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10

Speed, pu

StatorCurrent,Amp

Figure 3(d) Stator current against speed characteristics

D=1.0

D=0.7

D=0.9

D=1.0

D=0.9

D=0.4D=0.7

D=0.5

D=0.5D=

0.4


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motor) given below. The maximum output voltage of the

inverter used in the experimental set-up is 200 V, so the statorand rotor windings are connected in so that torque producedby the motor reaches maximum.

In this paper, PSPICE simulation results of line to line statorvoltage [v (12, 13)], line to line rotor voltage [v (15, 16)], statorcurrent [I(LS1)] and rotor current [I(R 1)] are shown in Figure 4for s = 0.2, d= 0.3; s = 0.5, d= 0.5; s = 0.9, d= 0.1 and s = 0.9,d= 0.9.

SPECIFICATIONS OF THE EXPERIMENTAL SET-UP

Specification of the Motor

Output 415 V or 240 V ac, 3-phase, 50 cycles, 1500 rpm.

Input 415 V or 240 V ac, 3-phase, 50 cycles, 1500 rpm.

Normal Current Rating 2.8 A for a voltage of 415 V

In all cases the volt-ampere rating is 2020 VA

Figure 4 PSPICE simulation

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Equivalent Circuit Parameters

(a) Stator resistance : 6.3 / phase

(b) Stator reactance : 15.953 /phase at 50 Hz

(c) Magnetizing reactance : 100.03 /phase at 50 Hz

(d) Rotor resistance : 5.2 /phase

(e) Rotor reactance : 15.953 /phase

Inverter Specifications

(i) Input supply : 230 V, single-phase,50 Hz ac/230 V dc

(ii) Output supply : 12200 V (line-to-line)variable voltage, 3-phase,

: 0320Hz variablefrequency, ac

(iii) Output power : 2.2 kW (4.0 kVA)

(iv) Output current : 10 A

(v) The dc link capacitor : 2 530 F (in parallel)

Chopper Specifications

POWER MOSFET IRFPG 50 is used as chopper switch, thevoltage and current rating of which is 1000 V and 8.0 A. The fastrecovery diode (IOR 16 FC) of voltage and current rating 1200 Vand 10 A is used.

CONCLUSIONAnalytical and experimental performance characteristics of thedrive are shown in Figures 3(a)-3(d). All these characteristicsexhibits that the drive has a particular range of speed for aparticular value of the chopper duty-ratio. In Figure 3(a),theoretical and experimental torque-speed characteristics aredrawn for different values of the duty-ratio of the chopper,which exhibits similarity with the torque-speed characteristicsof a V/fcontrolled induction motor drive. Maximum torqueslightly increases on increasing the duty-ratio of the chopper.Starting torque is high for the low values of the duty-ratios.Stable region of the torque-speed characteristics increases for

low values of the chopper duty-ratio. In Figure 3(b), theoreticaland experimental efficiency-speed characteristics are shown fordifferent values of duty-ratio of the chopper. This shows higherefficiency for large values of the duty-ratio at higher speed. InFigure 3(c), theoretical and experimental power factor-speedcharacteristics of the induction motor are shown for differentvalues of the duty-ratio of the chopper. The motor input powerfactor improves on increasing the duty-ratio of the chopper. Theline side power factor remains high due to the use of the diode-bridge rectifier. In Figure 3(d), theoretical and experimentalstator current-speed characteristics are shown. This figureexhibits that the stator current increases for increase in duty-ratio and it remains high for duty-ratios at low speeds.

Figure 4 gives PSPICE simulation results of the drive fors = 0.2,d= 0.3;s = 0.5, d= 0.5;s = 0.9, d= 0.1 ands = 0.9, d= 0.9.Simulation waveforms of line to line stator voltage, v (12, 13) isalmost sinusoidal for low values of the duty-ratio at lower slips.This voltage remains almost same for all slips at all duty-ratio asthis is the input voltage to the stator. Simulation waveforms of

line to line rotor voltage v (15, 16) is low and has almostsinusoidal waveforms for low values of slips and duty-ratios butits waveform distorts on increasing the duty-ratio as well as slip.Rotor voltage magnitude increases on increasing the slip but itremains same for all values of duty-ratios at a particular slip.The magnitude of the rotor current I (RL 1) increases onincreasing both the slip and duty-ratio. Distortion in the rotorcurrent waveform due to chopping action is more visible forlow slips and duty-ratio. The stator current magnitudeI(LS1)also increases on increasing both the slip and duty-ratio.

REFERENCES

1. A Lavi and R J Polge. Induction Motor Speed Control with StaticInverter in the Rotor. IEEE Transaction on Power Apparatus and Systems,

vol PAS-85, no 1, January 1966, pp 76-84.

2. W Shepherd and J Stanway. Slip Power Recovery in an Induction Motor

by the Use of a Thyristor Inverter. IEEE Transaction on Industry and

General Applications, vol IGA-5, no 1, January/February 1969, pp .74-82.

3. W Shepherd and A Q Khalil. Capacitive Compensation of Thyristor

Controlled Slip-energy-recovery System. Proceedings IEE, vol 117, no 5,

May 1970, pp 948- 956.

4. S K Pillai and K M Desai. A Static Sherbius Drive with Chopper. IEEE

Transaction on Industrial Electronics and Control Instrumentation, vol IECI-24,

no 1, February 1977, pp 24-29.

5. K Taniguchi and H Mori. Application of a Power Chopper to the

Thyristor Scherbius. IEE Proceedings,vol 133, Pt B, no 4, July 1986,

pp 225-229.

6. S R Doradla, S Chakravorty and K E Hole. A New Slip Power Recovery

Scheme with Improved Supply Power Factor. IEEE Transaction on Power

Electronics, vol 3, no 2, April 1988, pp 200-207.

7. L Refoufi, P Pillay and M R Harris. A Step-Down Chopper-Controlled

Slip Energy Recovery Induction Motor Drive. IEEE Transaction on Energy

Conversion, vol 8, no 3, September 1993, pp 396-403.

8. F Liao, JI Sheng and A L Thomas. A New Energy Recovery Scheme for

Doubly Fed, Adjustable-Speed Induction Motor Drives. IEEE Transaction

on Industry Applications, vol IA -27, no 4, July/August 1991, pp 728-733.

9. G D Marques and P Verdelho. A Simple Slip-power Recovery System

with a dc Voltage Intermediate Circuit and Reduced Harmonics on the

Mains. IEEE Transaction on Industrial Electronics, vol 47, no 1, February

2000, pp 123-132

10. Y Kawabata, E Ejiogu and T Kawabata. Vector Controlled Double-

Inverter Fed Wound Rotor Induction Motor Suitable for High Power

Drive. IEEE Transaction on Industry Applications, vol 35, no 5, September/

October1999, pp 1058-1066

11. A K Mishra. A Novel Scheme for Slip-Power Recovery Drive. Ph D

Thesis Submitted at IT, BHU, Varanasi, December 2000.

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12. A K Mishra and Dr A K Wahi. A New Slip-Power Recovery Scheme for

Inverter-Fed Induction Motor Drive. National Conference on Electric Drivesand Control for Transport Systems, January 16-18, 1997.

APPENDIX

Single-phase ac supply is input to a diode bridge rectifier. As a large

capacitor is connected across the rectifier output, the rectified dc voltageremains almost constant equal to the peak voltage of the supply, given by

Vo = 2E (A.1)

where, E is the rms value of the supply voltage.

This rectified voltage is input to the PWM controlled inverter. Assuming a

linear PWM modulation, the line to line rms output voltage of the inverter

is given by11

where, ma is amplitude modulation ratio.

E m V a o13

2 2= (A.2)

E Vo13

2 2=

= 0.612 Vo

= =3

208655E E. (A.3)

At 50 Hz and rated output voltage, the value of ma is assumed to be unity.

The fundamental line to line output voltage of the PWM inverter, input to

the stator is given by

Rotor terminal voltage is given by

E s En

s En

21 0 8655= = . (A.4)

Rotor rectified dc voltage is given by

Vr =3 6

2

sE

= 3 6

08655 1

sE

n.

= 14315.sn

Vo (A.5)

To feed power from the rotor terminal to dc link, the stepped-up rotor

rectified voltage should be equal to the dc link voltage.

Chopper output voltage Vo and input voltage Vc are related as

Vc = (1 d) Vo (A.6)

For developing dc model of the circuit, the stator and rotor resistances and

reactances are transferred to the dc side of the rotor rectifier. For

transferring the stator and rotor parameters to dc side of the rotor rectifier,

the equivalent voltage drop across them is considered. Their effect is

assumed to be resistive and their equivalent resistance Rm is given by11

R R X X R Rm cs= + + + +2 3 21 1 2 2c ho t/

= +R Rp Qs (A.7)

where R R X X P = + +2 31 1 2c h /

and

R R RQ c

= +22

From the Figure of dc model Figure (A-1), rectified dc current Id is given by

I R V V d m r c=

= 1 4315 1. ( / ) ( )V n d V o os

= 14315 1 14315. / . /V d n no s c ho t

or

IV d n

n Rdo

m

s=

1 4315 1 1 4315. / .c ho t(A.8)

Rm Lc

VoS CVc

Id

Vr

sE2

3-Phase DiodeBridge Rectifier

Figure (A-1) Equivalent dc model of the drive

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