mr.b.ravi and mr.e.narasimhulu comparison of performance ...iret.co.in/docs/itped/volume 1/issue1/2....

Mr.B.Ravi and Mr.E.Narasimhulu 8

IRET Transaction on Power Electronics and Drives (ITPED) Vol. 1, Issue. 1, Oct. 2013

Comparison of Performance Characteristics ofFive-Leg and Four-Leg Inverters Fed to Two

Different Induction Motor DrivesMr.B.Ravi and Mr.E.Narasimhulu

Abstract—Dual three-phase voltage source inverter (Three-VSI) system to drive two three-phase ac motors independentlyis generally used. The system connects one motor to one three-VSI and requires dual Three-VSIs. Recently, for the sake oflow cost, saving space, and reduction of inverter losses toreduce switching device counts, four-switch inverter to driveone motor, a four-leg inverter(FLI), a five-leg inverter and six-leg inverter to drive two three phase AC motors independentlyhas been studied. In particular, the four leg inverter consists offour legs and two capacitors connected in a series. One phase ofboth motors is shared and connected to the neutral point oftwo- sprit capacitors in common. The four leg inverter requireseight switching devices. Finally, the four leg inverter candecrease four switches compared with dual three-VSI systems.Also, the pulse width modulation technique in three phases VSIis not directly applicable for the four leg inverter because onlytwo phases must be modulated. The four leg inverter is a singleinverter that can drive two three-phase ac motorsindependently. The inverter consists of four legs and twocapacitors connected in a series. The U and V phases of bothmotors are connected in each leg, respectively, whereas the Wphase of both motors is connected in the neutral point of two-sprit capacitors. Then, this work also analyzes about theneutral point potential of two-sprit capacitors and inverteroutput voltage. Simulation of this project can be carried out byusing MATLAB/Simulink.

Index Terms— five-leg inverter, FLI, PWM, sprit-capacitors.

1.Introduction

At present, most of AC motors are driven with a three-leginverter. But two or more alternating current (AC) motorscannot be independently driven with a three-leg inverter.Recently, a single inverter for driving two AC motorsindependently has been studied for aiming a low-cost,saving space and reduction of inverter losses. For example,four-leg inverter and five-leg inverter so on. It

1Mr.B.Ravi PG-Student, Department of Electrical and Electronics Engg.RGMCET, Nandyal, India,E-Mail: [email protected], Assist. Professor, Department of Electrical andElectronics Engg. RGMCET, Nandyal, India,E-Mail: [email protected]

can be said that a five-leg inverter is the similar inverter. Inthis inverter, the w phase of both motors is connected to oneleg in common. Therefore, the five-leg inverter has aproblem that the switching losses in this leg are increasedcompared with the other legs. The four-leg inverter cansolve this problem by using a capacitor instead of aswitching device. The four-leg inverter is a single inverterthat can drive two motors independently. We show thestructure of the four-leg inverter to fig.1. The four-leginverter consists of four legs and two sprit capacitors. The uand v phases of a motor1 are connected in a leg1 and a leg2respectively, those of a motor2 is connected in a leg3 and aleg4 and w phase of both motors are connected in the neutralpoint of the two sprit capacitors.Moreover, this paper also analyzes about potential in theneutral point of two-sprit capacitors and inverter outputvoltage. This paper presents the simulation results of theindependent driving characteristics of two induction motors(IMs) fed by the four leg inverter and five leg invertersimulation results comparing, the PWM technique, and thevalidity of those analytic result2. Main Circuit Of FLI

Fig. 1. Main circuit of an FLI.

Fig. 1 shows the structure of the FLI to supply two three-phase ac motors [1]–[2]. The inverter consists of four legsand two capacitors connected in a series. Inverter U1 and V1phases are connected to U and V phases of IM1,respectively. Inverter U2 and V2 phases are connected to Uand V phases of IM2, respectively, whereas the W phase ofboth motors is shared and connected to the neutral point oftwo-sprit capacitors in common. vUNi, vVNi, and vWNi arethe phase voltages in the IM i (i = 1, 2). vxO (x = U1, V1,U2, V2, W) is the inverter x phase



TABLE 1

DESCRIPTION OF THE SYMBOLS

voltage. vWO indicates the neutral point potential of two-sprit capacitors. iUi, iVi, and iWi are the phase currents in theIM i, and iW is the inverter phase current. E expresses themagnitude of the dc-bus voltage. C is the capacitance oftwo-sprit capacitors. Table I presents the description of thesymbols. In this paper, a based point is chosen to thenegative side of dc-bus for the simplicity of analysis.

3. Characteristics Of FLI

A. Neutral point potential of two-sprit capacitors

Fig 2. Equivalent circuit of the four-switch inverter.

VWO is given by the following equation:

Vwo=E− ∫(iw1+iw2)dt E+ΔVwo (1)

Where Δvwo is the fluctuating component of vwo.From (1) vwo changes around E/2. The fluctuatedcomponent depends on the fundamental wave frequency andpeak value of both motor currents. In other words, it will beable to decrease when the motors are driven at lighter loadand higher speed condition and be also decrease by thecapacitor with larger capacitance. Finally, it is necessary to

select capacitance of capacity in the range that the motormay drive.

B. Inverter output voltageWe analyze the output voltage of the FLI to use a switchingfunction. Equation (2) defines a switching function. Thevoltage source and connects inductive loads (for example,RL loads, ac motors, and so on) fed to the FLI . Therefore,they should not make the open circuit path load currents tothe leg. In other words, the switches of one leg must not besimultaneously closed or opened. The switching constraintsdiscussed earlier can be expressed by

Switching function

Sji=1, switch is closedSji=0, switch is opened (j=1,2,3,4; i=1,2 (2)

Switching restriction

Sj1+Sj2=1 (3)

Employing the switching function and (1), the inverter phasevoltage can be expressed by the following equation:

VUiO = S2i-11EVViO=S2i1E (4)Vwo = E/2 + ΔVwo

The output line voltage level of the FLI differs from thethree-VSI because the modulation in W phase cannot beimpossible in the FLI. Therefore, the output line voltage canbe defined as follows from(4):

VUVi = VUiO−VViO = (S2i-11−S2i1) EVVWi = VViO−VWiO = (S2i1−1/2) E − ΔVwo (5)VWUi = VWiO−VUiO = (1/2−S2i-11) E + ΔVwo

Where VUVi, VVWi and VWUi are the U−V,V−W and W−U linevoltages in the motor i, respectively.

TABLE IIOUTPUT VOLTAGE LEVEL

Substituting (2) into (4), the output voltage level in bothmotors is obtained as table II. The VUVi is three levels.Otherwise, the VVWi, VWUi are two levels. It must be noted that– ΔVwo and + ΔVwo will be added to VVW and VWUrespectively.

Phase voltage in the IMi(i=1,2)

VUNi, VVNi ,VWNI

Inverter x phase voltage VXO(x=U1,V1,U2,V2,W)

The neutral pointpotentional of two-sprit

capacitors

Vwo

Phase current in the IM i iUi, iVi, iWiInverter phase current iw

Magnitude of the DC-busvoltage

E

Capacitance of two-spritcapacitors

C

U−V linevoltage

VUVi −E,0,E3 level

V−W linevoltage

VVWi −E/2− ΔVwo, E/2−ΔVwo2 level

(superposition of ΔVwo)

W−U linevoltage

VWUi −E/2+ ΔVwo, E/2+ ΔVwo2level

(superposition of ΔVwo)



4. PWM Technique Of FLI

A. ETAMSince inverter W phase is constructed in the two-spritcapacitors, the modulation in the phase is impossible.Therefore, the PWM technique in three-phase VSI is notdirectly applicable for the FLI. To obtain a balanced three-phase ac voltage, only the U and V phases must bemodulated in the FLI. Then, we apply an expanded two-armmodulation (ETAM) known as a modulation method of afive-leg inverter [4]. In the ETAM, inverter U (V) phasevoltage command in the IM i can be expressed as follows:

V*Ui = V*

UNi − V*WNi

V*Vi = V*

VNi − V*WNi (6)

Where V*ki is inverter k phase voltage command in the IM. i

“*” is the command value. V*kNi can be defined as follows:

V*UNi = M*

i E sin(ω*i t – φ*

i)

V*VNi = M*

i E sin(ω*i t − – φ*

i) (7)

V*VNi = M*


i)

Where M*i and ω*

i are the modulation index andfundamental angular frequency in the IM i, respectively. φ*

iis the initial phase angular to phase voltage in the IM i.

Substituting (6) into (7), we obtain

V*Ui = M*


i)

V*Vi = M*


i). (8)

The connection method in the FLI is equivalent to theV−connection of a transformer for one motor. In theV−connection of a transformer, if the phase differences ofeach phase voltage command are π/3 each other, we get abalanced three-phase voltage. As can be seen from (8), thephase difference between V*

Ui and V*Vi is π/3. Therefore,

employing the ETAM, it is possible to obtain a balancedthree-phase voltage in the FLI.

B. Neutral point potential of two-spritCapacitor compensation

When we analyze the FLI, we had better considerone motor to understand it easily. For one motor, it ispossible to think that the FLI connecting two motors isequivalent to the four-switch inverter as shown in Fig.2.From this reason, we use a model of the four-switch inverterto analyze the characteristic of the FLI [3],[4].Because only output voltage of the motors are remarkablevalues to be analyzed , each phase of the motor canapproximate to a load having impedance Z. Ni is the neutralpoint of the load. Applying Kirchhoff’s voltage law to Fig.2,it follows that

V*Ui = ZiUi − ZiWi + ΔVWO

V*Vi = ZiVi − ZiWi + ΔVWO (9)

iUi + iVi + iWi = 0.

From (9), iUi, iVi and iWi follow that

iUi =

iVi = (10)

iWi =

substituting (1) and (8) into(10), it follows that

iui= sin(

iui= sin( (11)

iui= sin(

Fig 3.Block diagram of carrier-based PWM

Employing the ETAM, the phase current in the IMibecomes unbalanced three-phase current. To obtainunbalanced three-phase current, it is necessary tocompensate.

As a compensation, is added to v* ki. V* ki follows that

(12)

Substituting (1) and (12) into (10), it follows that

(13)

Adding v*ki to Δvwo, the phase current in the IMi becomes a

balanced three-phase current [5].



C. Neutral Point Potential of Two-Sprit CapacitorCompensation

Fig. 3 shows the block diagram of PWM technique in FLI.The PWM strategy may apply carrier-based PWM. It is

noted that the amplitude of carrier signal is often chosen asone in the carrier-based PWM. In the FLI, defining the

reference signal of U (V) phase voltage in the IM icompared with the carrier signal as e∗ U(V)si can be

expressed as follows:

(14)

To obtain a balanced three-phase current, ΔvWO must beadded to reference signals in each phase (“unbalancedcompensation” in Fig. 3). vWO is detected with a voltagesensor. ΔvWO is calculated from (1). “ΔvWO driftcompensation” in Fig. 3 shows the control block diagram torestrain the drift of ΔvWO in steady state. Zero command isgiven because the drift must be restrained to zero.

The error between the command and ΔvWO isinputted to proportional–integral (PI) controller. Δvdrift_Comp,which is the output value of PI controller, is the value tocompensate the drift. The drift is compensated with theaddition of Δvdrift_Comp to reference signal as shown in (14). Itmust be noted that the drift will be able to compensate withonly the PI controller because it has a dc component insteady state. Comparing the reference signal in (14) with thecarrier signal, inverter U and V phases are modulated. If thedrift is not compensated, over modulation may be caused inconsideration that ΔvWO is added to each reference signal. Asa result, the VUF will be reduced, and it will be necessary torestrain the drift.

5. Dc-Bus VUF

To evaluate the inverter capacity, it is important tocalculate the VUF. The VUF is defined as the ratio of themaximum output voltage to the inverter and the dc-busvoltage. In the carrier-based sinusoidal PWM, this way,defining the VUF with the maximum modulation index hasthe advantage that can investigate the VUF more easily. Toconnect two motors in the FLI, it should be noted that theVUF must be defined for each motor. From the definition ofthe VUF, the VUF of the motor, which is VUFi, can beexpressed by

Fig 4. Independent V/F control system in the FLI.

Fig 5. Block diagram of V/F control.Where Mimax expresses the maximum modulation index.When the amplitude of the carrier signal is chosen as one,the constraint must be satisfied as

| e*usi | ≤ 1

| e*vsi | ≤ 1. (16)

Fig.6.Equivalent circuit of IM without a load

Substituting (7) and (14) into (16), we obtain

Mimax = - |ΔvWOmax| (17)

where |ΔVwomax| is the magnitude of maximum ΔVwo.Substituting (17) into (15), the VUF of FLI is understood tobecome 50% or less.

6. Independent Constant Volts Per Hertz Control

TABLE IIIRatings and parameters of tested IM



Fig. 4 shows the independent constant volts per hertz (V/f)control system of the FLI. Another V/f controller for eachIM is employed to realize two IM independent control. Fig.5 shows the block diagram of V/f control. The V/f controlsystem is the system to control the frequency fi in the IM i.Fig. 6 shows the equivalent circuit of IM without a load,where V is the phase voltage in IM. Io is the excitationcurrent, Rs is the stator resistance, ls is the stator inductance,E1 is the internal induced voltage, and M is the mutualinductance. From Fig. 5, it follows that

= ω ) 0+ 1 (18)where ω is the fundamental angular frequency in the IM. “•”represents phasor. When the size of both sides is squared, itfollows that

V 2 = (RsIo)2 + (ωlsIo + E1)2 (19)where E1 = kfn. fn is the rated frequency. Solving for k, weobtain

where Vn is the rated voltage. Solving for modulationindexM*

i in the IM i, we obtain

(21)

where f*i is the frequency command in the IM .

7. Simulation ResultsIn order to demonstrate the independent drivingcharacteristics of two IMs, an FLI to supply two three-phasesquirrel-cage IMs has been implemented. Table III showsthe ratings and parameters of tested IMs. Both IMs aredriven by V/f control. The ratings and parameters of bothIMs are identical. The dc bus voltage is 282 V. C is 9900 μF,and the carrier frequency is 5 kHz.The frequency commandsin the IM1 and IM2 are 20 Hz in the direction of orderrotation and 16 Hz in the direction of reverse rotation,respectively. Fig.9 shows the neutral point potentialwaveform of two-sprit capacitors with no compensation.The drift phenomenon of vWO can be observed at startingtime and load change. The potential in steady state is 149 V.The drift magnitude (ΔvWO−drift) is 149 − E/2 = 8 (V). Figs.23,27 shows the neutral point potential waveform of two

sprit capacitors. The drift phenomenon of vWO cannot bealmost observed at starting time and load change comparedwith no compensation. Moreover, the potential in steadystate can be maintained at 141 V = E/2. Figs. 26 and 27show the reference signal waveforms of the IM1 and IM2 atC = 9900 μF and C = 3300 μF with compensation. Thecentral point of the reference signals of and V phases can bemaintained at zero.

Fig 7. FLI Of Three-phase current waveforms of the IM1

Fig 8. FLI Of Three-phase current waveforms of the IM2

Fig 9. FLI Of W phase current waveforms

Rated output 0.75The number of poles 4

Rated output voltage 200 vrms

Rated current 3.1 ARated frequency 50 Hz

Rated speed 1410 r/minStator resistance 3.8 Ω

Rotator resistance 4.0 ΩStator inductance 254.6 mH

Rotator inductance 251.0 mHInertia ( IM + Load) 12.05 mkgm2



Fig 10. FLI Of Speed of IM1 (f*1 = 20 Hz in the directionof order rotation)

Fig 11. Five-Leg Inverter Of Three-phase currentwaveforms of the IM1

Fig 12. Five-Leg Inverter Of Three-phase currentwaveforms of the IM2

Fig 13. Five-Leg Inverter Of W phase current waveforms

Fig 14. Five-Leg Inverter Of Speed of IM1 (f*1 = 20 Hz inthe direction of order rotation)

Fig 15. FLI Of Speed of IM2 (f*2 = 16 Hz in the directionof reverse rotation)

Fig 16. FLI Of U–V phase line voltage of the IM1

Fig 17. FLI Of V–W phase line voltage of the IM1



Fig 18. FLI Of W–U phase line voltage of the IMl

Fig 19. Five-Leg Inverter Of Speed of IM2 (f*2 = 16 Hz inthe direction of reverse rotation)

Fig 20. Five-Leg Inverter Of U–V phase line voltage of theIM1

Fig 21. Five-Leg Inverter Of V–W phase line voltage of theIM1

Fig 22. Five-Leg Inverter Of W–U phase line voltage of theIMl

Fig 23. FLI of Neutral point potential of two capacitors

Fig 24. FLI of Reference signal Wwaveforms of inductionmotor 1 at c = 9900 µF





Fig 27. Five-Leg Inverter of Neutral point potential of twocapacitors

Fig 28. Five-Leg Inverter of Reference signal Wwaveformsof induction motor 1 at c = 9900 µF




Fig 32. Five-Leg Inverter of Reference signal Wwaveforms ofinduction motor 2 at c = 3300 µF

8. ConclosionThis paper has also analyzed about the neutral pointpotential of two-sprit capacitors and inverter output voltage.Next, a modulation technique in the FLI has been alsoshown. The simulation results demonstrated thecharacteristics of two IM independent drives and the validityof those analytic results. The simulation results of theindependent driving characteristics of two IMs fed by theFLI and the validity of the PWM technique and thoseanalytic results have been also demonstrated.



References

[1] C. B. Jacobina, E. R. C. da Silva, A. M. N. Lima, andR. L. A. Riberio, “Vector and scalar control of a fourswitch three phase inverter,” in Proc. IEEE IAS Annu.Meeting, 1995, pp. 2422–2429.

[2] F. Blaabjerg, D. O. Neacsu, and J. K. Pedersen,“Adaptive SVM tocompensate DC-link voltage ripplefor four-switch three-phase voltagesource inverters,”IEEE Trans. Power Electon, vol. 14, no. 4, pp. 743–752, Jul. 1999.

[3] D. T. W. Liang and J. Li, “FLux vector modulationstrategy for a component-minimized voltage sourceinverter,” IEEE Trans. Power Electron., vol. 14, no. 4,pp. 1331–1337, Jul. 1999.

[4] M. N. Uddin and M. A. Rahman, “Performanceanalysis of a four switch 3-phase inverter fed IMdrives,” in Proc. IEEE LESCOPE Conf., 2004, pp. 36–40.

[5] M. N. Uddin, T. S. Radwan, and M. A. Rahman,“Fuzzy-logic-controllerbased cost-effective four-switchthree-phase inverter-fed IPM synchronous motor drivesystem,” IEEE Trans. Ind. Appl., vol. 42, no. 1, pp.438–444, Jan./Feb. 2006.

[6] C. T. Lin, C. W. Hung, and C. W. Liu, “Positionsensorless control for four-switch three-phase brushlessDC motor drives,” IEEE Trans. Power Electron., vol.23, no. 1, pp. 438–444, Jan. 2008.

[7] J. S. Jang, B. G. Park, T. S. Kim, D. M. Lee, and D. S.Hyun, “Sensorless control of four-switch three-phasePMSM drive using extended Kalman filter,” in Proc.IEEE IECON, pp. 1368–1372.

[8] M.Monfared, H. Rastegar, and H.M. Kojabadi,“Overview of modulation techniques for the four-switch converter topology,” in Proc. IEEE PECon, pp.803–807.

[9] M. B. de Rossiter Correa, C. B. Jacobina, E. R. C. daSilva, andA. M. N. Lima, “A general PWM strategy forfour-switch three-phase inverters,” IEEE Trans. PowerElectron., vol. 21, no. 6, pp. 1618–1627, Nov. 2006.

[10] J. Kim, J. Hong, and K. Nam, “A current distortioncompensation scheme for four-switch inverters,” IEEETrans. Power Electron., vol. 24, no. 4, pp. 1032–1040,Apr. 2009.

[11] T. D. Nguyen, H. M. Nguyen, and H. H. Lee, “Anadaptive carrier-based PWM method for four-switchthree-phase inverter,” in Proc. IEEE Int. Symp. Ind.Electron., Jul. 5–8, 2009, pp. 1552–1557.

[12] K. Oka and K. Matsuse, “A performance analysis of afour-leg inverter intwo AC motor drives withindependent vector control,” IEEJ Trans. Elect.Electron. Eng., vol. 1, no. 1, pp. 104–107, May 2006.

[13] A. Furuya, K. Oka, and K. Matsuse, “A characteristicanalysis of four-leg inverter in two AC motor driveswith independent vector control,” in Proc. IEEEISEMS, pp. 619–624.

[14] N. Kezuka, K. Oka, and K. Matsuse, “Characteristicsof independent two induction motor drives fed by afour-leg inverter,” in Proc. IEEE-ECCE Annu.Meeting, Sep. 12–16, 2010, pp. 2114–2120.

[15] Y. Kimura, M. Hizume, K. Oka, and K. Matsuse,“Independent vector control of two induction motors

with five-leg inverter by the expanded two arm PWMmethod,” in Proc. IEEJ Int. Power Electron. Conf.,2005, pp. 613–616.

[16] N. Hoshi and M. Shibata, “Considerations on capacitorvoltage compensation schemes of a novel inverter fortwo PMSMs drive,” in Proc. Annu. Conf. Inst. Elect.Eng. Jpn., Ind. Appl. Soc., 2008, p. 1–78, I-357–I-358.

[17] M. Yamato and Y. Sato, “An investigation of a controlmethod for fault-mode inverters to drive inductionmotors,” IEEJ Trans. Ind. Appl., vol. 123, no. 12, pp.1430–1437, 2004.

B.Ravi was born in anantapur,in india.He received the B.Tech (Electricl andElectronics Engineering) degree fromJawaharlal Nehru TechnologicalUniversity, Anantapur in 2010 andpursuing M.Tech (Power Electronics)

from RGM College of Engineering and Technology(Autonomous), Nandyal, Jawaharlal Nehru TechnologicalUniversity Anantapur. His area of interesting powerelectronic industrial drives.(Email:[email protected])

Mr.E.Narasimhulu was born in Kurnool,in india. He received the B.Tech(Electricland Electronics Engineering)srivenkateswara university, Thirupathi in2009 and received the M.Tech in NITKSurathkal,manglore in 2011. His area of

interesting power electronic industrial drives.(Email:[email protected])

mr.b.ravi and mr.e.narasimhulu comparison of performance ...iret.co.in/docs/itped/volume 1/issue1/2....

Documents