POWER FACTOR IMPROVEMENT OF SINGLE_PHASE AC VOLTAGECONTROLLER EMPLOYING EXTINCTION ANGLE

CONTROL TECHNIQUE

Nabil A. Ahmed' and Emad H. El-Zohri2

Abstract - The general study of power converters is toimprove the overall efficiency of the power system by someadvanced methods of control techniques. Forcedcommutation of the power semiconductor switches leads toimproved power factor in dc converters. Similar techniquesmay be applied to improve the performance of accontrollers. For example, an ac controller can be used toadjust the stator voltage of an induction motor runningunder variable load in order to maintain better efficiency.With the present semiconductor technology (MOSFET's,IGBT's, GTO's and improved bipolar transistors), manysolutions exist to alter the powerfactor ofdynamic ac loadsthrough an ac controller. In this paper, the performanceevaluation of the extinction-angle control technique hasbeen illustrated as applied to a single-phase voltageconverter by examples of static load and the widely usedsingle-phase induction motor to verify the feasibility of theproposed technique. Observations on power factor,displacementfactor and motor efficiency make up the resultsofthis work.Index Terms - AC voltage controller, extinction angle controlpowerfactor, static load, single-phase Induction motor.

I. INTRODUCTION

AC voltage converters are widely used as one of the powerelectronics systems to control an output ac voltage; where avariable ac voltage is obtained from a fixed ac voltage, forpower ranges from few watts (as in light dimmers) up tofraction of megawatts (as in starting systems of largeinduction motors). Phase-angle control (PAC) linecommutated voltage controllers and integral-cycle control ofthyristors have been extensively employed in this type ofregulators for many applications. Such techniques offersome advantages as simplicity and the ability of controllinglarge amount of power economically. However, they sufferfrom inherent disadvantages such as; retardation of the firingangle causes lagging power factor at the input side especiallyat large firing angles and high low order harmonic content inboth of load and supply sides [1-3]. Moreover, adiscontinuity of power flow appears at both input and outputsides [4-8].

On the other hand, the efficiency of a squirrel cageinduction motor often reaches its maximum value at ratedload. At light loads the machine becomes quite inefficientand since the phase of the stator current is lagging, the

power factor becomes poor. Such a behavior can beexplained by the fact that a constant supply voltage isusually applied to the machine and under decreasing loadconditions; the internal impedance of machine becomesmainly reactive [9].

Trying to solve this problem with simple ac voltageconverters is an almost impossible task. On the other hand,adding a complex controller costing several times the priceof the motor, is in itself an objectionable solution. Thus, thepresent paper describes a simple scheme to generate avariable ac voltage directly from the line with fewcommutations at each period for speed control of single-phase induction motors. This is achieved by using theextinction-angle control (EAC) control technique to controlthe extinction angle by making use of forced commutationnature of the switches while introducing a freewheeling pathin parallel with the load terminals. The proposed ac voltageconverter employs only two controlled switches with the aidof two diode bridge rectifiers. Reducing the number ofcontrolled switches is essential for the point of view of thesimplicity, cost, reliability, decreasing the switching losses;a matter which enhances the converter efficiency.

The extinction-angle control (EAC) is similar to that ofphase-angle control wlhere there is only one pulse pcr lialf-cycle. In the phase-angle control the conduction is started atthe required delay angle (a ) and continues until the currentreaches zero value naturally, while in the EAC control theconduction is started at zero crossing of the supply voltageand forced commutated at a certain angle before the nextzero crossing (/). Fig. I shows the voltage and currentwaveforms in the EAC. Of course a freewheeling path isprovided for the load current to discharge the stored energyof the load inductance. The output voltage is controlled byvarying the extinction angle /. The fundamental componentof input current leads the input voltage, and the displacementfactor (and power factor) is leading. In some applications,this feature may be desirable to simulate a capacitive loadand to compensate for line voltage drops. Therefore, theperformance of ac voltage converters with extinction-anglecontrol is similar to that with phase-angle control, except thepower factor is leading while in phase-angle control, thepower factor is lagging.

This study is concerned with the investigation of acvoltage controllers with EAC technique applied to a staticload and the widely used squirrel cage single-phase

' Nabil A. Ahmed, Assuit University, Faculty of Engineering, Electrical Engineering Depattment, Assuit 71516, Egypt, Nabil eacc.aun.edu.eg2 Emad H. EI-Zohri, Egyptian Electricity Holding Company, Assiut 500 kV Substation, emadelzohri@yahoo.com

0-7803-8294-3/04/$20.00 ©2004 IEEE1075

induction motor. The operation of this controller as avariable voltage source employing EAC is evaluated andcompared with conventional PAC from supply power factorimprovement point of view. The simulation results areverified experimentally.

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Fig. I Input voltage, output voltage, Input current and outputcurrent waveforms with extinction-angle control.

11. CIRCUIT DESCRIPTION AND PRINCIPLE OFOPERATION

Fig. 2 shows the schematic representation of the powercircuit configuration which consists of a single-phaseinduction motor connected to an ac source through an acvoltage controller. The forward switch S I is usedperiodically to connect and disconnect the load to the supplyin the EAC technique, i.e. regulates the power delivered tothe load. The parallel switch S2 provides a freewheelingpath for the load current to discharge the stored energy of theload inductance when the forward switch SI is turned off.Switch S I is turned on at at = 0 and is turned off by forcedcommutation at (t0 = - p as shown in Fig. I to supplypower to the load during the active mode. For an inductiveload, a freewheeling path for the load current must beprovided for the period from 1= ir_--fl toot=,r by thefreewheeling switch S2 to discharge the load inductance

stored energy during the freewheeling mode. Thefundamental component of the input current leads the inputvoltage, and the displacement factor (and hence the powerfactor) is leading.

The switching pulses shown in Fig. I are synchronizedwith the supply voltage and the output voltage is controlledby varying the extinction ariile p from 0 to nc to vary therms value of the output voltage from rated input voltage tozero. The gating pulses of thw freewheeling switch S2 are thecomplemenit of the gating pulsp of the forward SI.

Fig. 2 Configuration of ac voltage converter fedsingle-phase induction motor.

Due to non ideality of the switching devices, a dead time isrequiisite to avoid comimiliutationi problcim. A by-passcapacitor Cb is added parallel to the load in order to providea path for the current during the dead tiime whien botliswitches are opened. The operation is divided ilto tlhreemodes: active, dead time and freewheeling modes. Thecurrent flows through the input and output sides, providingenergy to the load during the active mode, freewheelsthrough the freewheeling path during the freewheeling modeand bypasses during the dead time mode. Detailed analysisof the operation modes and circuit description can be foundin [101

111. PERFORMANCE OF THE TEST CONVERTER

This section presents the performance evaluation ofthe proposed ac voltage converter with the EACtechnique by simulation using MATLAB Simulinkpackage and experimentally by using a prototypemodel where the used power electronic switches areMG5OJ2YS1 (600V, 50A) IGBT'S and PSB 35/14(1400V, 35A) diode bridge rectifier. An experiment on1.1 KVA (220V, 5A) laboratory model was performedin order to verify the feasibility of the circuit and toinvestigate the validity of the simulated results.

The ac voltage controller with the EAC technique isapplied to a static load and a dynamiiic load represented by a

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squirrel cage single-phase induction motor. The obtainedsimulated and experimental results will be discussed in thefollowing:(A) Static R-L Load

Table I shows the simulated and experimental circuitparameters for the ac voltage converter with aninductive load.

TABLE ISIMULATED AND TEST CIRCUIT PARAMETERS

Parameter Symbol ValueMaximum supply voltage Vsm 220-li [V]Supply frequency f 50 [Hz]Switching frequency As 100 (Hz]

By-pass capacitor Cb 1 pFBleeding resistance Rb 2.1 kfiLoad resistance Ro 18 [Q]Load inductance Lo 99 [mH]

Fig. 3 shows the variation of the simulated andexperimental rms values of the output voltage V0 with theextinction angle J over the complete range of control.

0

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as

vr

X)s

- e - - --1- -- - z - - - e -- -- - - w1- t-

xI

t_

1 ..40 60 80 100 120 140 160 1

Extnction angle (p)Fig. 3 Variation of the rms output voltage with the extinction angle.

Fig. 4 shows the variation of the supply power factorversus the extinction angle over the complete range ofcontrol for three different load conditions, resistive (O = 00)and two inductive loads (D = 450, 60° ). The measurement ofthe input power factor is done by two ways, by using thepower analyzer and for verification by using a digitalwattmeter to measure the values of the input power and nmsvalues of the voltage and current at the supply side. Dividingthe measured input power by the product of the rms valuesof the supply voltage and current results the average inputpower factor and the two ways lead to the same results. A

good agreement between the simulated and measured resultsis evident.

It is worth to mention that the input power factor isimproved gradually from its lagging value and becomesleading after the extinction angle exceeds the load angle andas the extinction angle increases, the input power factor getsmore improved and this feature gives more importance forEAC technique. The positive sign in Fig. 4 refers to laggingpower factor while the negative sign refers to leading powerfactor.

Fig. 5 shows the simulated and measured variation ofthe supply power factor with the rms output voltage for thesame loading conditions as in Fig. 4. As the rms outputvoltage decreases, i.e. the extinction angle increases), thesupply power factor is improved gradually from its laggingvalue and becomes leading after the extinction angle exceedsthe load angle and goes more leading as the extinction angleincreases. This feature in EAC technique is mainly due tothe fact that the fundamental component of the supplycurrent leads the supply voltage because the displacementangle goes from lag to lead as the extinction angle increases.

0.50

S.

00.

00o6

0

-0.5

.1Extinction angle (A)

Fig. 4 Variation of the Input Power factor with the extinction angle.

0.50

S.

0

4-.5

-1

Output volage [V]Fig. 5 Variation of the Input power factor with the otitput voltage.

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Fig. 6 demonstrates the simulated and mecasuredvariation of the total harmonic distortion factor TIlDF of theinput current versus the rms output voltage for different loadconditions. In this Figure as the output voltage decreases; theextinction angle increases and hence the THDF in the supplycurrent increases. Also, as the load power factor decreases,the THDF increases. However, it is a compromise betweenthe increase in THDF and the improvement in displacem-entfactor. It can be concluded that for the same load condition,although the increase in the THDF with increase in theextinction angle, the input power factor goes from lag to leadas well as displacement factor. Althoughi the increase inTHDF will reduce the value of the supply power factor,however, it will not affect its leading nature.

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varying the extinction anigle, a wide range of motor speedconitrol can be achieved.

200ISO.....

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420 . . . . . . . .:. . . ... . ... . . . . . . . . . .

200 . .. .. ..... . . ... ............

0 18 36 54 72 90 log 126 144 162 180Extinction angle ( 0 )

Fig. 7 varilation of the motor applied voltage.

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20 40 60 80 100 120 140 160 180 200 220Output voltage [VI

Fig. 6 Variation of TI-IDF of Input current with the outpuit voltalge.

(II) Induction Motor Control

The proposed extinction-angle control technique hasbeen applied, also, to a dynamic load represented by a1/6 Hp, 180 V, 1.8 A, 50 Hz split-phase squirrel cageinduction motor coupled to a fan as a mechanical loadto obtain variable speed operation. The test motorparamneters are:

Rm=7.25 C Xm =7.8lI 2M l01.81 C2 P= 2Rr= 7.08 (l Xr 3.9flR 15.6 Ql Xa IL.2Detailed analysis of the motor performance equations

can be found in literature [I11]. The obtained simulated andexperimental results will be discussed in the following:

Fig. 7 depicts the variation of the computed andmeasured rms value of the motor applied voltage with theextinction angle. A significant improvement in the inputpower factor with the extiniction angle control is showni inFigs. 8 and 9. Fig. 10 gives the variation of the computedand mneasured THDF of the input current withi the mnotorapplied voltage.

The computed and measured variation of the motorspeed versus the extinction angle is shown in Fig. Il1. By

0.

0.

to04.0

-0

. I I.

0

&Extinction angle ( fi)

Fi'g. 8 Variation of the input power factor with the extinction angle.

0.1

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06

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.8

RMS notor voltage IV]Fig. 9Variation of the Input power factor with the niotor voltage.

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1.6

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..... . ..... . ...... ......... 200*100

0 I 1 1 1 § 1 1 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.420 40 60 80 100 120 140 160 180 slpl

RMS motor voltage [VJFig. 13 Variation of the input power with slip.

Fig. 10 THDF of the input current with the output voltage. I1600___________________________________________ _ 0.8 _ $

140 .7........ . . . .. ...........G5-

Measured -Computed1200 ->@ 8 i > <,, Q~ ~ ~ ~~~~~~~~~~~~.2*Measured

goo ............ ............ .............I.............I........... ' ....-0.20.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

,600t-teX -@--------- ........ ... .0.4 *- ,,,-¢

-0.4 ,*4 .... t

200 -........................-...........- ........... .........................-.........--- .8SlipI p.u

V .1 . . f . .Fig. 14 Variation of the input power factor with slip.0 18 36 54 72 90 108 126

Extincton angle ( p ) 2.5 -

Fig. IiVariation of the motor speed loaded by a fan with 2 -Computedlthe extinction angle. Meaured

The steady state characteristics of the test motor Z 1.5with extinction angle control technique, such as motorinput current, input power, input power factordeveloped torque and motor efficiency, are shown in

0.5Figs. 12-16. It must be mentioned that the supplypower factor is improved gradually and becomes 0

leading after the extinction angle exceeds the load 0' 0 0-- - 0 0 0.~~~~~~~~~~~~~~~~0.0 0.1 0 15o .l o0.2 0.25 0.3 0.35 0.4angle as shown in Figs. 8 and 14. SlipIpuI

Fig. 15 Torque-slip characteristics.Copue 0.7

0.1-Computed 0 7 -

0 0.05 0.1 00.

0O

0 O.OS 0.1 0.1S 0.2 0.25 0.3 0.35 0.4Slip [ p.u I

Fig. 16 Variation of the motor eMciency with slip.

1079

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4Slip [P.UI

Fig. 12 Variation of the motor input current with slip.

V. CONCLUSIONS

The paper presents the EAC technique for single-phase ac-ac voltage converters. The EAC techniqueprovides a considerable improvement in the inputpower factor. This improvement is mainly due to theimprovement in the displacement factor. The acvoltage controller with the EAC technique has beenapplied to a static load and a dynamic load which is asquirrel cage single-phase induction motor loaded by afan. A full control range of the ac output power and awide control range of the motor speed can beachieved. Thus, this technique is suitable for speedcontrol of single-phase induction motor as it isrequired a simple and less costly technique. A goodagreement is obtained between the simulated and theexperimental results.

REFERENCES

[11 S. Willliams, "Reduction of the Voltage and Current HarmonicsIntroduced by a Single-Phase Triac ac Controller," IEEE Trans. Ind.Electron. Contr. Instrum. Vol. 28, no. 4, 1981.

[2] M. H. Rashid, Power Electronics: Circuits, Devices and applications,2nd edition, Upper Saddle River, New Jersey: Prentice-Hall, 1993.

[31 1. 1. Barbi and J. C. Fagundes, "A Compact AC/AC Voltage RegulatorBased on AC/AC High Frequency Flyback Converter," in Proc. IEEEPESC'91, 1991, pp. 846-852.

[4] K. S. Lock, Thyristor Control of Shaded-Pole Induction Motors",Electric Machines and Power Systems, pp. 185-193, 1987.

[5] C. V. Abroal and S. S. Sharama, "Integral-Cycle Control of Single-Phase Induction Motor Using SCR's", International Conference onI.lectrical Machines, ICEM, pp. 35-37, 1988.

[61 A. M. Hashem, "An Efficient Approach to Dynamic Modeling andTransient Analysis of SCR Controlled Induction Motors",International Conference on Electrical Machines, ICEM, pp. 365-370, 1988.

[7] E. El-Bidweihy, K. Al.-Bidweihy M. S. Metwally and M. El.-Bedwiehy, "Power Factor of AC Controllers for Inductive Loads,"IEEE Trans. Ind. Electron. Contr. Instrum., vol. IECI-27, no. 3, pp.210-212, June 1980.

[8] Longya Xu., "Dynamic Model of an Integral-Cycle Controlled Single-Phase Induction Machine", IEEE Transactions on EnergyConversion, vol. 7, no. 4, pp. 761-767, December 1992.

[91 G Roy, P. Poitevin and G. Olivier, "A Simple Topologies for Single-Phase AC Line Conditioning," IEEE Trans. Ind. Applicat., vol. IA-30, no. 2, pp. 406-412, March./April 1994.

[10] A-R A. M. Makky Nabil A. Ahmed Emad H. El-Zohri., "Ac VoltageController-Fed Siingle_phase Induction Motor with Power FactorImprovement Employing Modified Phase Angle Control Technique,"Proceeding of the Ninth International Middle-East Power SystemsConference, Mepcon'2003, Minoufiya University, Shebin El-Kom,Egypt, December 16-18,2003.

[II] S. S. Shokralla, "A New Modeling Approach for Speed Control ofSingle-Phase Capacitor Motor Using Proportional Controller,"ICECS'94, Dec. 19-22, 1994, Cairo, Egypt.

[12] A-R A. M. Makky, G. M. Abdel-Rahim, and N. A. Ahmed, "A NovelDC Chopper Drive for Single-Phase Induction Motors," IEEE Trans.Ind. Electron., vol. 42, no. 1, pp. 33-39, Feb. 1995.

[131 Nabil A. Ahmed, K. Amei and M. Sakui, "A New Configuration ofSingle-Phase Symmetrical PWM AC Chopper Voltage Controller,"IEEE Trans. Ind. Electron., vol. 46, no. 5, pp. 942-952, Oct. 1999.

[14] Nabil A. Ahmed, Kenji Amei and Masaaki Sakui, "AC ChopperVoltage Controller-Fed Single-Phase Induction Motor EmployingSymmetrical PWM Control Technique," Electrical Power SyslemsResearch Journal, Vol. 55, no. 1, pp. 15-25, July 2000.

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