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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Operation of Induction Motor at Optimum Efficiency Ratheesh C.K, Devarajan A.T, T.N.P.Nambiar, Sasi K.K, Isha T.B. Department of Electrical and Electronics Engg., Amrita Viswavidyapeetham, Ettimadai, Coimbatore-641 105 e-mail: isha_t_b@yahoo.co.in; ratheeshck@gmail.com, Abstract— In this paper an 8 switch PWM three phase AC chopper is used to control the stator voltage of a three phase induction motor. Three different control strategies viz., keeping the power factor constant, keeping the stator current minimum and keeping the input power minimum for a given load, are used to operate the induction motor at maximum efficiency. The results obtained from a closed loop simulation using MATLAB and experimental validation in open loop, are presented here. Also, a comparative study of the harmonics with a three phase AC regulator is done.

B. Operation with constant power factor This method is based upon the optimal-efficiency power factor tracking by adjusting the input voltage. In a conventional drive system; the induction motor is driven under constant rated voltage. There is only one torquespeed curve which meets a load torque curve at a given reference speed. Thus, the power factor is fixed. However, if the induction motor is driven under variable voltage, there are many torque-speed curves which meet a load torque curve at a given reference speed. Therefore, each power factor is different. Power factor corresponding to the maximum efficiency point from the equivalent circuit is chosen as the reference value.

Index Terms— AC chopper, Induction motor, efficiency, search method, current minimization.

I. INTRODUCTION

C. Minimization of input current A method based on the minimization of input current is discussed here. A steady-state change in load is sensed by change in current. The amplitude of the motor voltage is adjusted to obtain the new optimum voltage when the motor current is minimum. Whenever there is a load change, the new value of the optimum voltage is determined as a function of optimum current at the previous optimum point and steady-state change in current after load change. V.V. Sastry [6] presents a phase controlled AC voltage regulator controller using thyristors.

Induction motor is a highly efficient machine when working close to its rated operating point. However with torques below the rated values, the efficiency is greatly reduced. Efficiency of an induction motor is maximum when the copper loss is equal to the fixed loss which comprise mainly of core loss. Voltage reduction is to maintain a better balance between the load dependent losses (i.e. stator and rotor losses) and the load independent losses (core losses). Three phase AC voltage regulator or PWM AC chopper can be used for controlling stator voltage. In this paper, a three phase AC chopper is used whose output voltage is varied by varying the duty ratio.

D. Voltage control using PWM AC chopper A three-phase voltage controller (AC chopper) using IGBTs in the power circuit is presented in [5]. The circuit allows Pulse Width Modulation (PWM) control. Performance of both the SCR phase controlled regulator and PWM AC chopper are compared and it is seen that IGBT based PWM chopper gives better result as far as the energy savings is concerned.

II. EFFICIENCY IMPROVEMENT Various methods used for efficiency improvement of induction motor are discussed here. The three strategies use power factor, stator current and input power as independent control variables. With rated stator voltage, efficiency of an induction motor reduces as the load torque reduces. The three methods used are discussed next.

E. A control scheme to maximize efficiency A comparison between inverter and AC chopper for a constant speed drive is that, if the drive starts frequently or the required starting torque is high, the inverter is more attractive, providing large starting torque and low starting losses. When frequent starting is not needed, AC chopper is preferred.

A. Minimization of input power R J Spiegel [1] describes a fuzzy-logic-based energyoptimizing controller to improve the efficiency of induction motor drive operating at various load (torque) and speed conditions. Air gap flux of the motor is decremented in steps until the measured input power reaches the lowest possible value. The rotor air gap flux is decreased by reducing the magnetizing component of the stator current as a result of lowering the stator voltage in steps.

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010 TABLE I.

III EFFECT OF VARIATION OF SUPPLY VOLTAGE

: PARAMETERS OF THE MOTOR USED

ON TORQUE-SLIP CHARACTERISTICS 230V, 2.2kW, 50Hz, 1440 rpm, 3 phase, class A, delta connected induction motor stator resistance R1

9.36Ω

rotor resistance, R2

2.04Ω

Stator and rotor leakage reactance, X1&X2

9.625Ω each

Magnetizing reactance, Xm

220.265Ω

Fig.1: Effect of variation of input voltage on torque-slip characteristics

The effect of variation of input voltage on the torqueslip characteristics is shown in Fig.1. It is observed that any reduction in the voltage will lead to increase in slip, to meet the load requirements. If the stator voltage reduces, a corresponding reduction in stator current and stator core loss component current will occur, causing a reduction in stator copper loss and core loss. But to maintain the load torque, slip will increase. Increase in slip leads to an increase in the rotor copper loss and poor efficiency. It is seen from the MATLAB simulation results in Fig. 2 that the current drawn by induction motor is of sinusoidal nature (for 20% full load torque). As seen in Fig. 3, lower order harmonics are absent with AC chopper control which offering a better torque response.

(a)

Fig.2: Stator current when driven from PWM AC chopper (b) Fig.3: Harmonic current spectrum (a) with PWM chopper; (b) with Phase control

IV. COMPARISON OF DIFFERENT CONTROL METHODS USING IDEAL VOLTAGE SOURCE

Three control methods viz., minimizing the input power, minimizing the input current and keeping the power factor constant are used for maximizing the efficiency. Performance of the induction motor with these strategies is studied. Simulation block diagram used for this study is shown in Fig.4. Parameters of the motor used were determined experimentally and is given in Table 1. In Fig.4, three phase V-I measurement block is used to measure the input voltage and current. Above experiment was repeated for different load torques. Fig.4: Simulation Block diagram for comparing different schemes with ideal voltage source

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010 TABLE II.

The maximum efficiency obtained for each torque condition is plotted and is shown in Fig.5.

COMPARISON OF EFFICIENCY

Fig.5: Efficiency at rated input voltage and at optimum voltage for different loads

CONCLUSIONS

stator current Vs applied voltage

A detailed study on the efficiency optimization of induction motor was done both by simulation and experimentation. Three methods viz., operation at constant power factor, minimum stator current and minimum input power, were tried for a given load. Of all the other methods of control used, the method of keeping minimum stator current is seen to give better efficiency improvement, at the same time with reduced complexity for implementation. Current minimization technique was implemented using dsPIC30F3011 microcontroller and verified on a laboratory model.

3.635 3.63

stator current(A)

3.625 3.62 3.615 3.61 3.605 3.6 3.595 3.59 3.585 3.58 340

350

360

370

380

390

400

410

420

stator voltage(V)

Fig.6: current Vs input voltage for 40% of full load torque

Fig.6 shows the plot of input current as a function of stator voltage for 40% full load torque.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support received from the funding agency of Govt.of India, New-Delhi. REFERENCES [1] R J Spiegel, M W Turner, V E McCormick “Fuzzy-logicbased controllers for efficiency optimization of inverter – fed induction motor drives”, Fuzzy Sets and Systems 137 (2003) 387-401. [2] Bimal K.Bose, Nitin R.Patel and Kaushik Rajashekhara “ A Neuro-Fuzzy-Based On-Line Eficiency Optimaization Control of a Stator Flux-Oriented Direct Vector-Controlled Induction Motor Drive ”, IEEE Transactions on Industrial Electronics, vol 44 no 2 April 1997. [3] D S Kirschen, D W Novotny and T A Lipo “Optimal Efficiency Control of an Induction Motor Drive”, Wiscons in Electric Machines and Power Electronics Consortium. Research report. [4] Gilberto C. D. Sousa, Bimal K. Bose, and John G. Cleland “Fuzzy Logic Based On-Line Efficiency Optimization Control of an Indirect Vector-Controlled Induction Motor Drive”, IEEE Transactions On Industrial Electronics, Vol. 42, No. 2, April 1995 [5] Matyas Hunyar And Karoly Veszpremi “Pulse Width Modulated IGBT AC Chopper”, Periodica Polytechnica Ser. El. Eng. Vol. 45, No. 3–4, Pp. 159–178 (2001) [6] Venkata V. Sastry, M. Rajendra Prasad, and T. V. Sivakumar “Optimal Soft Starting of Voltage-ControllerFed IM Drive Based on Voltage Across Thyristor ”, IEEE Transactions On Power Electronics, Vol. 12, No. 6, November 1997.

Fig. 7: Flowchart of the control used