Simple Reluctance Motor Drive System

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SIMPLE RELUCTANCE MOTOR DRIVE SYSTEM

08 !i&!!JJ *BRIOHIDON. 13.169 I593

M Morimoto and K Aiba Mitsubishi Heavy Industries,Ltd. JAPAN Abstract

A simple control scheme of the reluctance motor is presented. The use of only real time current waveforms and a position sensor will be enables to realize a simple reluctance motor drive system. The controller of the dc brushless motor driver keeps the mmf phase angle constant at any load torque. The controlling of the motor torque can be done only by controlling the voltage. Experimental results show that the proposed scheme is much more practical than that of the conventional one. The proposed control scheme is also applicable to the control of switched reluctance motor.

Keywords. Blushless motor, mmf phase angle, Switced reluctance motor, Synchronous machine, inverter.

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tem [3]. Generally, synchronous reluctance motors are driven on the basis of the internal phase angle control, owing to the fact that the reluctance torque is proportional to the sine of the internal phase angle 6. The torque is expressed as; T = k12 sin 26. (1)

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Therefore, the conventional control of reluctance motors . . has been done by varying the internal phase angle 6 as well as the amplitude of the current I. If the voltage is Internal phase angle 26 constant, the operating- point of the motor moves along . the sinusoid4 line according t o the operating torque, as (a) The conventional control shown in Fig.l(a). The control of both the current and internal phase angle 6 is complicated, because the d-q axis W a transformation, the 2-3 phase transformation, or the vecP tor control should be taken into consideration. However, the simple decoupling control of the internal phase angle 6 and the current I was not available. In this paper, the solution of the above problem, a simple drive system of the reluctance motor d l be presented; the decoupling control of the internal phase angle and the current amplitude can be realized by the use of a dc brushless motor controller. Moreover, the proposed scheme can be used to drive a switched reluctance motor. Internal phase angle 26 ’ The control of the dc brushless motor is based on the (b) The proposed control phase angle of the magneto motive force(mmf) control. The uee of the dc brushless controller realizes the mmf TllE TORQUE 017 TIIE RELUCTANCE MOTOR. phase angle constant control. The mmf phase angle con- Fig.1

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0 1993 The European Power Electronics Association

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mmf Phase Angle Constant Control

In the dc brushleas motor control, the stator current is supplied at fixed mmf phase angle in order to keep the stator mmf orthogonal to the rotor flux. Therefore, the use of the dc brushless controller makes it possible to keep the mmf phase angle of the reluctance motor at fixed value. The mmf phase angle constant control of the synchronous motor has been introduced by Ishizaki et a1 [5]. The mmf phase angle 9 of the reluctance motor is the space angle between the magneto-motive force of the armature reaction and the salient pole of the rotor, and is expressed as: r 9 =6 - Pl, (2) 2 where d is the mmf uhase anele. 6. the internal phase

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The relation of Equation (2) . . shows how the internal phase angle 6 can be nearly constant at any load torque when the mmf phase angle - is constant. Therefore, in the f" phase constant the Of the motor torque can be done Only by the current amplitude. AS a result, the operating Point Of which moves vertically on the figure, (see Fig.1 (b)), while the conventional one moves sinusoidally (see Fig.1 (a)).

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Fig.3

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The DC Brushless Controller

Fig, 3 shows the block diagram of the dc brushless motor controller. The controller consists of three control blocks, they are, a sinusoidal reference generation, a current control, and speed control, The d-axis of the rotor is sensed by the rotor position sensor. The detected position is used to generate the sinusoidal reference at the fixed mmf phase angle 9 according to the position of the rotor. This sinusoidal reference is multiplied by the speed command. The current command is generated by the speed command and the current feedback signal. Finally, PWM signal is generated by the current command a t the PWM controller. The PWM strategy of the controller is subharmonic method. The inverter used is topologically Voltage Source Inverter though it acts as the current source. The speed controller is a conventional P I controller. B

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THE BLOCK DIAGRAM OF THE DC BRUSHLESS CONTROLLER

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The driver supplies the current whose amplitude is varied by the speed command at the fixed mmf phase angle. The difference between the dc brushless motor control and the reluctance motor control lies in the value of the mmf phase angle; In the dc brushless motor drive, the mmf phase angle has small value, while the mmf phase angle of the reluctance motor has relatively large value. The difference comes from the fact that the mmf phase angle of the dc brushless drive is decided in order to be orthogonal to the rotor flux, while, the one of the reluctance motor is decided by the relation of Equation (2). Therefore, the mmf phase angle of the reluctance motor can be decided by the drive purpose, such as, maximum torque operation(6 = f ) , high efficiency drive or high powerfactor operation [6].

Experimental Results

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In the experiments described hereafter, the mmf phase angle is kept constant a t 50 electrical degrees. Fig.4 shows the speed-torque characteristics when the applied voltage is constant. In this case, only the current controller is used. The speed controller is not used. The speed characteristics is markedly drooping with the increase of the load torque. The characteristics is so-called the "series characteristics" of the conventional dc motor. This result shows that when the mmf phase angle is constant the speed can be controlled only by varying the voltage. Fig.5 shows the result of the speed control experiment. In this case, the speed is controlled by the controller. The speed-torque characteristics change into the "shunt characteristics" of the dc motor. Increased torque makes only very small decrease of the speed, which can be adjustable by the gain of the speed controller. Fig.6 shows the result of the constant speed operation at 150Drpm. The result shows that the current is almost proportional to the torque. The liniarization of the torque is realized only by varying the voltage of the mmf phase angle constant control.

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SPEED-TORQUE ClIARACTERlSTICS

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Application to Switched Reluctance Drive

The proposed scheme is based on the alternate current synchronous machine theory. However, the proposed scheme is applicable to the drive of the switched reluctance motor. F i g 3 shows the block diagram of the switched reluctance motor drive system. The differences from the dc blushless one are the current reference and the PWM strategy. The current reference is the square wave, and the PWM strategy is the hysteresis PWM method.

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The experimental result is shown in Fig.8. In this experiment, the mmf phase angle is kept at 71 electrical degrees, and the speed of the motor is 1800 rpm. The current is almost proportional to the load torque, though, the difference of the voltage between no-load condition and the full-load condition is small. This result implies that the torque of the switched reluctance motor and the synchronous reluctance motor has quite different principle.

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Conclusion

In this paper, experimental results of the reluctance motor drive system by the use of the dc-brushless controller is presented. The principle of the proposed drive system is the mmf phase angle constant control. The experimental results show that the decoupling control of the internal phase angle 6 and the current I was successfully achieved. This simple and practical drive system will serve widely in the application of the synchonous and the switched reluctance motors.

Acknowledgement The author is grateful to Professor Ishizaki of Tokyo Denki University for his valuable discussions.

VOLTAGE

7 References [l] Fukao,T., Chiba,A. and Matsui,M, 1989, IEEE 'Ilans. Ind. Appl.IA-25, 119-125. [2] Xu,L., Xu,X. and Lipo,T.A., 1991, IEEE Trans. Ind. Appl. IA-27, 977-985. [3] Miller,

T.J.E., Hutton,A., Cossar,C. and Staton,D.A., 1991, IEEE Trans. Ind. Appl. IA-27, 741749.

[4] Civii,M., Juffer,M. and Reinman,T., 1991, EPE FIRENZE, 4, 7-11.

[5] Ehizaki,A., Mino,T and Saioh,K., 1990, Proc. of IPEC-Tokyo'90, 964-970.

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[6] Morimoto,M., 1992, Proc of IEEJ-IAS'92 E60-E63.

THE SWITCHED RELUCTANCE MOTOR CHARACTERISTICS

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THE BLOCK DIAGRAM OF THE SWITCHED RELUCTANCE DRIVE