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Three-Phase SRM Drive Using DC-Link Current

engineering

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Three-Phase SRM Drive Using DC-Link Current



Abstract—The paper proposes the SRM drive system using the information of DC-link current and rotor position, from which the phase currents of SRM can be easily estimated and also they can be used in driving SRM instead of the three-phase currents. Comparing to the general drive system based on the three-phase currents, it is verified through the simulation that the SRM drive system based on the proposed estimation has also the good performance in dynamic and steady-state responses of the speed control. Using the DC-link current and the rotor position, the multi-phase currents of SRM can be simply estimated and the number of current sensors can be at minimum decreased to a single current sensor.

I.                     INTRODUCTION

Switched reluctance motor (SRM) drives are gradually penetrating the market with applications already developed or being developed for the consumer products, aerospace, home appliances, and automobile industries [1]-[6]. The SRM drives have the attractive characteristics of fault tolerance and absence of magnets. However, the control of an SRM depends on the phase current and the commutation of the stator phases in synchronism with the rotor position. The current-sensing and position-sensing requirements increase the overall cost and calls for extra space and complexity [1][2]. This drawback excludes the SRM from many cost sensitive industrial applications. Sensorless operation is a key requirement for the success of SRM drives in various industries. Nowadays sensorless field in study has been progressed with much interest [1][2].

In detecting the stator current of the SRM, the number of current sensor is increased in proportional to the number of stator phase. The algorithm using the minimum sensor or the pure sensorless algorithm is required for keeping the overall cost lower.

Solving the above mentioned, this paper proposes the SRM drive system using the information of DC-link current and rotor position, from which the phase currents of SRM can be easily estimated and also they can be used in driving SRM instead of the three-phase currents.

RMxprt is used for designing the SRM model with nonlinear characteristics. Simplorer available to multi-domain of power, electronics, and system is used for analyzing the overall system consisting of power circuit, control, and mechanical system. [7][8].

 It will be verified through the simulation using RMxprt and Simplorer that the proposed algorithm based the information of DC-link current and rotor position is very effective.

II.                   Principle of Switched reluctance motor

A.       Voltage and torque equation of the SRM

The voltage equation for one phase is followed as;

      (1)

where  is phase voltage,  is stator winding resistance,  is self-inductance, and  is phase current.

Introducing the concept of co-energy such as equation (2), the developed torque of the SRM in equation (3) can be obtained by partial derivatives.

     (2)

                                    (3)

where the developed torque of the SRM is proportional to the current square. And the sign of torque is dependant on the rotor position, because the sign of torque is varied according to the up- or down-slope of the inductance [4][5].

B.       Excitation current control according to rotor position

Fig. 1. Relation between phase current and inductance according to rotor position.

The torque is independent of the direction of the current. Its direction depends only on the sign of . When the rotor poles are approaching the aligned position, this is positive, and the positive torque is produced, regardless of the direction of the stator current. When the rotor poles are leaving the aligned position and approaching the unaligned position, the torque is negative, regardless of the direction of the stator current. Therefore the ideal motoring current waveform is a rectangular pulse (in practical, the trapezoidal waveform in Fig.1 is used for reducing the torque ripple) that coincides with the rising inductance. Similarly, the ideal braking current waveform is a rectangular pulse that coincides with the falling inductance. The implication is that the current should be switched on and off in synchronism with the rotor position; in other words, the SRM is a shaft-position-switched machine like the square-waveform BLDC motor.

III.                 Estimation Algorithm of phase currents Using DC-Link current

Fig. 2. Relations of phase current and DC-link current by switch state.

When the switch is turn-on, the DC-link current flows as shwon in Fig.2(a). a-phase current and DC-link current have the same flow and the relation are followed as;

                                           (1)

Otherwise when the switch is turn-off, DC-link current flows as shown Fig.2(b). a-phase current and DC-link current have the reverse flow and the relation are followed as;

                                                     (2)

Therefore, using the relations of phase current and DC-link current shown in Fig.1, phase current can be estimated as the following.

                                                     (3)

where  is the estimated a-phase current.

These relations are equally available to the estimation of other phase currents. Therefore, using the information of the rotor position and DC-link, each phase current can be estimated as followings;

                             (4)

                                           (5)

                                                (6) where is the estimation function of each phase current,  and  are b- and c-phase estimated currents.

IV.                Simulation Analysis

Using Simplorer and the RMxprt available to the multi-domain simulations of the power-electronics-system, the virtual system is implemented to verify the effectiveness of the proposed algorithm.

A.       Model of Switched Reluctance Motor

Fig.3(a) and (b) shows the appearance and the cross-section of the SRM used in simulation.

The specification of SRM is shown in TABLE I. Its information and shape are used for modeling the SRM by RMxprt, which can consider a nonlinear-characteristic of the SRM and produce the practical model. Fig.4 and 5 show the current-flux curve of SRM and the inductance profile of SRM, which be produced by RMxprt.

TABLE I

SPECIFICATION OF SRM USED IN SIMULATION

Items

unit

value

rated power



W

80

rated voltage

V

220

rated speed

rpm

815

resistance / phase

ohm

113

inductance / phase

mH

9.678

number of stator / rotor pole

12/8

air gap

mm

0.45

number of turns / phase

turn

650

outer diameter of stator

mm

132.0

inner diameter of stator

mm

74.5

Fig. 3. Appearance and cross-section of SRM used in simulation (12/8).

Fig. 4. Current-flux curve of SRM used in simulation.

Fig. 5. Inductance profile of SRM used in simulation.

B.       System Configuration

After producing the SRM model with a nonlinear characteristic by RMxprt, the overall speed control drive system of the SRM in Fig.6 and 7 is implemented by Simplorer. DC-Link voltage is 220V and proportional gain and integral time of the speed controller are set as 100Hz and 1ms, individually. In addition, the current controller is configured by hysteresis controller.

Fig. 6. SRM drive system by Simplorer.

Fig. 7. Block-diagram of SRM control system.

C.      Simulation Result

Fig.8 and 9 shows the simulation results, individually based on actual phase currents and estimated phase currents. Through the simulation results, the good responses about the reference speed and disturbance can be confirmed. And also the phase currents can be well estimated using DC-link current and rotor position.

V.                  conclusion

The paper proposes the algorithm that three phase currents of SRM can be estimated using only the DC-link current sensor. Simplorer and RMxprt, which can support the multi-domain simulation of power, electronics, and system, are used to verify the proposed algorithm applied to SRM drive. Comparing to the general drive system based on the phase currents, it is verified through the simulation that the SRM drive system based on the proposed estimation has the good performance in dynamic and steady-state responses of the speed control. Using the DC-link current, the multi-phase currents of SRM can be easily estimated and the number of current sensors can be decreased to a single current sensor. RMxprt is used to model the SRM under consideration of the nonlinear characteristics, and the model has the nearly equal characteristics of the practical motor.

References

[1]     RR. Krishna, “Sensorless Operation of SRM Drives: R & D Status,” Proceedings of IEEE IECON’01, pp.1498-1503, 2001.

[2]     M. Ehsani, B. Fahimi, 'Elimination of Position Sensors in Switched Reluctance Motor Drives: State of the Art and Future Trends,' IEEE Trans. on Industrial Electronics, vol.49, no.1, pp.40-47, Feb. 2002

[3]     K. Ohyama, 'Recent Advances of Reluctance Torque Asisted Motors (in Japanese),' IEEJ Trans. on Industry Application, vol.123, no.2, pp.63-66, 2003

[4]      PJ Lawenson et al., 'Variable-Speed Switched Reluctance Motor,' IEE, vol. 127, pp.253-165, July 1980

[5]     V.N. Walivadekar, V.V.,Sr. Athani, G.N. Acharya, 'Equivalent Circuit for Switched Reluctance Motor,' Proceedings of IEEE TENCON '93, pp.568-571, Oct. 1993

[6]     Swithced Reluctance Motor Drives, Interec Communications Inc., T.J.E. Miller, 1988

[7]      http://www.ansoft.kr, (Ansoft Korea HP)

[8]      http://www.ansoft.com, (Ansoft Headquarter HP)

Fig. 8. Simulation result based on phase current information.

Fig. 9. Simulation result based on estimated phase currents.

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