Switched-Reluctance Motor Controls -- Drives and Controls

Switched-reluctance motors have been available for over 100 years, but the means to control them have made significant strides in the last decade. The availability of high-power high-frequency semiconductor switches combined with the power of microcontrollers has made these motors a strong contender in the race to provide elegant motion solutions. Switched-reluctance motors (SRMs) are known for their unique capability to work at high ambient temperature conditions.

7.1 Basic Principles

ill 31 shows the cross-sectional view of a four-phase SRM. When the coils wound around diametrically opposing poles are energized by a dc source, the rotor poles have a tendency to align with the corresponding stator poles. Continuous torque production is achieved by energizing the phase coils in sequence. As the rotor moves, the phase coil inductance changes from a maximum value when the stator and rotor poles are aligned to a minimum value when the corresponding poles are unaligned.

ill. 31 Cross-sectional view of an 8/6 four-phase switched-reluctance motor (SRM).

ill 32 shows the idealized inductance profile of one phase of an SRM. The placement of coil current in the rising slope of inductance produces positive torque, while the placement of current in the falling slope of inductance produces opposing or negative torque. For optimal torque production, the coil current should be initiated prior to the rising slope of inductance and must be commutated before the onset of the falling slope of inductance. Such a control is conventionally accomplished by the use of a rotor-position sensor that indirectly indicates the occurrence of the inductance slope.

7.2 Control Methods

Power Inverter. ill 33 shows the power inverter circuit used to energize the phase coils of a four-phase SRM. A full-wave diode rectifier is used to provide a dc source for the SRM. The inverter is comprised of an upper switch and a lower switch between which one phase coil is connected. The semiconductor switches are implemented by metal-oxide semiconductor field-effect transistors (MOSFETs) in the case of low-voltage systems (under 100 V) and insulated-gate bipolar transistors (IGBTs) in the case of high-voltage, high-power systems. The two-switch-per-phase inverter is used to energize the phase coils such that when both switches are closed, the current in the phase coil starts rising. When the current in the phase coil reaches a predetermined limit, the upper switch is controlled in such a manner that the coil current can be made to remain within a predetermined band. The magnitude of the band-limited current is proportional to the torque produced by the motor. When the rotor reaches a position of alignment with the stator poles, the switches in series with the phase coil are commutated, disconnecting the dc source from the phase coils. The stored electrical energy in the phase coil is circulated back to the dc source capacitor through the diodes. The capacitor across the dc bus voltage assists in recovering the energy after a cycle of commutation.

ill. 32 Idealized inductance and torque profiles.

ill. 33 Configuration of the classic converter.

Current Control. The method of controlling the current within a band using high-frequency switching of the semiconductor devices is called hysteresis control or cur-rent control. In this method, a current sensor provides a feedback of the motor current, which is compared to the commanded current, and the controlling signals to the power switches are realized. Torque control is realized by varying the commanded level of current. When both the switches in series with the phase coil are turned on, the current starts rising to the commanded level. The voltage to the phase coil is equal to the dc bus voltage. When the motor current reaches the commanded current, both the switches are turned off, causing the diodes to freewheel the motor current back to the dc source. As a result, the voltage across the phase coil is reversed in polarity due to the action of the diodes, and the current starts falling down. When the current reaches the lower level of the hysteresis band, the switches are turned on again, and the switching process continues. ill 34 shows the corresponding waveforms during current control. Current control is typically used in variable-speed automotive applications, such as radiator cooling fans and cooling pumps. A typical control block diagram for a current-controlled system appears in ill. 35. The angle calculator, commutator, and speed calculator are implemented using a microcontroller. The angle calculator typically calculates the turn-on and turn-off angles for the coil current after measuring the motor speed and knowing the commanded current. As an example, for high motor speeds, the turn-on angle is advanced with respect to the onset of the rising slope of inductance in order to permit the current to rise to the commanded value. Similarly, the turn-off angle is calculated such that there is no negative torque when positive torque is commanded, which would occur due to the presence of current in the falling slope of inductance.

ill. 34 Low-speed operation phase inductance, phase voltage, flux linkage, and phase current.

ill. 35 Typical SRM drive system with position feedback.

The commutator sequences the phase coil currents in the correct sequence of commanded direction. For a three-phase motor, if the energization sequence of ABC produces a clockwise direction, then the energization sequence of ACB would rotate the motor in a counterclockwise direction. Position feedback is achieved by using a shaft encoder or a position estimator.

Voltage Control. Voltage control is the method of switching semiconductors by providing a high-frequency gate signal to control the voltage input to the phase coil.

Torque control is achieved by varying the duty cycle of the control signal while keeping the frequency constant. This scheme is typically implemented by generating the duty cycle signal using a PWM channel from a microcontroller. This method is commonly used to vary the duty cycle within one commutation cycle such as to profile the current waveform. Such control leads to minimizing the torque ripple that's often common in SRMs. Voltage control is used in servo-positioning automotive applications, such as electric power steering and electronic throttle control.

The torque-speed characteristics of a typical SRM drive show a flat torque capability from zero speed to base speed, after which the curve follows a constant power curve. The high-speed operation follows a natural characteristic, as indicated in ill. 36.

7.3 Open-Loop and Closed-Loop Control

ill. 36 Torque-speed characteristics of the SRM drive. The cost of position sensors and the complexity associated with position estimators have resulted in economical open-loop controllers that sequence the phase coil com-mutation without any position feedback at low operation frequencies, resulting in low speeds of operation. The greatest advantage of such systems is that a motor with gear reduction can achieve extremely accurate positioning capability with high torque output. An example of such a system is found in automotive cruise control, where the throttle plate is positioned in real time to match the commanded speed set by the driver. The motor operating speed required to achieve this positioning is relatively low; hence, an open-loop system can be used.

In variable-speed drives, the operational speed is high; hence, the use of closed-loop feedback for commutation is mandatory. Position feedback mechanisms using opto-interrupters with slotted disks, resolvers, and encoders are commonly employed at present. However, research in position estimators has led to the development of sensorless commutation methods that use the variation in the coil inductance to estimate commutation instants.

7.4 Conclusion

Advanced control methods for elimination of position sensors, minimization of torque ripple, and reduction of acoustic noise have made SRMs suitable for various applications. However, control cost and complexity have prevented them from replacing conventional dc motors in several other applications. The capability of withstanding high temperatures and the ability to perform servo applications with-out any brushes or magnets is seen as a significant advantage as far as automotive applications are concerned. As reliability and performance become crucial over the next few years, the use of Sims will start to dominate in the industry.