Electrical

Switched reluctance motors

The switched reluctance motor is designed for high-performance servo applications. The switched reluctance motor is particularly suitable for a wide range of applications due to the robustness of the mechanical and electrical design.

In a reluctance machine, the torque is produced by the moving component moving to a position such that the inductance of the excited winding is maximised. The moving component is typically the machine’s rotor which can be either internal or external depending on the design or a linear component in the case of a linear reluctance motor.

 

The switched reluctance motor is topologically and electromagnetically similar in design to the variable-reluctance stepper motor. The key differences lie in the details of the engineering design, the approach to control, and hence its performance characteristics.

The switched reluctance motor is operated under closed loop control, with a shaft mounted encoder being used to synchronise the phase currents with rotor position. In comparison, the variable reluctance stepper motor is operated open loop.

Circuit working:

The rate of change of inductance as a function of rotor position is one of the design parameters of the switched reluctance machine. it is clear that the torque does not depend on the direction of current flow, however, the voltage
must be reversed to reduce the flux-linkage to zero. A suitable power circuit for a single winding is shown:

The circuit is capable of operating the motor as either a motor or a generator, as vi can either be positive or negative, and the power flow is determined by the switching pattern of the power bridge relative to the rotor’s position.

Switched reluctance motors Controller:

A block diagram of a suitable controller for a basic switched reluctance motor is shown :

It is recognised that although this type of drive is simple, and gives adequate performance for speed control, it is incapable of providing instantaneous torque control as required by a servo or similar application.

To achieve a performance similar to a conventional servo-drive, commutation as a function of rotor position has to be replaced by a control strategy that produces the desired total motor torques by controlling the individual phase currents.

It is clear that the switched reluctance machine is a very robust machine, that could offer the designer of high performance application an additional choice in the drive selection. While the switched reluctance machine is becoming widely used in high speed applications is has not been seen in the high performance position control application.

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