Shaft Position Sensing

SHAFT POSITION SENSING

^v   Commutation requirement of the SR motor is very similar to that of a PM brushless motor.

^v   The shaft position sensor and decoding logic are very similar and in some cases it is theoretically possible to use the same shaft position sensor and the same integrated circuit to decode the position signals and control PWM as well.

^v   The shaft position sensors have the disadvantage of the associated cost, space requirement and possible extra source of failure. Reliable methods are well established. In position sensors or speed sensors, resolvers or optical encoders may be used to perform all the functions of providing commutation signals, speed feedback and position feedback.

^v   Operation without position sensor is possible. But to have good starting and running performance with a wide range of load torque and inertias, sensor is necessary.

^v   When the SR motor is operated in the 'open-loop‘ mode like a stepper motor in the slewing range, the speed is fixed by the reference frequency in the controller as long as the motor maintains 'step integrity‘. (i.e) stay in synchronism. Therefore like an ac synchronous motor, the switched reluctance motor has truly constant speed characteristics.

This open-loop control suffers from two dis-advantages.

(a)  To ensure that synchronism is maintained even though the load torque may vary.

(b) To ensure reliable starting.

^v   Because of the large step angle and a lower torque/inertia ratio, the SR motor usually does not have reliable ‗starting rate‘ of the stepper motor.

^v   Also some form of inductance sensing or controlled current modulation (i.e) such as sine wave modulation may be necessary in the control at low speeds.