Method and apparatus detecting an operating state of a motor

A controller (10) measures an induced voltage (203) across at least one phase of a motor, such as a stepper motor, a permanent magnet motor, a brushless DC motor or similar, during an operational cycle of the motor. The measured voltage is compared with a stored value indicative of an expected voltage for that part of the operational cycle of the motor to derive a difference. The controller determines that a particular operating state (e.g. stall, degree of wear) has occurred when the difference exceeds a threshold value (204). The induced voltage can be measured when the motor has been brought up to a constant operating speed and while the phase is de-energised. The controller can use this method during an initialisation cycle, to detect stall caused by the rotor or external load hitting a mechanical stop or during normal operation. The controller can determine a reference position associated with stall of the motor caused by a mechanical stop with an accuracy that is greater than level of one step of the step motor.