Injection of a carrier with higher than the PWM frequency for sensorless position detection in PM synchronous motors

The injection of an alternating carrier allows detecting the rotor position in PM synchronous motors. It is usually injected through the PWM, being therefore limited to about 1/5 of the switching frequency. By increasing the carrier frequency beyond the switching one, the bandwidth of the position observer can be significantly increased and audible noise can be reduced, among other advantages. A method is proposed to inject a carrier with a higher than the PWM frequency by coupling an external source through a transformer and a filter. An analysis of the advantages and difficulties, as well as experimental results, are presented.