a method and circuit for monitoring phase voltages of a polyphase tachometer generator to detect a phase failure , where the tachometer generator is associated with a rotor position transmitter . the phase voltages are added up by a summing stage to form an auxiliary voltage . the auxiliary voltage is sampled at every commutation instant determined by a rotor position signal generated by the rotor position transmitter during a predetermined measuring time . this sampled voltage value is compared with a predetermined voltage value range . an indication of a tachometer fault is displayed as soon as a sampled voltage value is outside the predetermined voltage value range . by this method and circuit , a fault which occurs between two commutation instants can be detected at the next commutation instant .