an electric motor having an electromagnetically operated clutch automatically engageable to lock the motor shaft against rotation in at least one direction . embodiments are described in which interruption of the power to the motor causes engagement of the clutch and in which application of power to the motor causes disengagement of the clutch . also disclosed is an automatic sliding door assembly operated by an electric motor having an electromagnetically operable clutch which is engaged on interruption of power to the motor to lock the motor shaft against rotation in one direction .