Refer to the simplified diagram of the AC synchronous electric motor drive system as shown in the illustration. The propulsion motor operates from maneuvering speed to full ahead speed in the ahead direction. The propulsion motor will not operate at any speed in the astern direction. What is most likely the problem? Illustration EL-0155

Correct Answer: Option D — The output of the controlled inverter fails to achieve a reversal in phase sequence.

Controlled inverter drives for AC synchronous motors vary frequency and phase sequence to control speed and direction. A failure to achieve astern operation indicates that the solid-state inverter output fails to execute the necessary phase sequence reversal to alter rotating magnetic field direction.