Under what condition does the power factor of a synchronous motor become less than 100%?

The correct condition under which the power factor of a synchronous motor becomes less than 100% is when the rotor is locked. In this scenario, the motor cannot rotate because its rotor is essentially 'stuck' due to a mechanical issue or load demand exceeding its capability. When a synchronous motor is locked, it continues to draw current (often reactive) from the supply without doing any useful work (i.e., without producing mechanical power). This scenario leads to a scenario where the motor operates with a lagging power factor, meaning that the current drawn is not purely in phase with the voltage.

In contrast, high or low DC current may impact other motor parameters, but it does not directly result in the power factor of a synchronous motor falling below unity in the same straightforward manner. Fluctuations in motor speed could affect performance but are not a direct cause of a less than 100% power factor, as synchronous motors are designed to operate at a constant speed based on the supply frequency. Finally, an interrupted power supply would halt operation altogether, so the consideration of a power factor becomes moot in that case.