What is the immediate effect when DC power is cut off from the rotor of a synchronous motor?

When direct current (DC) power is cut off from the rotor of a synchronous motor, the immediate effect is that the motor loses its magnetic field, which consists of the north and south poles created by the current flowing through the rotor. Synchronous motors operate by using the interaction between the rotating magnetic field produced by the stator and the magnetic field of the rotor. When DC power is interrupted, the rotor's magnetic poles vanish as the current ceases, leading to a loss of synchronism.

This phenomenon essentially allows the rotor to no longer interact effectively with the rotating magnetic field of the stator, which can lead to the rotor slowing down or stopping entirely, but the key aspect here is the loss of the magnetic poles. This loss is what fundamentally alters the operation of the motor and results in a breakdown of the magnetic field necessary for continuous rotation.