A synchronous motor drives a constant torque mechanical load. Which statement about speed control is true under these conditions? A) The excitation current is directly proportional to the speed. The speed can be adjusted using the excitation rheostat. B) The only method for speed regulation is to change the frequency applied to the motor. C) Sub-nominal speed adjustment is possible with the starting resistor. D) The magnetic flux is inversely proportional to the velocity. E) Under constant torque load, speed can be adjusted by changing the voltage.

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Synchronous Motor

The synchronous motor is an AC electric type motor where at a steady-state the shaft rotation is synchronized with the supply current variable-frequency. To an integral number of AC cycles, the rotation period is exactly equal. The stator of the motor in the synchronous motor contains multi-phase AC electromagnets due to which a magnetic field is created that rotates in time with the oscillation of the line current. The rotor contains a permanent magnet and the rotor rotates in synchronization with the stator field at the same rate and provides the second synchronized magnetic field which is rotating in nature as a result. When the synchronous motor is supplied with independently excited multi-phase AC electromagnets on both the rotor and the stator, it is termed as a doubly-fed machine. The most widely used AC-type motors are the synchronous AC motor and the induction AC motors.