A synchronous AC motor is an electric motor that operates at constant speed and is synchronized with the AC line that provides electricity to it. These motors are extremely useful in applications where accurate timing is needed, such as the timers of clocks.
The rotor of a synchronous AC motor comprises a two-pole, diametrically magnetized, permanent cylindrical magnet. It spins at the same speed as the alternating current that powers the motor, which keeps its rotational output speed in synchronization with the power line's frequency. The motor's speed is thus a ratio of the frequency of the AC line.
Synchronous AC motors can be small and nonexcited, such as those used in clock timers. In such motors, the rotor does not have a rotor winding that is excited by DC current. These motors therefore have a reduced output torque and are used only for their synchronous speed abilities.
The large synchronous AC motors have DC current-excited rotor windings over and above the AC line that supplies power to the stator windings. As a result, these motors not only maintain constant synchronous speed, but also show increased torque output even in varying electricity load conditions. Such motors find use in machines used in industries. At the home level, synchronous motors are used in appliances that contain a clock timer, such as air conditioners, washers, refrigerators and dryers.