The above circuit diagram is represents? 

The correct answer is option 3):(Synchronous motor )

Concept:

The Synchronous motor:

• It is an AC motor in which at a steady state, the rotation of the shaft is synchronized with the frequency of supply current; the rotation period is exactly equal to an integral number of AC cycles.
• The operation of a synchronous motor is due to the interaction of the magnetic fields of the stator and the rotor. Stator winding is provided with a 3-phase supply and rotor winding is provided with a DC supply.
• It is called a synchronous motor because the rotor rotates at the same speed as that of the rotating magnetic field.
• It is a constant-speed motor because even if there is an increase in load, the motor still runs at the same synchronous speed.
• Synchronous motors are used where precise and constant speed is required such as ball mills, clocks, record players, etc.

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