Questions You Should Know about Working Principle of 3 Phase Synchronous Motor

Three-Phase Synchronous Motor – Construction and Working Principle

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A 3-phase synchronous motor converts electrical energy input into mechanical energy output. Its key characteristic is that it operates at a constant speed equal to synchronous speed at any load, provided the load does not exceed a certain limit. If the load exceeds this limit, the motor stops, and the torque becomes zero, making the synchronous motor non-self-starting.

This motor is doubly-excited, with its stator winding connected to an AC supply and the rotor winding excited by a DC source.

Construction of Three-Phase Synchronous Motor

A synchronous motor consists of two main parts:

Stator

The stator is the stationary part of the machine, built of sheet steel laminations with slots on its inner periphery. A three-phase winding, called armature winding, is placed in these slots and receives power from a 3-phase supply.

Rotor

The rotor has salient poles with a field winding supplied with direct current through slip rings by a DC source to form alternate N and S poles. Generally, a small DC shunt generator mounted on the shaft of the motor serves as the DC source.

Note – The stator is wound for the same number of poles as the rotor poles.

Working Principle of Synchronous Motor

Consider a 3-phase, 2-pole synchronous motor with rotor poles NR and SR, and a stator also wound for two poles NS and SS. A three-phase AC supply is connected to the stator winding, and a DC voltage is applied to the rotor winding.

The stator winding produces a rotating magnetic field at synchronous speed, while the rotor field remains stationary until the rotor starts running. This setup creates revolving stator poles (NS-SS) and stationary rotor poles (NR-SR).

Initially, the stator poles and rotor poles exert repulsive forces leading to an anticlockwise torque on the rotor. However, after a half-cycle of AC supply, stator pole polarities reverse, creating attractive forces that induce a clockwise torque. Due to high inertia, the rotor does not start, making the starting torque zero, so the synchronous motor is not self-starting.

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Recently, advancements have been made to improve the self-starting capabilities of synchronous motors. New hybrid synchronous motors integrate induction motor principles to allow for an initial start, overcoming the challenge of zero starting torque.

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