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The starter of a synchronous motor is fed my three phase AC current while the rotor is fed by DC current. At some instance the north pole of starter magnetic field will coincide with the north of rotor magnetic field and the very next second the poles will be opposite, that is N S, so this will make the rotor to stand still until an external force is applied to start the rotation.
Sirnels answered the question on April 30, 2018 at 22:03
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