SOLUTION

In a synchronous motor, the magnitude of back EMF depends on DC excitation.

• In the case of synchronous motor, once the rotor starts rotating at synchronous speed, the stationary stator conductors cut the flux produced by the rotor. The conductors are stationary and flux is rotating.
• Due to this, there is an induced e.m.f. in the stator which according to Lenz’s law opposes the supply voltage.
• This induced e.m.f. is called back emf in case of synchronous motor.
• This back emf is due to the rotation of the rotor field at synchronous speed, this rotor field will be cut by stator conductors & as per electromagnetic induction, due to changes in magnetic field emf is generated which will oppose the applied voltage to the stator. This back emf magnitude will depend upon flux per pole and the speed of the motor.
• It is denoted as E_bph i.e. back e.m.f. per phase. This gets generated as the principle of an alternator and hence alternating in nature and its magnitude can be calculated by the equation,

$E_bph = 4.44K_cK_dφf T_ph$

or

E_bph = φ

As speed is always synchronous, the frequency is constant, and hence the magnitude of such back e.m.f. can be controlled by changing the flux φ produced by the rotor.

So back e.m.f. in case of the synchronous motor depends on the excitation given to the field winding and not on the speed, as speed is always constant.

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