Question

Please typed by computer not hand write..

1. What happens to the rotor of a synchronous motor when load torque is applied to the motor?
2. What is the pull-out torque of a synchronous motor?
3. At which angle and relative position (leading or lagging) does the rotor of a synchronous motor pull out of synchronization with respect to the rotating magnetic field?
4. What is the relationship between the pull-out torque of a synchronous motor and the motor field current?

Answer 1

Ans)

1) Assume that  the rotor is rotating at synchronous speed on no load, when load torque is applied to the motor the rotor slips back in space because of the increase in the load and then increased torque increases the rotor speed, and the motor comes back to  the synchronous speed, but with the larger torque angle(it is the angle between rotor magnetic field and stator magnetic filed )

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2)

Pull out Torque is the maximum torque that motor can develop without pulling out of synchronism

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3)

the syncronous motor will remain in synchronous and doesn't pull out of step untill the angle between between the rotor and stator mangnetic filed ,the rotor is retarded by an maximum angle of beyond which if we increase the load torque motor will step out of synchronism (,you can say rotor lags behind stator by 90 degrees)

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4)

The pullout torque is given as

where E is induced EMF and V is applied voltage and and Xs is reactance of stator and ws is speed in rad/s

the induced EMF or back emf is proportional to field current hence

pull out torque is proportional to field current

whenever field current reduced pull out torque also reduced in proportion