SOLUTION

• In a synchronous motor, we provide DC excitation to produce require a magnetic field.
• This magnetic field generates the back emf in the synchronous motor.
• The low field current is the main reason when the synchronous motor fails to pull into synchronous after applying the D.C. field current.
• Lowering the field strength lowers the internally generally generated voltage.
• So if the value of excitation is low we can’t generate the required magnetic field and hence back emf will is low.
• When the machine fails to pull into synchronism it operates as an induction motor at a speed slightly less than synchronous speed.
• Therefore the synchronous motor can’t attend the synchronous speed.

Pull-in torque

• Pull-in (synchronizing) torque is required to accelerate the motor and driven load from the maximum induction motor speed to synchronous speed.
• It is the critical stage in the starting of a synchronous motor rotor. Whether or not the rotor pulls into step depends on the position of the rotor poles relative to the poles of the rotating stator field.
• If like poles are aligned, they tend to repel each other, so that the pull-in torque is at a minimum and fails to pull the rotor into synchronism.
• But if unlike poles are aligned, they attract each other and the pull-in torque is a maximum.
• When the direct current excitation is applied to the field system and because the rotation is running below synchronous speed, the torque is a combination of the pulsating torque due to the d.c. excitation of the field system and the induction motor torque which also varies with speed.
• If the slip is low enough, the attraction between the unlike poles in the stator and rotor will pull the rotor into step with- the rotating stator field.
• For given excitation, system inertia, and load, there is a slip below which the synchronous motor will not pull into step. This is termed the critical slip.