Torque in the Synchronous Motor

Starting torque

• It is torque developed by the synchronous motor when name plate voltage is applied to its armature winding. It is also called as breakdown torque. 
• It may as low as 10% in the case of centrifugal pump.

Running torque

• It is torque developed by the synchronous motor under running condition. It is determined by horse power and speed of the motor.

        P = Tω

Pull in torque

• A synchronous motor is started as induction motor during starting condition and its speed below 2 to 5% of the synchronous speed. 
• When the excitation ( or DC supply ) to the field winding is applied, the motor pull into synchronism resulting stator and rotor magnetic field rotates at same speed.
• The amount of torque requires for synchronous motor to pull into synchronism is called as pull in torque.

Pull out torque

• It is maximum torque which the synchronous motor can develop without pulling out of step. 
• When the synchronous motor is loaded, the rotor falls back by some angle a is called as load angle. 
• The stator and rotor magnetic rotates at synchronous speed in spite of some load is applied on the rotor. 
• The synchronous motor developed maximum torque when the rotor falls back by angle 90⁰. 
• When the load increases beyond its maximum rating, the rotor steps out of synchronism and synchronous motor stop.

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