In a Synchronous Motor, damper windings are provided on the pole faces of the rotor. These windings consist of short-circuited copper bars embedded in the pole faces and connected by end rings, similar to the rotor construction of an Induction Motor. The damper winding serves several important purposes in the operation of a synchronous motor.
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Starting the Synchronous Motor
A synchronous motor cannot start by itself because at standstill the rotor field cannot immediately lock with the rotating magnetic field of the stator.
When the stator is energized with a three-phase AC supply, it produces a rotating magnetic field at synchronous speed. The damper windings act like the squirrel-cage rotor of an induction motor.
Working during starting:
- At standstill, the rotating magnetic field cuts the damper bars.
- This induces currents in the damper windings due to electromagnetic induction.
- The interaction of these currents with the stator field produces induction motor torque.
- The rotor accelerates and reaches near synchronous speed.
- Once close to synchronous speed, DC excitation is applied to the rotor field winding, and the rotor locks with the stator field.
Thus, the damper winding enables the synchronous motor to start as an induction motor.
Damping Rotor Oscillations (Hunting)
Another important purpose is preventing hunting.
Hunting refers to oscillations of the rotor around the synchronous position due to sudden load changes.
In systems such as power plants or industrial drives, when load varies suddenly:
- The rotor may accelerate or decelerate temporarily.
- This causes oscillations in rotor angle.
When this happens:
- Relative motion occurs between the rotor and rotating magnetic field.
- This induces currents in the damper bars.
- These currents produce damping torque opposing the oscillation.
Hence, the damper windings stabilize the motor and quickly suppress oscillations.
Improvement of Transient Stability
Damper windings also improve the dynamic and transient stability of the motor.
During disturbances such as:
- Sudden load application
- Voltage fluctuations
- System disturbances
the induced currents in damper bars create stabilizing torque, helping the rotor return to the synchronous position quickly.
This is particularly important in large synchronous machines used in power systems.
Reduction of Torque Pulsations
Damper windings help in smoothing torque variations by:
- Reducing torque ripple
- Improving steady operation
- Maintaining smoother rotation of the rotor