Short Answer:

The purpose of damper windings in a synchronous motor is to help the motor start and to maintain its stability during operation. Since synchronous motors cannot start on their own, damper windings provide the starting torque like an induction motor until the rotor locks in with the stator’s rotating magnetic field.

Damper windings also prevent hunting, which is the oscillation of the rotor around the synchronous position when the load changes suddenly. They act like a damper in mechanical systems, absorbing shocks and improving smooth and stable operation of the motor.

Detailed Explanation:

Purpose of damper windings in a synchronous motor

Damper windings, also called amortisseur windings, are additional windings placed in the rotor slots of a synchronous motor. They play an important role in both starting the motor and stabilizing its operation. A synchronous motor by itself cannot produce starting torque and needs help to accelerate the rotor up to synchronous speed. That is where damper windings become useful.

These windings resemble the squirrel cage structure found in induction motors and are made of copper or aluminum bars inserted in the rotor pole faces and short-circuited at both ends.

1. Helping in Starting the Motor:

One of the main limitations of a synchronous motor is that it cannot start on its own. This is because, at standstill, the rotor does not have a rotating magnetic field to interact with the stator’s rotating field. So, no torque is developed initially.

When damper windings are added, they behave just like a squirrel cage rotor of an induction motor. When three-phase AC supply is given to the stator, the rotating magnetic field induces current in the damper bars. This induced current produces torque, which accelerates the rotor like an induction motor.

As the rotor speed increases and reaches close to synchronous speed, DC supply is applied to the rotor field winding. The rotor then locks with the stator’s rotating magnetic field and starts rotating at synchronous speed. At this point, the damper winding no longer carries current and does not affect normal operation.

Key Point:
Without damper windings or an external starting method, a synchronous motor cannot self-start.

2. Preventing Hunting and Improving Stability:

Hunting is a condition where the rotor oscillates back and forth around the synchronous position due to sudden load changes. It can cause vibrations, instability, and even damage the motor if not controlled.

Damper windings act like a damping system in mechanical setups. When the rotor begins to oscillate, the movement causes the magnetic field to cut the damper bars, inducing currents in them. These currents produce an opposing torque that resists the oscillation, bringing the rotor back to its stable position.

Thus, damper windings:

• Reduce rotor vibrations
• Absorb sudden mechanical shocks
• Prevent long-lasting oscillations
• Enhance smooth performance under varying loads

3. Continuous Role in Operation:

While damper windings are inactive during steady-state synchronous operation, they come into action whenever there is a disturbance. This makes them an automatic and built-in feature that improves the reliability and smoothness of the motor.

They do not interfere with the DC excitation system or create losses when the motor is running steadily, but they are ready to act during disturbances or at startup.

Conclusion:

The purpose of damper windings in a synchronous motor is two-fold: they allow the motor to start like an induction motor and help prevent hunting during load variations. These windings make synchronous motors more practical and reliable in real-world applications by ensuring smooth starting and stable operation. Without them, the motor would not start on its own and would be unstable under dynamic load conditions.