Synchronous Motor Startup Methods

A synchronous motor is an AC electric machine where the rotor rotates at the same speed as the stator’s magnetic field.

Working Principle

• The stator generates a rotating magnetic field when three-phase AC power is applied.
• The rotor produces a constant magnetic field, either through electromagnets or permanent magnets.
• The interaction between the rotor’s and stator’s fields ensures that the rotor spins at the exact synchronous speed.

Key Characteristics

• Constant Speed: The rotational speed is always equal to the synchronous speed, given by:

• where:

  • is the supply frequency (Hz),

  • is the number of pole pairs.

• Cannot Start on Its Own: The rotor needs assistance to reach synchronous speed before it can lock into the rotating field.
• High Efficiency & Precise Speed Control: Suitable for applications requiring constant speed under varying loads.

Applications

Synchronous motors are used where precise speed and high efficiency are required, such as:

• Precision machinery and clocks (constant speed operation)
• Large industrial applications (pumps, compressors, turbines)
• Energy-efficient drives (electric power plants, industrial automation)

Synchronous Motor Startup Methods

A synchronous motor cannot start on its own because its rotor must rotate at the same speed as the stator’s magnetic field from the beginning. Therefore, various methods are used to start it:

1. Asynchronous Start with Damper Winding

• The rotor is equipped with damper windings, similar to the squirrel-cage rotor of an induction motor.
• When power is applied, the stator’s rotating magnetic field induces currents in the damper windings, creating an asynchronous starting torque.
• Once the rotor reaches near-synchronous speed, the excitation system is activated, and the motor locks into synchronism.

2. Start with an External Motor

• A separate motor (such as an induction motor, diesel engine, or turbine) accelerates the synchronous motor to near-synchronous speed.
• Once the speed is reached, the excitation is switched on, and the synchronous motor takes over the load.

3. Self-Synchronization (Direct-On-Line Start)

• Used for large synchronous machines in power systems.
• The motor is directly connected to the power supply without a dedicated starting system.
• The high inertia of the rotor helps it synchronize with the stator field after initial disturbances.

4. Startup with a Frequency Converter

• A variable frequency drive (VFD) gradually increases the supply frequency, allowing the rotor to accelerate smoothly.
• Once the desired speed is reached, the excitation system is engaged, and the motor enters synchronous operation.

The choice of the startup method depends on the motor’s design, power rating, and operational requirements.