Starting Methods for Synchronous Machines
Synchronous machines, while efficient and reliable in steady-state operation, present a unique challenge: they cannot self-start. This is because, at standstill, the rotor poles are stationary, and the rotating magnetic field of the stator induces no torque. Therefore, special starting methods are required to bring the synchronous motor up to synchronous speed before it can be connected to the supply or energized for synchronous operation.
Why Synchronous Machines Don't Self-Start
Synchronous motors require an external force to reach synchronous speed.
At zero speed, the stator's rotating magnetic field does not induce any torque on the stationary rotor poles. The rotor poles are attracted to the stator poles, but this attraction is static and does not result in rotation.
When a synchronous motor is energized, the stator winding produces a rotating magnetic field (RMF). However, the rotor, with its DC-excited field winding, is initially stationary. At standstill, the RMF sweeps past the stationary rotor poles. While there's an instantaneous attraction between the stator poles and rotor poles, this attraction is static and does not create a continuous torque to accelerate the rotor. The rotor remains stationary until it is brought close to synchronous speed by some external means.
Common Starting Methods
Several methods are employed to start synchronous motors, each with its advantages and disadvantages. These methods primarily aim to provide initial torque to accelerate the rotor to near synchronous speed.
1. Using the Damper (Amortisseur) Windings
This is the most common method. Damper windings, embedded in the pole faces of the rotor, are essentially short-circuited bars similar to those in a squirrel cage induction motor. When the stator is energized, these windings act like an induction motor rotor, producing torque and accelerating the rotor. As the rotor speed approaches synchronous speed, the DC field excitation is applied, and the rotor pulls into synchronism.
To act like a squirrel cage rotor, allowing the synchronous motor to start as an induction motor.
2. Using an External Prime Mover
In this method, an external source, such as a small induction motor or a DC motor, is used to drive the synchronous motor rotor up to synchronous speed. Once the rotor is rotating at synchronous speed, the DC field excitation is applied, and the synchronous motor is connected to the supply. The external prime mover is then disconnected.
This method is less common for general-purpose synchronous motors but might be used in specific applications where a reliable external drive is readily available.
3. Using a Pony Motor
This is a specific case of using an external prime mover. A small induction motor (the 'pony motor') is coupled to the shaft of the synchronous motor. The pony motor starts the synchronous motor and brings it up to near synchronous speed. At this point, the synchronous motor is energized, and it pulls into synchronism. The pony motor is then switched off and can be used to start another synchronous motor.
4. Reduced Voltage Starting
Similar to induction motors, synchronous motors can be started at reduced voltage to limit the starting current. This is typically achieved using auto-transformers or star-delta starters. However, reduced voltage starting also reduces the starting torque, which is a critical factor for synchronous motors, especially when starting with load. This method is often used in conjunction with damper windings.
| Starting Method | Mechanism | Pros | Cons |
|---|---|---|---|
| Damper Windings | Acts as induction motor rotor | Simple, common, no external motor needed | Starting torque depends on damper design; potential for rotor oscillations |
| External Prime Mover | External motor drives rotor | Controlled acceleration, can start under load | Requires an additional motor, more complex setup |
| Pony Motor | Small induction motor coupled to shaft | Reliable starting, can be shared | Requires space for pony motor, additional starting circuit |
| Reduced Voltage | Lower stator voltage | Limits starting current | Significantly reduces starting torque, may not be sufficient for all loads |
Key Considerations for Starting
When selecting a starting method, several factors are crucial:
- Starting Torque: The motor must be able to develop sufficient torque to overcome inertia and any connected load.
- Starting Current: High starting currents can cause voltage dips and stress the power system.
- Rotor Oscillations (Hunting): The transition from induction motor starting to synchronous operation can induce oscillations in the rotor. The damper windings help to damp these oscillations.
- Complexity and Cost: The chosen method should be practical and cost-effective for the application.
The process of starting a synchronous motor using damper windings can be visualized as a transition. Initially, the motor behaves like a squirrel cage induction motor, accelerating due to the interaction between the stator's RMF and the induced currents in the short-circuited damper bars. As the rotor speed approaches synchronous speed, the DC field is applied. The rotor's magnetic poles are then 'pulled in' and locked with the stator's RMF, achieving synchronous operation. This transition requires careful control to avoid excessive oscillations.
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Summary
Synchronous motors require specific starting methods because they cannot self-start. The most common method utilizes damper windings, which allow the motor to start as an induction motor. Other methods involve external prime movers or reduced voltage techniques. The choice of method depends on factors like required starting torque, acceptable starting current, and system complexity.
Learning Resources
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