Starting Synchronous Motors

Synchronous motors are widely used in industrial applications where precise speed control and high efficiency are required. Unlike induction motors, synchronous motors run at a constant speed determined by the supply frequency and the number of poles, making them ideal for applications such as large compressors, pumps, and fans. However, starting synchronous motors presents unique challenges that require specific methods to overcome.

Basic Principles of Synchronous Motors

Synchronous motors operate based on the principle of magnetic locking between the rotating magnetic field of the stator and the rotor field. The rotor, typically fitted with electromagnets, locks into step with the rotating magnetic field and rotates at the same speed, hence the term "synchronous."

Unlike induction motors, synchronous motors cannot self-start. This is because the rotor, due to its inertia, cannot immediately follow the rotating magnetic field of the stator. If the rotor were connected directly to a power source, it would simply vibrate due to the alternating torque without building up enough speed to lock into synchronism. Therefore, various starting methods are employed to bring the rotor close to synchronous speed before applying DC excitation to the rotor.

Starting Methods for Synchronous Motors

1. Using Induction Motor Start

One common method to start synchronous motors is to begin operation as an induction motor. Synchronous motors are often constructed with damper windings (also known as amortisseur windings) in the rotor. These windings function similarly to the squirrel-cage rotor of an induction motor.

The starting procedure with this method involves:

Initially applying three-phase AC power to the stator windings
Keeping the rotor DC excitation off
Allowing the motor to accelerate as an induction motor using the damper windings
Once the motor reaches approximately 95-98% of synchronous speed, DC excitation is applied to the rotor
The motor pulls into synchronism with the rotating magnetic field

This method is popular as it doesn't require additional equipment beyond the motor itself. However, the starting current can be several times the rated current, which may require special starting arrangements to minimize voltage disturbances in the supply system.

2. Using Pony Motor

The pony motor method involves using an auxiliary motor to bring the synchronous motor near synchronous speed before connecting it to the power supply. The pony motor is typically a smaller induction motor or DC motor mechanically coupled to the synchronous motor.

The starting process includes:

Starting the pony motor and allowing it to accelerate the main motor
When the synchronous motor reaches approximately 95-98% of synchronous speed, the pony motor is decoupled or turned off
DC excitation is then applied to the rotor
The synchronous motor is connected to the AC supply and locks into synchronism

This method reduces the inrush current compared to direct starting and is particularly useful for very large synchronous motors where starting current limitation is critical. However, it requires additional equipment and space for the pony motor.

3. Using Reduced Voltage Starting

Reduced voltage starting methods can be applied to synchronous motors just as with induction motors. These techniques reduce the initial inrush current and torque during the starting period.

Common reduced voltage methods include:

Auto-transformer starting: Uses an auto-transformer to reduce the voltage applied to the motor
Star-delta starting: Connects the motor windings in star configuration during start, then switches to delta configuration for running
Reactor starting: Places reactors in series with the motor during start
Soft starters: Electronic devices that gradually increase voltage to the motor

These methods reduce the starting torque in proportion to the square of the voltage reduction, which must be considered when selecting a method for a specific load.

4. Using Static Frequency Converter

Static frequency converters provide a modern approach to starting synchronous motors. These electronic devices can vary both voltage and frequency, allowing the motor to start smoothly from zero speed.

The starting procedure with a static frequency converter involves:

Initially supplying low voltage and low frequency to the motor
Gradually increasing both voltage and frequency to maintain constant torque
The rotor DC excitation remains on throughout the starting process
When the motor reaches rated speed, the frequency matches the supply frequency
The motor is then transferred to direct online operation if required

This method provides excellent control over starting characteristics, minimizes inrush current, and is suitable for loads with demanding starting requirements. However, it involves higher initial costs and requires specialized control systems.

5. Using Damper Windings

As mentioned earlier, most synchronous motors are equipped with damper windings that serve dual purposes. During operation, these windings help dampen oscillations during load changes. During starting, they allow the motor to operate as an induction motor.

The damper windings typically consist of:

Copper or aluminum bars embedded in the rotor surface
End rings that short-circuit the bars
Configuration similar to the rotor of a squirrel-cage induction motor

The effectiveness of damper windings in starting depends on their design and resistance. Low-resistance windings produce high torque but may not be suitable for applications requiring high starting torque with low starting current.

Comparison of Starting Methods

Starting Method	Advantages	Disadvantages	Applications
Induction Motor Start	No additional equipment required; Simple operation	High starting current; Limited starting torque	Most common; Medium-sized motors
Pony Motor	Low starting current; Good for very large motors	Additional motor required; More complex system	Very large motors; Limited starting current applications
Reduced Voltage Starting	Reduced inrush current; Less stress on power system	Reduced starting torque; Longer acceleration time	Where starting current must be limited
Static Frequency Converter	Excellent control; Smooth torque; Low inrush	Higher initial cost; Complex control system	High-performance applications; Variable speed
Damper Windings	Integrated design; Dual-purpose functionality	Design compromises between starting and operation	Most synchronous motors

Safety Considerations

When starting synchronous motors, several safety considerations must be observed:

Proper grounding: Ensure all equipment is properly grounded to prevent electrical shock hazards.
Protective devices: Install appropriate protective relays to detect and respond to faults such as overcurrent, overtemperature, and underexcitation.
Interlocks: Implement electrical interlocks to prevent accidental energizing of the DC excitation before the motor reaches proper speed.
Testing: Regularly test starting circuits and protective devices to ensure proper operation.
Training: Ensure personnel are properly trained on safe starting procedures for specific synchronous motors.
Arc flash protection: Use appropriate personal protective equipment when working with energized equipment.

Conclusion

Starting synchronous motors requires careful consideration of the motor size, load characteristics, power system capabilities, and cost constraints. While induction motor starting using damper windings is the most common method due to its simplicity, alternative methods like pony motors, reduced voltage starting, and static frequency converters offer solutions for specific applications where performance requirements demand different approaches.

Regardless of the starting method chosen, proper design, implementation, and maintenance of starting systems are essential for reliable operation of synchronous motors. With appropriate starting techniques, synchronous motors can provide efficient, precise speed control for a wide range of industrial applications.

Reference Files For Starting Synchronous Motors

Screenshoot