How to Start a Synchronous Motor: A Comprehensive Guide

How to Start a Synchronous Motor: A Comprehensive Guide

Starting a synchronous motor can be challenging due to its non-self-starting nature. However, with the right methods and techniques, you can efficiently start a synchronous motor and ensure reliable operation. This article provides a comprehensive guide on how to start a synchronous motor, covering different starting methods, practical tips, and best practices.

Why Synchronous Motors Are Not Self-Starting

Synchronous motors require their rotor to reach the synchronous speed before the motor can operate effectively. This is because the rotor needs to lock into the rotating magnetic field created by the stator windings. Without achieving this speed, the rotor cannot generate the necessary torque to start.

Methods to Start a Synchronous Motor

Several methods can be employed to start a synchronous motor, each with its advantages and applications. Here are the most common methods:

1. Direct-On-Line (DOL) Starting

Direct-On-Line (DOL) Starting is a straightforward method where the motor is connected directly to the power supply. However, this method is rarely used for synchronous motors due to high inrush currents and the inability to self-start.

Procedure:

• Connect the motor directly to the power supply.
• Use a starter to manage the high inrush current.

Limitations:

• High inrush current can damage the motor and power supply.
• Not suitable for large motors or high-load applications.

2. Using a Separate Starting Motor

This method involves using a smaller auxiliary motor to bring the synchronous motor up to synchronous speed before it is connected to the main power supply.

Procedure:

• Connect the starting motor to the synchronous motor's shaft.
• Use the starting motor to accelerate the synchronous motor to near-synchronous speed.
• Once the synchronous speed is reached, engage the synchronous motor to the main power supply and disengage the starting motor.

Advantages:

• Reduces inrush current.
• Suitable for large motors and high-load applications.

3. Using a Variable Frequency Drive (VFD)

A Variable Frequency Drive (VFD) gradually increases the supply frequency, allowing the synchronous motor to accelerate smoothly to synchronous speed.

Procedure:

• Connect the synchronous motor to the VFD.
• Gradually increase the VFD's frequency to accelerate the motor.
• Once the motor reaches synchronous speed, switch the power supply from the VFD to the main AC supply.

Advantages:

• Smooth acceleration.
• Reduces mechanical and electrical stress on the motor.
• Suitable for various motor sizes and applications.

4. Using Damper Windings

Damper windings, also known as amortisseur windings, are additional windings placed in the rotor. These windings provide initial torque similar to that of an induction motor, helping to start the synchronous motor.

Procedure:

• Ensure the motor is equipped with damper windings.
• Connect the motor to the power supply.
• The damper windings generate initial torque, bringing the rotor close to synchronous speed.
• The motor then synchronizes with the stator magnetic field.

Advantages:

• Effective for motors with damper windings.
• Reduces starting complexity.

Practical Tips for Starting Synchronous Motors

1. Pre-Start Checks:

   • Ensure all connections are secure and correctly configured.
   • Check the excitation system for proper functioning.
   • Verify that the starting method chosen is suitable for the specific motor and application.

2. Load Management:

   • Gradually apply the load after the motor reaches synchronous speed to avoid sudden stress on the motor.
   • Use load monitoring systems to ensure the motor is not overloaded during the starting process.

3. Monitoring Systems:

   • Install monitoring systems to track the motor's performance during startup, including temperature, current, and vibration levels.
   • Use these systems to detect any anomalies and take corrective actions promptly.

4. Maintenance:

   • Regularly maintain and inspect the motor, especially the excitation system, to ensure reliable starting and operation.
   • Lubricate bearings and check for any signs of wear and tear.

FAQs

Why can't synchronous motors self-start?

• Synchronous motors require the rotor to reach synchronous speed to lock into the stator's rotating magnetic field. Without this initial speed, the motor cannot generate the necessary torque to start.

What is the most efficient way to start a large synchronous motor? 

• Using a Variable Frequency Drive (VFD) is often the most efficient method for starting large synchronous motors, as it allows smooth acceleration and reduces mechanical and electrical stress.

Can damper windings be retrofitted to existing synchronous motors?

• In some cases, damper windings can be added to existing synchronous motors. However, this depends on the motor's design and the feasibility of retrofitting.

Conclusion

Starting a synchronous motor involves understanding and implementing the appropriate method for your specific application. Whether you use a starting motor, a VFD, or damper windings, each method has its advantages and considerations. By following the procedures and practical tips outlined in this guide, you can ensure a smooth and efficient startup for your synchronous motor, leading to reliable and long-term operation. Proper maintenance and monitoring are also crucial to avoid issues and optimize performance, making synchronous motors a valuable asset in various industrial applications.