Chuck Yung
Senior Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "Synchronous Machine Repair Tips" by Chuck Yung, presented at the EASA Convention 2010, provides comprehensive guidance on the repair and maintenance of synchronous machines, which are a subset of AC motors. While the stator of a synchronous machine is similar to that of a squirrel cage induction motor (SCIM), the rotor is substantially different. Synchronous machines offer several advantages, including the ability to operate exactly at synchronous speed, control power factor, improve plant-wide power factor, lower operating costs, and reduce current, which can result in savings in switchgear and power cable routing.
Synchronous machines are commonly used in lower-speed applications and can be constructed with multiple poles. They can also operate as generators, making them suitable for specialized applications such as hydroelectric power plants. Testing synchronous motors requires special treatment of the rotor windings, and brushless synchronous motors pose additional challenges.
The paper discusses the importance of retaining rings, which are made from exotic alloys and are critical for constraining rotor windings at high operating speeds. Proper handling and inspection of retaining rings are essential to prevent catastrophic failure. The rotor body and shaft are forged from a single billet of carbon steel for high strength and fatigue resistance. The rewind process for edge-wound concentric coils is time-consuming and requires specialized equipment.
Test running synchronous motors involves measuring winding resistances, performing polarization index tests, and conducting AC voltage drop tests on rotor pole circuits. The paper emphasizes the importance of using a field discharge resistor to protect the rotor field winding during startup. Brushless synchronous motors use an exciter to power the field, and testing these motors requires additional steps.
The paper also covers amortisseur cage testing and repair. The amortisseur cage functions as a squirrel cage to accelerate the synchronous motor to speed and helps keep the rotor locked at synchronous speed. Testing for open rotor bars can be challenging, but tools such as growlers, core loss testers, and flexible current probes can be useful. The paper provides detailed instructions for removing and replacing amortisseur bars and end rings, emphasizing the importance of matching material resistance and ensuring tight fits.
Handling large motor frames requires careful attention to avoid twisting the frame, which can lead to alignment issues, increased vibration, and reduced bearing life. The paper advises using all lifting points and supporting the frame properly during assembly and burnout processes.
Cutting out damaged coils is a method used for emergency repairs on large low-speed machines. The paper outlines the principles for safely removing coils and minimizing negative side effects. Slip rings with helical grooves can reduce brush and ring temperatures, and the paper provides guidelines for machining and testing slip rings.
Root cause failure analysis (RCFA) for synchronous machines involves examining factors such as bond strength, ventilation, dovetail and wedge fits, and cooling improvements. The paper includes examples of common failures and their causes, emphasizing the importance of proper maintenance and handling.
Key Points Covered:
- Advantages and applications of synchronous machines
- Importance of retaining rings and proper handling
- Rewind process for edge-wound concentric coils
- Test running synchronous motors and using field discharge resistors
- Amortisseur cage testing and repair
- Handling large motor frames to avoid twisting
- Cutting out damaged coils for emergency repairs
- Slip ring machining and testing
- Root cause failure analysis for synchronous machines
Key Takeaways:
- Synchronous machines offer several advantages, including precise speed control and power factor improvement.
- Proper handling and inspection of retaining rings are crucial to prevent catastrophic failure.
- Rewinding edge-wound concentric coils requires specialized equipment and careful attention to detail.
- Testing synchronous motors involves multiple steps, including using field discharge resistors.
- Amortisseur cage testing and repair require specific tools and techniques to ensure proper function.
- Handling large motor frames correctly is essential to avoid alignment issues and increased vibration.
- Cutting out damaged coils can be a viable emergency repair method for large low-speed machines.
- Slip rings with helical grooves can reduce operating temperatures and improve brush life.
- Root cause failure analysis helps identify and address common issues in synchronous machines.
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