Chuck Yung
Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO

This paper, authored by Chuck Yung, provides a comprehensive overview of recent innovations in mechanical repair procedures for rotating electrical equipment. It assumes the reader has a basic understanding of mechanical repair and aims to introduce new techniques that have evolved in response to industry changes. The paper covers various topics, including bearing housing repair, insulating housings, and balancing rotors, highlighting their interconnectedness.

One significant issue addressed is shaft currents, particularly those caused by VFD-fed motors. Traditional methods of insulating bearings are ineffective against these currents, necessitating new approaches such as ceramic bearings, grounding brushes, and insulating housings. The paper explains the difference between traditional shaft currents and those induced by VFDs, emphasizing the need for insulating both bearings to prevent current flow.

The paper also discusses rotor balancing, noting that ceramic coatings on bearing journals can be damaged during the process. It recommends supporting rotors on the nearest shaft fit to avoid damaging the ceramic material. Additionally, it introduces Nylon as a new material for insulating bearing housings, detailing its application and potential issues related to bearing failure and warranty concerns.

Insulating sleeve bearings presents unique challenges due to high temperatures involved in rebabbitting. The paper outlines various insulation materials and methods, including resin-impregnated fiberglass banding tape, 3M Scotchply, micarta bond, and epoxies. Each method's advantages and drawbacks are discussed, providing practical insights for service centers.

The importance of balancing rotors to tight limits is emphasized, as unbalance is a major component of vibration spectra. The paper explains the relationship between magnetic forces, air gap, and shaft deflection, highlighting the need for precise rotor balancing to prevent excessive vibration and potential damage. Special cases, such as balancing rotors with flywheels, are also covered, underscoring the importance of maintaining perpendicularity to the rotating axis.

In summary, Chuck Yung's paper offers valuable insights into mechanical repair techniques for rotating electrical equipment, addressing key issues such as shaft currents, rotor balancing, and bearing insulation. It provides practical recommendations for service centers to improve repair quality and efficiency.

Key Points Covered:

• Shaft currents and their impact on bearings
• Insulating methods for bearings and housings
• Rotor balancing techniques and challenges
• Insulating sleeve bearings with various materials
• Importance of precise rotor balancing to prevent vibration

Key Takeaways:

• VFD-fed motors require new insulation methods for bearings
• Ceramic coatings on bearing journals need careful handling during balancing
• Nylon is a promising material for insulating bearing housings
• Various insulation materials have specific advantages and drawbacks
• Precise rotor balancing is crucial to prevent excessive vibration and damage

Categories: Technical topics, AC motors, Rotors, Balancing, Bearings, Ball bearings, Roller bearings, Sleeve bearings, Repair procedures & tips

Tags: Shaft currents

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