Tom Bishop
Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO

In his presentation at the EASA Convention 2006, Tom Bishop outlines the best practices for repairing electric motors to maintain or improve their efficiency and reliability. Bishop emphasizes that repair processes should not increase overall losses and should ideally reduce them. He draws on findings from the EASA/AEMT Rewind Study to provide practical guidance for repairers.

Bishop begins by discussing the types of energy losses in induction motors, which include core losses, stator and rotor I²R losses, friction and windage losses, and stray load losses. He explains that core losses can be increased by mechanical factors such as excessive pressure on the stator core, distortion of laminations, and damage to interlaminar insulation during burnout. Stator I²R losses, which can be the largest component of losses, can be reduced by increasing the conductor cross-sectional area and minimizing the mean length of turn (MLT).

Rotor I²R losses are influenced by the resistance of rotor bars and end-rings, and can be increased by changes to their cross-section or material. Friction and windage losses can be affected by bearing fits, lubrication, seals, and fans. Stray load losses, typically 10-20% of total motor losses, are caused by high-frequency harmonic fluxes near the air gap surfaces of the stator and rotor core.

Bishop outlines the major motor repair processes, including preliminary inspection, dismantling, removing the old winding and cleaning the core, rewinding, mechanical repairs, and re-assembly. He stresses the importance of recording nameplate data, conducting external and internal inspections, and documenting findings during dismantling. Proper handling of the rotor and careful removal of the old winding are crucial to avoid damage to the core.

During the burnout process, it is essential to control the temperature to prevent damage to the interlaminar insulation. Bishop recommends a maximum core temperature of 680°F unless the core plate is known to be C-5, in which case 750°F is acceptable. After removing the old winding, the core should be cleaned using methods such as scraping, high-pressure washing, or mild abrasive blasting.

Rewinding involves either copying the original winding or changing the style to improve performance. Bishop advises minimizing the coil extension length and increasing the copper cross-section to reduce I²R losses. He explains the advantages of lap windings over concentric windings, noting that lap windings can maintain or improve efficiency and provide better cooling.

Mechanical repairs to components such as the stator core, rotor, shaft, frame, bearings, and seals can also impact efficiency and reliability. Bishop emphasizes the importance of using equivalent replacement bearings, avoiding overgreasing, and ensuring proper fan and fan cover placement.

In conclusion, Bishop highlights the importance of following best practices throughout the repair process to maintain or improve motor efficiency and reliability. He encourages repairers to use the guidelines from the EASA/AEMT Rewind Study to achieve optimal results.

Key Points Covered:

• Types of energy losses in induction motors
• Factors affecting core losses, stator and rotor I²R losses, friction and windage losses, and stray load losses
• Major motor repair processes and their impact on efficiency and reliability
• Importance of recording nameplate data and conducting thorough inspections
• Proper handling of the rotor and careful removal of the old winding
• Controlling burnout temperature to prevent damage to interlaminar insulation
• Methods for cleaning the core after winding removal
• Rewinding techniques to minimize coil extension length and increase copper cross-section
• Advantages of lap windings over concentric windings
• Impact of mechanical repairs on efficiency and reliability

Key Takeaways:

• Repair processes should not increase overall losses and should ideally reduce them
• Proper handling and documentation are crucial during dismantling and winding removal
• Controlling burnout temperature is essential to prevent damage to interlaminar insulation
• Cleaning the core using appropriate methods can prevent increased losses
• Rewinding techniques can improve efficiency by reducing I²R losses
• Lap windings offer advantages in terms of efficiency and cooling
• Mechanical repairs must be done carefully to avoid increasing losses and reducing reliability
• Following best practices from the EASA AEMT Rewind Study can help achieve optimal repair results