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

The paper "Rewinding of DC Field Coils: Synchronous and Stationary" by Chuck Yung, presented at the EASA Convention 2004, provides a comprehensive guide on the procedures and considerations for rewinding the rotating poles of synchronous machines and stationary field coils. The process of rewinding field coils is crucial for maintaining the performance and reliability of rotating electrical equipment, and this paper outlines the steps to ensure successful rewinding.

The paper begins by emphasizing the importance of understanding the forces acting on rotating poles, which can cause catastrophic failure if the coils are not wound using appropriate materials and procedures. It provides a formula to calculate centrifugal force and highlights the need for special handling of large poles. The first step in the rewinding process is to draw the connection diagram to avoid misconnecting the rotor. Proper marking of the poles is essential to ensure they are reinstalled in the correct position and orientation, preventing irregular air gaps and unbalance.

The paper discusses the removal of poles, including tips for handling bolted and wedged poles. It recommends saving a sample coil for dimensioning and comparison to the rewound coils. The importance of documenting the turns per layer, number of layers, and total turns per coil is stressed, especially when changing wire size. The paper provides guidelines for selecting replacement materials for collars, considering factors such as insulation class, mechanical strength, and temperature rating.

Wire size is a critical factor in maintaining coil resistance and field strength. The paper explains how to calculate the cross-sectional area of the wire and the importance of matching the original wire area as closely as possible. It suggests converting round wire to rectangular wire for easier stacking and improved heat transfer. The process of winding the coils is detailed, including tips for maintaining wire tension, using insulation strips, and wet-winding with epoxy.

The paper also covers the installation of rewound poles, ensuring they are mounted in their original position and verifying the connection and polarity. It highlights the importance of controlling heat during the brazing process and supporting jumpers to manage centrifugal forces. The final steps include measuring the runout of the poles and adding bracing between adjacent poles if necessary.

Key Points Covered:

• Understanding forces acting on rotating poles
• Drawing connection diagrams and marking poles
• Removing poles and saving sample coils
• Documenting turns per layer and selecting replacement materials for collars
• Calculating wire size and converting round wire to rectangular wire
• Detailed winding process and maintaining wire tension
• Installing rewound poles and verifying connections and polarity

Key Takeaways:

• Proper documentation and marking are essential for successful rewinding.
• Matching the original wire size and cross-sectional area is crucial for maintaining coil resistance and field strength.
• Using appropriate materials and techniques for winding and insulating coils ensures mechanical strength and heat dissipation.
• Verifying connections and polarity prevents issues such as noise, vibration, and unbalance.
• Controlling centrifugal forces and adding bracing between poles can enhance the reliability of the rewound coils.