Chuck Yung
Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "Armature Repair Tips" by Chuck Yung, presented at the EASA Convention, provides detailed guidance on the critical aspects of repairing DC equipment, particularly focusing on the commutator. Yung emphasizes the importance of maintaining proper clamping pressure, understanding the minimum commutator diameter, and ensuring the hardness of the commutator bars. These factors are crucial for the reliable operation of DC machines but are often overlooked.
The commutator, which is clamped using through-bolts or a large clamping nut, requires significant closing pressure during assembly. This pressure is maintained by tightening the bolts while the commutator is under hydraulic press pressure. Over time, the bolts may need to be re-tightened, especially after the armature has been processed through a service center and is still hot from the bake oven. This step helps prevent loosening of the bars and subsequent arcing caused by brush chatter.
Machining the commutator to ensure concentricity with the shaft and roundness is essential. The bearing journals should be indicated within 0.0005 inches to avoid commutator runout when the machine is assembled. The recommended turning speed is 500 feet per minute for carbide tools and 750 feet per minute for diamond tools. The commutator should be inspected for signs of stalls, which can cause flat spots on the bars due to overheating and annealing. Bars with varying hardness should be checked and potentially replaced if they are significantly softer than others.
Undercutting the commutator is another critical step. The undercutter blade should be appropriately sized to clean the slot without removing excess copper. The slot depth should be between one and one-and-a-half times the slot width. A U-cutter is preferred for most applications as it maintains the bar top width and prevents mica from flaking. Chamfering the bar edges is necessary to prevent brush chipping and excessive noise.
The back and front ends of the bars should be cleaned and chamfered to reduce flashover. The mica insulation extending past the bar ends should be protected with string banding, which can be covered with varnish, Teflon bands, or epoxy coatings to prevent carbon tracking and flashover. Protective coatings should be applied carefully to avoid contaminating the commutator surface or slots.
In summary, Yung's paper provides comprehensive tips for ensuring the reliability and performance of DC armatures through proper maintenance and repair of the commutator. These practices include maintaining clamping pressure, machining for concentricity and roundness, undercutting, chamfering, and protecting the commutator with appropriate coatings.
Key Points Covered:
- Importance of clamping pressure for commutators
- Machining commutators for concentricity and roundness
- Inspecting and addressing bar hardness
- Proper undercutting techniques
- Chamfering bar edges to prevent brush chipping
- Protecting mica insulation with string banding and coatings
Key Takeaways:
- Proper maintenance of commutators is crucial for DC machine reliability.
- Clamping pressure should be periodically checked and maintained.
- Accurate machining ensures commutator performance.
- Undercutting and chamfering are essential for preventing operational issues.
- Protecting mica insulation helps prevent flashover and carbon tracking.
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