Tom Bishop, P.E.
Technical Support Specialist
Electrical Apparatus Service Association
St. Louis, MO
The paper "DC Motor Testing & Commutation Issues" by Tom Bishop, presented at the EASA Convention 2008, provides a comprehensive guide to testing DC motors and addressing commutation issues. The paper covers various tests to perform during different stages of motor repair, including incoming, post-disassembly, in-process, and final testing. It also discusses the interpretation of DC motor nameplates, testing armatures, commutators, field coils, and interpoles, as well as diagnosing and solving commutation problems.
DC motor nameplates can be confusing, especially when dealing with dual voltage fields and field-weakening capabilities. The paper explains that the first field current rating and the first rpm correspond to the high field voltage rating at operating temperature and rated load. Field weakening increases motor speed by reducing the voltage applied to the fields, which decreases the current carried by the field conductors. Understanding the nameplate information is crucial to avoid misinterpretation and ensure proper motor operation.
Armature testing begins with a careful inspection for obvious faults. If no issues are found, electrical tests such as the megohmmeter test, low-resistance bar-to-bar test, growler test, high-frequency bar-to-bar test, surge comparison, and high potential test are performed. The megohmmeter test checks insulation resistance to ground, while the low-resistance bar-to-bar test measures resistance between adjacent commutator bars. The growler test detects shorted coils, and the high-frequency bar-to-bar test compares impedance between winding sections. The surge comparison test uses surge pulses to identify winding faults, and the high potential test ensures groundwall insulation adequacy.
Commutator testing involves checking bar tightness, inspecting for cracked or broken risers, and performing electrical tests such as the megohmmeter test, hipot test, and bar-to-bar insulation test. Field coil testing includes surge testing, measuring resistance, and performing drop tests. The drop test can be done using AC or DC voltage, with AC being the preferred method due to its ability to detect shorted turns more effectively.
Interpoles are tested using AC drop tests, surge tests, and current comparison tests. The paper emphasizes the importance of verifying field and interpole polarity, brush grade and size, brushholder alignment, and brush spacing. Proper brush seating and establishing a commutator film are essential for optimal motor performance.
Commutation issues such as streaking, threading, grooving, pitch bar marking, slot bar marking, bar edge burning, copper drag, and stall conditions are diagnosed based on the appearance of the commutator. The paper provides detailed descriptions of these conditions and their causes, along with corrective actions.
Before assembling the motor, all windings should be checked for resistance to ground using a megohmmeter. The paper outlines steps to protect the commutator, verify polarities, clean brush boxes, check bearing insulation, label brushholder leads, align brushholders, and adjust brush neutral. Final testing includes no-load testing, dynamometer testing, and load testing using methods such as the saltwater rheostat and back-to-back testing.
In conclusion, the paper offers valuable insights into DC motor testing and commutation issues, providing practical guidelines for service centers to ensure reliable motor performance.
Key Points Covered:
- Interpretation of DC motor nameplates
- Armature testing methods
- Commutator testing procedures
- Field coil testing techniques
- Interpole testing methods
- Diagnosing commutation issues
- Steps before assembling the motor
- Final testing procedures
Key Takeaways:
- Understanding DC motor nameplates is crucial for proper operation.
- Various electrical tests are essential for diagnosing armature faults.
- Commutator testing ensures bar tightness and insulation adequacy.
- Field coil testing includes surge tests and drop tests.
- Interpole testing involves AC drop tests and current comparison tests.
- Diagnosing commutation issues requires careful observation of commutator appearance.
- Proper steps before assembling the motor ensure reliable performance.
- Final testing includes no-load, dynamometer, and load testing methods.
Print
