Chuck Yung, Cyndi Nyberg & Tom Bishop
Technical Support Specialists
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "Final Testing of DC Machines" by Chuck Yung, Cyndi Nyberg, and Tom Bishop, presented at the EASA Convention, provides a detailed guide to ensuring the quality repair of DC machines through specific tests and procedures. While load-testing every DC machine is ideal, it is not always feasible due to limitations in service centers. The authors outline essential tests that can be routinely performed to prevent issues such as arcing and ensure reliable operation.
Brush seating is crucial for conducting armature current effectively. The brushes must be fully seated to avoid increased current density and potential overheating. The process involves using non-conductive sandpaper to shape the brushes to conform to the commutator, followed by final seating with a brush-seating stone at reduced armature voltage. Establishing a commutator film is also important for reducing friction and indicating machine performance.
Brush neutral adjustment is necessary whenever the commutator end bracket is removed. The preferred method is the AC method, which involves applying AC voltage to the field leads and measuring the induced voltage across adjacent brush posts. The brushholder assembly is shifted to obtain the lowest voltage reading, ensuring a near-perfect neutral position. Other methods include the inductive kick method and comparing clockwise and counterclockwise rpm.
Brushholder spacing should be checked to ensure equal spacing around the commutator circumference. Uneven spacing can lead to selective arcing and poor performance. The authors provide guidelines for measuring and adjusting brush spacing, including considerations for designs with staggered brushes.
Compound field polarity must be verified to ensure cumulative connections, where the polarity of the shunt and series fields sharing a pole are the same. Two methods are outlined for verifying polarity: using a low voltage analog DC meter and flashing the shunt field, or comparing the direction of rotation when operating the machine as a shunt motor and series motor.
Interpole polarity is checked to ensure it opposes the magnetic flux of the armature. The preferred method involves applying low-voltage AC to the armature and interpole circuit and measuring the output voltage. If the output voltage is less than the input voltage, the interpole polarity is correct.
Final testing includes no-load testing with rated voltages, checking for sparking, vibration, and ensuring the motor operates at approximately 10% above rated speed. When rated voltages are not available, the authors provide guidelines for maintaining the field-to-armature voltage ratio. Load testing with a dynamometer or using the Kapp Test method for identical machines is recommended to detect problems that may not appear on static or no-load tests.
The paper concludes with additional testing methods such as the saltwater rheostat for load testing DC generators, the brush pencil neutral test, and the black band test for determining commutating field strength and brush neutral position.
Key Points Covered:
- Importance of brush seating and establishing commutator film
- Methods for adjusting brush neutral position
- Checking and adjusting brushholder spacing
- Verifying compound field polarity
- Checking interpole polarity
- Final testing procedures, including no-load and load testing
- Additional testing methods: saltwater rheostat, brush pencil neutral test, black band test
Key Takeaways:
- Proper brush seating and neutral adjustment are crucial for preventing arcing and ensuring reliable operation.
- Brushholder spacing must be checked to avoid selective arcing and poor performance.
- Compound field polarity and interpole polarity must be verified to ensure correct connections.
- Final testing procedures, including no-load and load testing, are essential for detecting potential issues.
- Additional testing methods provide valuable insights into machine performance and commutating field strength.
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