Chuck Yung
Senior Technical Support Specialist
Electrical Apparatus Service Association
St. Louis, MO
The paper "Troubleshooting DC Motors" by Chuck Yung, presented at the EASA Convention 2006, provides a comprehensive guide to diagnosing and resolving issues with DC motors. The successful repair or overhaul of a DC motor begins with a basic understanding of the nameplate data and components. This paper emphasizes the importance of correctly interpreting the DC motor nameplate, which includes information on field strength, field current, and resistance. Understanding these details is crucial for avoiding mistakes during the repair process.
DC motors operate based on the principles of magnets, where opposite poles attract and like poles repel. The paper explains the properties of magnetic fields, including their bipolar nature, the formation of closed loops connecting each pair of poles, and the measurement of field strength by flux density. The operation of DC motors involves the attraction and repulsion of strategically placed magnets, with the armature's polarity being reversed to create rotational motion. This process, known as commutation, is facilitated by the commutator and carbon brushes.
The paper discusses the importance of brush neutral, which is the position where the polarity of the armature coil reverses, resulting in minimal induced voltage. Proper brush neutral adjustment is essential for preventing arcing and ensuring smooth motor operation. Interpoles are added to DC motors to counteract the distortion of field flux caused by armature flux. The correct polarity of interpoles is crucial for maintaining stable field flux and preventing excessive arcing.
Magnetic field strength in DC motors is described in terms of ampere-turns, which can be increased by adding turns to the coil, increasing current, or adjusting the air gap between the pole iron and armature. The paper also covers compound fields, which combine shunt and series fields to provide both speed control and increased torque. The relative strengths of these fields determine the percent compounding, which can be cumulative or differential.
The assembly and final testing of DC motors involve several steps, including verifying field and interpole polarity, checking brush grade and size, aligning brushholders, and seating brushes. Proper brush seating is critical for ensuring full contact with the commutator and preventing arcing. The paper outlines methods for establishing commutator film, adjusting brush neutral, and verifying compound field polarity.
On-site troubleshooting of DC motors requires basic testing equipment such as a megohmmeter, AC/DC voltmeter, DC clamp-on ammeter, ohmmeter, and tachometer. The paper provides detailed procedures for diagnosing common issues, including motors that do not start, run faster or slower than rated speed, or exhibit sparking at the brushes. It also covers troubleshooting generators that do not produce voltage or the correct voltage.
Key Points Covered:
- Importance of correctly interpreting DC motor nameplate data
- Principles of magnets and properties of magnetic fields
- Operation of DC motors through commutation and brush neutral adjustment
- Role of interpoles in maintaining stable field flux
- Description of magnetic field strength in terms of ampere-turns
- Compound fields and percent compounding
- Steps for assembly and final testing of DC motors
- On-site troubleshooting procedures for common DC motor issues
- Troubleshooting generators
Key Takeaways:
- Correct interpretation of nameplate data is crucial for successful DC motor repair.
- Understanding magnetic principles and field properties is essential for DC motor operation.
- Proper brush neutral adjustment and interpole polarity are critical for preventing arcing.
- Magnetic field strength can be modified by adjusting turns, current, and air gaps.
- Compound fields provide both speed control and increased torque.
- Thorough assembly and testing procedures ensure smooth motor operation.
- Basic testing equipment and detailed troubleshooting procedures are necessary for diagnosing DC motor issues.
- Effective troubleshooting of generators involves checking field excitation and connections.
To read the full technical paper and view the slides, download the PDF below.
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