Tom Bishop, P.E.
EASA Technical Support Specialist
Electrical Apparatus Service Association, Inc.
The document "DC Troubleshooting and Repair Tips" by Tom Bishop, P.E., presented at the EASA Convention 2009, provides a comprehensive guide to the operation, troubleshooting, and repair of DC motors. It begins with an introduction to the fundamentals of DC motor operation, emphasizing the importance of understanding nameplate data and the components of DC motors, such as the armature, frame, and bearing systems. The document explains how to interpret the DC motor nameplate, including details on field strength, field current, and the implications of field weakening on motor speed.
The guide delves into DC motor theory, covering the principles of magnetism, magnetic force, and the properties of magnetic fields. It explains the construction and operation of DC motors, including the roles of the armature, field poles, interpoles, and compensating windings. The document highlights the importance of proper commutation and the factors affecting it, such as brush neutral, interpole polarity, and the impact of magnetic field strength on motor performance.
The section on disassembly and inspection provides detailed steps for dismantling a DC motor, inspecting its components, and identifying potential issues. It covers the importance of documenting the condition of the motor upon receipt, checking covers and ventilation, assessing the commutator and brushes, and evaluating bearing condition. The guide also includes a comprehensive inspection report template to ensure thorough documentation.
Testing procedures for DC motors are extensively covered, including megohmmeter tests, low-resistance bar-to-bar tests, growler tests, high-frequency bar-to-bar tests, surge comparison tests, and high potential tests. The document emphasizes the importance of these tests in identifying faults in the armature, commutator, and field coils. It also provides guidelines for performing drop tests on field coils and interpoles to ensure their proper functioning.
The assembly and final testing section outlines the steps to be taken before, during, and after assembling a DC motor. It includes verifying field and interpole polarity, checking brush grade and size, aligning brushholders, and seating brushes. The guide also explains how to establish a commutator film, set brush neutral, and perform no-load and load testing to ensure the motor operates correctly.
The troubleshooting section addresses common issues encountered with DC motors, such as the motor not starting, running at incorrect speeds, sparking at the brushes, and overheating. It provides a systematic approach to diagnosing and resolving these problems, including checking the control circuit, inspecting mechanical components, and verifying electrical connections.
Finally, the document includes a section on failure analysis, which discusses the various stresses that can affect DC motor components and the importance of identifying the root cause of failures. It provides a methodology for conducting failure analysis, including examining the appearance of the motor and system, considering application factors, and reviewing maintenance history. The guide is supplemented with numerous photographs and diagrams to illustrate common failures and their causes.
Key Points Covered:
- Fundamentals of DC motor operation and nameplate interpretation
- DC motor theory and principles of magnetism
- Disassembly and inspection procedures
- Testing methods for DC motors
- Assembly and final testing guidelines
- Troubleshooting common DC motor issues
- Failure analysis methodology
Key Takeaways:
- Understanding nameplate data is crucial for successful DC motor repair.
- Proper commutation and magnetic field strength are vital for motor performance.
- Thorough inspection and documentation are essential during disassembly.
- Comprehensive testing can identify faults in motor components.
- Correct assembly and final testing ensure reliable motor operation.
- Systematic troubleshooting can resolve common motor issues.
- Identifying the root cause of failures helps prevent future issues.
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