Tom Bishop
Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "Squirrel Cage Rotor Testing" by Tom Bishop, presented at the EASA Convention 2003, provides a comprehensive guide to various methods for testing squirrel cage rotors in electric motors. The primary challenge addressed is determining whether a rotor is defective, which is crucial for service centers and end users. The paper outlines a wide range of tests that can be performed both in the service center and at the end user site, with some tests applicable to assembled motors and others requiring disassembly.
The paper begins by explaining the principles of squirrel cage rotors, likening them to the secondary winding of a transformer, with the motor stator acting as the primary winding. The rotor consists of bars and end rings, typically made of aluminum or copper alloys. The integrity of these components is vital for the rotor's performance, as any faults can lead to issues such as torque pulsations, speed fluctuations, vibration, noise, overheating, and arcing.
Visual inspection is the first step in rotor testing, where obvious faults like cracked end rings or broken bars can be identified. If no defects are found initially, the rotor should be cleaned and re-inspected. The paper also discusses the tap test, where tapping on the bars can reveal loose or broken bars, and the dye penetrant test, which uses dye to highlight cracks in the rotor components.
Temperature indicating paints can be applied to the rotor to identify areas of high resistance or open circuits based on color changes. Ultrasonic testing is another method for detecting voids in die cast end rings, similar to testing for cracks in shafts. Electrical tests, including the growler test, high current excitation, and digital low resistance ohmmeter, are also covered. These tests help identify open bars or high resistance joints by measuring the magnetic field patterns or resistance variations.
For assembled motors, the paper outlines several tests that can be performed on-site. The single phase rotor test involves applying a lower than rated voltage to the motor and manually rotating the shaft to detect current variations. The two ammeter comparison test uses identical ammeters to monitor current variations, which can indicate mechanical issues or open rotors. A stroboscope can detect speed variations caused by torque fluctuations, while vibration analysis and motor current signature analysis (MCSA) can identify rotor faults based on frequency patterns.
Stray flux measurement is another technique discussed, where a coil attached to the motor detects magnetic fields that leak to the exterior. The output is analyzed using a spectrum analyzer to identify changes in amplitude related to rotor condition.
Key Points Covered:
- Principles and construction of squirrel cage rotors
- Visual inspection and cleaning of rotors
- Tap test and dye penetrant test for detecting cracks
- Temperature indicating paints for identifying high resistance areas
- Ultrasonic testing for detecting voids in end rings
- Electrical tests including growler, high current excitation, and low resistance ohmmeter
- On-site tests for assembled motors, including single phase rotor test, two ammeter comparison, stroboscope, vibration analysis, and MCSA
- Stray flux measurement for detecting rotor faults
Key Takeaways:
- Determining rotor defects is crucial for motor performance and reliability.
- A variety of tests can be performed both in the service center and on-site to assess rotor condition.
- Visual inspection, tap test, and dye penetrant test are effective for identifying obvious faults.
- Temperature indicating paints and ultrasonic testing provide insights into high resistance areas and voids.
- Electrical tests help detect open bars and high resistance joints.
- On-site tests for assembled motors can identify rotor faults based on current variations and frequency patterns.
- Stray flux measurement is a valuable technique for detecting changes in rotor condition over time.
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