Tom Bishop, P.E.
Senior Technical Support Specialist
Electrical Apparatus Service Association
St. Louis, MO

The paper "Electrical Testing of Three-Phase Motors" by Tom Bishop, presented at the EASA Convention 2011, provides a comprehensive guide to electrical testing of three-phase squirrel cage induction motors in the service center. The paper covers various tests performed at different stages of motor repair, including incoming tests, post-disassembly tests, in-process tests, pre-assembly tests, and post-assembly tests. These tests are categorized as routine, optional, or conditional, depending on their necessity and the specific conditions of the motor.

Bishop begins by emphasizing the importance of safety during electrical testing, including the use of personal protective equipment, insulating electrical connections, and isolating the test area. The paper then outlines the incoming tests, which are performed before disassembling the motor. These include the insulation resistance (IR) test, polarization index (PI) test, insulation power factor test, lead-to-lead resistance test, high potential (hipot) test, surge test, single-phase rotor test, and shaft magnetization test. The IR test is crucial for assessing the condition of the winding insulation, and the PI test helps determine the cleanliness and dryness of the insulation.

The post-disassembly tests focus on identifying issues with the rotor and stator. The growler test is used to detect open rotor bars, while the bearing insulation test checks for proper insulation of the bearings. The stator core loss test assesses the condition of the insulation on the electrical steel laminations, and the surge test identifies shorted turns in the stator windings. The phase balance test ensures that the circuits and currents in the three-phase stator winding are balanced.

In-process tests are performed during the overhaul and rewind stages. After cleaning and baking, the insulation resistance and surge tests are repeated to ensure the winding insulation is in good condition. Before and after burnout, the stator core loss or loop test is conducted to assess the condition of the core. Before impregnation, the winding is subjected to insulation resistance, lead-to-lead resistance, high potential, and surge tests. After impregnation, the winding is tested again to confirm its integrity.

Pre-assembly tests include the bearing insulation test, shaft magnetization test, and insulation power factor test. These tests ensure that the motor components are in good condition before final assembly. Post-assembly tests involve repeating the insulation resistance, high potential, and surge tests to verify that the assembly process did not cause any issues. The motor is then test run under no-load conditions to assess its operating performance.

Finally, the paper discusses performance testing, including full load tests to establish benchmarks for future repairs. These tests measure electrical parameters such as voltage, current, and wattage, as well as non-electrical parameters like speed and torque.

Key Points Covered:

• Importance of safety during electrical testing
• Incoming tests: IR, PI, insulation power factor, lead-to-lead resistance, hipot, surge, single-phase rotor, shaft magnetization
• Post-disassembly tests: growler, bearing insulation, stator core loss, surge, phase balance
• In-process tests: insulation resistance, surge, high potential, stator core loss, lead-to-lead resistance
• Pre-assembly tests: bearing insulation, shaft magnetization, insulation power factor
• Post-assembly tests: insulation resistance, high potential, surge, no-load test
• Performance testing: full load tests to establish benchmarks

Key Takeaways:

• Electrical testing is crucial for assessing the condition of three-phase motors at various stages of repair.
• Safety measures must be strictly followed during testing.
• Insulation resistance and polarization index tests are essential for evaluating winding insulation.
• Growler and stator core loss tests help identify issues with the rotor and stator.
• In-process tests ensure the integrity of the winding during overhaul and rewind.
• Pre-assembly and post-assembly tests verify the condition of motor components before and after assembly.
• Performance testing establishes benchmarks for future repairs and assesses motor efficiency.

Categories: Technical topics, AC motors, Stators/windings, Testing, Motor testing

Tags: Testing

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