Tom Bishop, P.E.
Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "Field Testing and Inspection of Electric Motors" by Tom Bishop, presented at the EASA Convention 2005, provides a comprehensive guide to the electrical testing and inspection of installed electric motors. The primary focus is on three-phase squirrel cage induction motors, but the paper also addresses wound rotor, synchronous, and direct current (DC) motors. The main purposes of testing installed motors are to assess their condition for continued service or to diagnose suspected faults.
Bishop begins by discussing routine tests such as insulation resistance (IR), polarization index (PI), and dielectric absorption (DA), which are performed with the motor at rest. These tests indicate the adequacy of the winding ground insulation and can detect insulation weakness due to moisture or contaminants. However, they do not detect internal winding defects, which require specialized tests like the surge test.
For three-phase squirrel cage induction motors, routine online tests include measuring current and voltage for each phase. Other offline tests include lead-to-lead resistance, hipot, surge test, single-phase rotor test, and mechanical tests like output shaft runout and alignment checks. Specialized online tests include motor current signature analysis, vibration analysis, partial discharge detection, thermal imaging, and noise level analysis. Conditions during startup, such as locked rotor starting current and time to accelerate to full speed, should also be checked.
Bishop emphasizes the importance of visual inspections, which vary depending on the motor enclosure. If the interior is accessible, inspections can include checking the windings, air gap, and other internal components. Tools like borescopes and mirrors can aid in these inspections.
The paper also covers testing of synchronous motors, which includes additional tests like drop testing of rotating field coils, air gap measurement, and inspection of brush and collector ring conditions. For wound rotor motors, offline tests focus on mechanical components, while online tests include measuring rotor voltage and current. The open circuit rotor test is used to detect faults in the rotor winding.
For DC motors, offline tests include drop testing of field coils and inspection of the commutator and brushes. Online tests involve measuring field and armature voltage and current. The paper also discusses the importance of checking brush neutral and interpole polarity.
Bishop concludes with case histories that illustrate common motor issues, such as motors running hot, motors failing to start high inertia loads, and motors emitting smoke. These examples highlight the importance of thorough testing and inspection to diagnose and address motor faults effectively.
Key Points Covered:
- Routine tests: insulation resistance (IR), polarization index (PI), dielectric absorption (DA)
- Specialized tests: surge test, hipot, motor current signature analysis, vibration analysis, partial discharge detection, thermal imaging, noise level analysis
- Visual inspections: checking windings, air gap, internal components
- Testing of synchronous motors: drop testing, air gap measurement, brush and collector ring inspection
- Testing of wound rotor motors: mechanical and electrical tests, open circuit rotor test
- Testing of DC motors: drop testing, commutator and brush inspection, field and armature voltage and current measurement
- Case histories: diagnosing common motor issues
Key Takeaways:
- Thorough testing and inspection are crucial for assessing motor condition and diagnosing faults.
- Routine tests like IR, PI, and DA indicate winding insulation adequacy but do not detect internal defects.
- Specialized tests and visual inspections provide a comprehensive assessment of motor condition.
- Synchronous and wound rotor motors require additional specific tests.
- Case histories illustrate the practical application of testing and inspection techniques to diagnose and address motor issues.
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