A split-phase induction squirrel-cage motor will not start and come up to speed, even though the rated voltage and frequency are applied. Assuming that the motor hums without rotating, which of the following troubles would MOST likely be suspected?
• Split-phase induction squirrel-cage motors and how they start (start winding vs. run winding) • The purpose and operation of the centrifugal switch in starting circuits • Typical symptoms of open vs. shorted windings and rotor bar faults
• Think about what normally gets a split-phase motor turning from a standstill, and what would happen if that part of the circuit failed. • Ask yourself: if the motor is humming but not rotating, does that sound more like a starting problem, a running problem, or a protection problem? • Consider which fault would prevent starting, but still allow the motor to draw current and hum when power is applied.
• Identify which components are in series with the start winding and how their failure (open or short) would affect starting torque. • Recall what a shorted rotor bar typically does to motor performance (starting vs. running, noise, vibration, heating). • Verify which fault condition would leave the motor energized (humming) yet unable to develop enough torque to begin rotation.
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