The three-phase squirrel cage induction motor is a very robust machine that can operate over a wide range of voltage conditions; it is the workhorse of the industry. However, for optimum performance and life expectancy, the voltage supply should be a balanced three-phase, sine-wave voltage, and its magnitude should be as close to the nominal voltage stated on the motor nameplate as practical. 

Any deviations from these conditions may cause a depreciation in the motor performance and expected life, and increase energy costs. Benchmarking the service entry voltage and the motor performance may be a useful tool to assist in knowing when reasonable limits have been exceeded and that corrective action needs to be taken. 

The following detrimental conditions may alone or in combination exist at various times or continuously: 

1. Voltage variation (+/- nominal)
2. Voltage Unbalance
3. Transient voltage (surges)
4. Voltage waveform distortion (harmonics)

Voltage surges come in many forms, all of which can be devastating to an electric motor. Transient conditions, rapid bus transfer, ungrounded systems, reclosure, improperly located power factor correction (PFC) capacitors, accidental connection of a new dual-voltage motor for the wrong voltage, and lightning are all sources of damaging surges. Here, we would like to discuss two of these sources: rapid bus transfer and reclosure.