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How to determine rotor bar current

Publication date: 
August 2006
Chuck Yung, EASA Senior Technical Support Specialist

Occasionally, someone asks how much current a squirrel cage rotor bar carries. That's an interesting question, and the answer depends on several factors. The rotor kVA* of a wound rotor motor is typically about 0.8 times the stator kVA. The rotor rated voltage is open circuit - a condition than cannot exist in a functional squirrel cage rotor and the amps are at rated-load; the two don't "coincide," thus the 0.8 factor. For a squirrel-cage induction rotor, a multiplier of 0.96 is used because the magnetizing current comes from the stator rather than from the rotor. Since the rotor is the secondary winding to the primary stator winding, the turns ratio of the stator to the rotor can be calculated if we know the stator winding data and the number of rotor bars.

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