This process can be used with either new or repaired electric motors. Using the nameplate information of a three phase induction motor, you can obtain the current, horsepower and kVA/HP Code. These values can be used to calculate the inrush, or locked rotor, current of the motor. The ammeter and voltmeter must each have the ability to capture min/max readings. Connect the ammeter to Phase A and the voltmeter leads between Phase A and B with the ammeter set to capture max current and the voltmeter set to capture minimum voltage. Start the motor and obtain the results from both instruments.

The results are applied to the following:

LRA

Table: kVA/HP Code AC Induction Motors

Table: kVA/HP Code AC Induction Motors

Results falling outside the locked rotor current range indicates rotor bar or stator winding faults, including workmanship issues.

Howard W Penrose, Ph.D.

Howard W Penrose, Ph.D., CMRP, is the President of MotorDoc LLC, the Executive Vice President of MotorSight Corp, Treasurer of SMRP and Web Editor-in-Chief of the IEEE Dielectrics and Electrical Insulation Society. Howard has over 30 years in the electric machinery repair, design, materials, energy and systems industry and is an award-winning author. www.motordoc.com

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