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Calculating Motor Load With A Voltmeter and Ammeter

Accurately evaluating electric motor load can be useful when troubleshooting conditions. In machines over 50% loaded this can be performed with a relatively good degree of accuracy with a True RMS voltmeter and ammeter, if more advanced instruments are not available. If you are using an ammeter or voltmeter with min/max capabilities, you can obtain peak loading, in variable loads, by taking your max readings across a full cycle of operation. Otherwise, the results are a snapshot of the point in time and cycle of operation.

Take voltage and current for each phase of the motor (Va-b; Va-c; Vb-c; Aa; Ab; Ac). Calculate the average of each:

Calculating Motor Load

Using the nameplate voltage (Vn) and current (An), the true load can be estimated as:

Calculating Motor Load

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.

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