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:

Vave = (Vab + Vac + Vbc)/3

Aave = (Aa + Ab + Ac)/3

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

%Load = (Vave/Vn)x(Aave/An) x 100%

For more motor diagnostics tech tips, see the Motor Diagnostics and Motor Health News eZine at http://www.motordoc.com/subscribe.htm. January tips on p. 9