Energized Electric Motor Testing is an essential ingredient in any World Class maintenance program. The evaluation of acquired data should follow a set sequence. Voltage should always be evaluated first. Evaluating current is the second step. Current should be evaluated second. Current can provide a great deal of data to asset motor and driven components. From the monitoring of In-Rush start current to the modulation effects of electrical and mechanical anomalies, current is a significant parameter to monitor.

The first step in evaluating current is for balance and proper phasing. As with voltage, symmetrical phasing and balance should be observed for phase to phase and phase to neutral currents. In a distribution system, balanced voltage does not necessarily mean balanced current. Bus load balance can be affected by, power factor correction circuitry, harmonics, individual load types, and individual impedances, just to mention a few, all affect current balance. Energized motor test equipment is primarily designed for analysis of motor loading but can be utilized to conduct bus balance checks. Phase to neutral current must be observed for RMS, Peak, and Crest Factor.

Normal Current and Impedance

For analysis of motors, the load of the motor is critical for assessing current balance. Motors running at minimal load normally will exhibit unbalances that would not be within acceptable parameters. As loading on the motor increases unbalance will decrease on un-faulted motors. A good rule of thumb is, if the current and impedance data are comparable, the data should be valid, even when not operating at optimal loads above 70%. It is also useful in determining if a connection or stator fault, or parallel impedance fault is present.

Current Unbalance Caused by Defective PF Cap

As displayed on the left: Current should exhibit 120-degree phasing and balance with symmetrical distortion free time domains.

Anything that causes a voltage unbalance will also affect current. Connection anomalies, stator faults, and parallel unbalanced impedance are the most common cause of current unbalance that is non-voltage related.

Further evaluation of current must be done with a comparison with impedance data when the unit is operating at an appreciable load.

For more information on evaluating current and power quality, consider attending one of the Snell Groups electric motor training courses.  Don’t forget to enquire about our new Energized Electric Motor Data Analysis Wall Chart, a 36” x 48” laminated wealth of information, that will make energized testing fault identification a breeze.  We also have a De-Energized Electric Motor Testing Data Analysis Wall Chart available to assist with your data analysis. Read more.