July 19, 2005
12 Motor Testing and CBM
Combined Voltage and Current testing
VP, Electrical Reliability Group
T-Solutions, Inc.
Voltage and Current Measurements
Voltage and current readings can be used to provide some value as to the condition of your electric motor.
Voltage readings should be measured from phase to phase, such that you measure phase A to B, then phase A to C, then phase B to C. The primary reason is that different types of transformers will provide different phase to ground values that are not as effective in determining supply condition. The values will provide over/under voltage condition as well as voltage unbalance condition. The present limits are that the over/under voltage condition must not be more than 10% from the nameplate voltage, with the recommendation, for energy and reliability purposes, not to exceed 5% from nameplate. Voltage unbalance is the % unbalance from the average to largest difference from average voltage. This value should be less than 5% with a recommended limit being 2% and a de-rating factor provided for unbalances greater than 2%. For instance, a voltage unbalance of 5% requires a derating of the motor of 25% (ie: a 10 hp motor would be rated as 7.5%) to compensate for additional heating.
Current readings provide an indication of percentage of load, when measured above 50% of nameplate current. Current unbalance is a normal course of operation in motor circuits (exception: power factor correction will cause balanced current) that results from phase circuit: Impedance; Power factor; Loading; Voltage unbalance; and, Possible motor faults. Loading using current and voltage can be estimated:
Load = (I/Ir) x (V/Vr) x 100%
Where I is the RMS average current, Ir is nameplate current,
V is RMS average voltage and Vr is nameplate voltage.
RMS current readings, by themselves, can only provide an indicator as to potential condition, but should not be counted as a pass/fail indicator by itself, due to the number of conditions that contribute to current unbalance. For instance, a small motor drawing 10.1, 7.1 and 9.9 amps would have an unbalance of 22%, whereas a motor with a current draw of 100, 87, 97 amps would have an unbalance of 8%. In this example, the first motor was a 10 horsepower motor in good condition and the second motor was a 150 horsepower motor with a developing problem.
Understanding the abilities and limitations of the tools that you have available will assist you in making the right call to correct an existing or impending problem. When using voltage and current tests only, you have some limited capabilities. Utilization of other tools, such as power analysis, electrical signature analysis or motor circuit analysis techniques can be used to verify the actual condition of the system.
