Use voltage, current readings to check conditions.
Measure voltage readings phase to phase: A to B, then A to C, and B to C. Why? Because different types of transformers provide different phase-to-ground values that aren’t very effective in determining supply condition. The values will provide over/under voltage and unbalance condition. Over/under must not be more than 10% of the nameplate voltage, with the recommendation – for energy and reliability purposes – not to exceed 5%.
Unbalance is the percent from the average to largest difference from average voltage. This value should be less than 5% with a recommended limit of 2% and a derating factor for unbalances greater than 2%. For instance, a voltage unbalance of 5% requires a derating of the motor of 25% (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, possibly, motor faults. Loading using current and voltage can be estimated.
An RMS current reading, by itself, only provides an indicator for potential conditions. It 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 8% unbalance.
Understanding the abilities and limitations of available tools will assist you in making the right call to correct an existing or impending problem. When using voltage and current tests only, you have limited capabilities. Using other tools, such as power, electrical signature or motor circuit analysis, verify the actual condition of the system.
Source: MotorDoc LLC Newsletter