VOLTAGE UNBALANCE IMPACT ON MOTORS

Voltage unbalance and its most severe form – single phasing cause up to 14% of motor failures. It's important to understand that maximum current unbalance isn't defined in standards for motors in the field . Voltage unbalance, which has direct impact on the operating life, reliability and characteristics of an electric motor, is defined. The National Electrical Manufacturers Association (NEMA) standard MG-1 defines the maximum voltage unbalance to be applied to an electric motor as 5%, and notes that a derating factor must be applied to a motor operating with a voltage unbalance. Voltage unbalance is relatively simple to calculate. It requires measurement of the phase-to-phase voltage of the supply to a three-phase motor. The first step is to take all three measured voltages, add them together and divide by three. This will be your Vave. Next, subtract the voltage furthest from the Vave. Change it to a positive value, then divide by Vave., and multiply by 100%. With the data, you can contruct a graph or chart of voltage unbalance derating factor from 0.0 – 1.0 and voltage unbalance (%). Applying the chart, you obtain the optimum voltage unbalance derating factor and voltage unbalance (%) This will help to determine and select a suitable motor, operating conditions and its critical parts. The voltage unbalance derating factor can be determined with the help of Technological Inheritance Coefficient through component quality factor. This coefficient is also used to find quality growth and degradation. The control limit of both quality growth and degradation is set with Technological Inheritance Coefficient, which is used to determine the voltage unbalance, failure, life and reliability threshold points. With the help of Technological Inheritance Model, you can identify the cause of failure, unbalance and misalignment as well as variations from set-points. At this point, if you see current unbalance with low-voltage unbalance, try rotating the phases, then recheck current. (Rotate phases by moving the lead from phase A to phase B, phase B to phase C and phase C to phase A. This doesn't change the direction of rotation). If upon rechecking current, the unbalance moves, it's motor, part and process-related. If the unbalance stays in the same location or disappears, it's supply-related.