Analysis of a three-phase induction motor under voltage unbalance using the circle diagram method

Protective functions of an induction motor are mainly intended to prevent overheating of its windings. Owing mainly to overloads or unbalanced voltages, there is increase in consequent overheating of the windings. Three-phase voltage unbalance has long been a problem in power system. In fact, the large-scale negative sequence currents in induction motor result from slight unbalanced voltage, causing overheating, shaft vibration, noise, derating and additional losses, and hence reduce its lifetime and performance. This paper tries to analyze an induction motor supplied by unbalanced voltage utilizing the circle diagram method. Results obtains from the mathematical method are compared to validate the proposed graphical approach.