Investigating the effects of unbalanced voltages on Three-phase induction motor performance

Abstract

The purpose of this study is to examine and demonstrate the effects that unbalanced supply voltages have on the operation and performance of a three-phase induction motor. Unlike a single-phase supply, three-phase supply voltages can become unbalanced in a number of ways, whereas a balanced condition maintains constant voltage magnitude and angles in all three phases, though in practice it is impossible to have a perfectly balanced condition. Unbalanced conditions can appear in different scales and can be potentially damaging to the motor. A three-phase induction motor is designed to operate under balanced conditions, but can encounter different and complex alterations in its electrical and mechanical variables under unbalanced conditions. To understand the level of such alterations, this study compares different balanced and unbalanced conditions in terms of their electrical and mechanical responses. These responses include stator and rotor currents, electromechanical torque and rotor speed. A simulated model of a three-phase induction motor along with different software generated subsystems is used to examine these responses and the motor operation under different unbalanced conditions. The final throughput of the motor is analysed from torque-speed characteristics and derating factor curves for different unbalanced voltage conditions. A general observation is that a complete loss of a phase is much more detrimental to the motor’s performance than a certain percentage rise or drop in voltage magnitude or phase. In addition, a change in the phase angle of the voltage has been found to yield further alterations in the responses than would a change in the voltage magnitude. The quantitative relationship between voltage imbalance and its resulting electrical and mechanical responses will facilitate devising proper protection to mitigate the effects of voltage imbalance. This work would also help to develop derating factors for motors operating under various unbalanced conditions