Abstract
The performance of a three-phase squirrel cage induction motor fed by an unbalanced voltage supply has been investigated in this article. Matlab/Simulink software is used for simulation of the motor using simple (conventional) and exact models under different unbalanced voltage conditions. Simulation results are verified by performing experimental tests on a 1.5-kW standard three-phase induction motor. With the aid of simulation and experimental results, variations of stator current, motor efficiency, power factor, and rotor ripple have been investigated under various steady-state unbalanced voltage conditions. It is shown that, in addition to the voltage unbalance factor, the magnitude of positive and negative sequence components of the unbalanced supply voltage have an important role on motor performance. For instance, motor power factor decreases with the increase of positive sequence voltage component, even if the voltage unbalance factor is constant. Also, the motor efficiency decreases with the increase of the voltage unbalance factor, even if the positive sequence component of the supplied voltage is constant. It is shown that the stator current unbalance factor and motor losses will increase (decreasing efficiency) with the increase of the voltage unbalance factor under constant load conditions. Moreover, the voltage unbalance factor has a negative impact on the electromagnetic torque peak due to the decrease in the positive sequence voltage component. Using different voltage unbalance factors, the motor derating factor is also calculated, where four different limits are considered. The results imply conservative limits set by standards for motor operation under unbalanced voltage conditions. A comparison between the experimental and exact model simulation results confirmed a maximum discrepancy of 4% between the results, indicating the validity of the employed exact model for motor analysis in unbalanced voltage conditions.
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