Estimating Torque-Speed Characteristic of Three-Phase Induction Motor Operating under Unbalance Supply

This paper presents a competent approach to explain torque-speed, torque-time and speed-time characteristic of an induction motor operating under different degree and conditions of voltage unbalance. Dependence of breaking torque and torque pulsation with condition and degree of unbalance has been revealed. It has been shown that, for the same degree of unbalance, the magnitude of breaking torque is more in case of over-voltage than in under-voltage while torque pulsation increases with increasing of degree of voltage unbalance. Further, the operating point of load has been investigated under different conditions and degree of unbalance through torque-slip characteristic. It has been revealed that, as the condition of voltage unbalance changes from over- to under-voltage, the operating point shifts towards lesser speed for fixed load characteristic. Additionally, a speed-time characteristic has been also investigated and it is found that, steady state obtained in case of over-voltage is quicker than under-voltage for fixed degree of voltage unbalance and is slightly dependent on degree of unbalance. Additionally, shortfalls of different interpretations of unbalances are also explained in this paper.