Induction motors are the commonly used motors in power industries, due to its low price, maintenance and easy to control, it consumes about 40% of the electrical energy all over the world. It is very common to find problems of power quality in electric supply systems, among of these problems unbalanced phase voltage and under / over voltage deviation. The developments in the power electronics field have led to an ever-increasing use of static switching devices to control the torque and speed of ac motors. Invariably, the output voltage and current waveforms of these devices are harmonically distorted in shape due to the existence of numerous harmonic orders; they primarily affect the performance operation of induction motors. Symmetrical component approach is adopted to estimate the performance of a three phase induction motor operating with unbalanced supply system or other factors. Further MATLAB/SIMULINK has been used simultaneously for simulation purpose. A new approach is proposed to de-rate the motor operating with these external factors in order to safe induction motors and increase its life time. This paper, therefore, presents a comprehensive study the influence of these external factors on induction motors behavior. A comparative study of induction motor de-rating factors is presented.