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This paper investigates consequences of the differences between the actual, practical operating conditions of in-service induction motors and the ideal operating conditions that apply for the Standard Type Test, following the IEEE 112-B method. In these tests, the significance of each factor influencing motor efficiency, including operating temperatures, temperature rise, voltage supply magnitude, voltage balance and different load conditions is assessed upon the motor performance. The preliminary analysis shows the ambient temperature has a noticeable influence on motor losses, especially on the conductor losses and thus efficiency. The actual part load efficiency is higher than catalogue part load efficiency figure when the motor is running at partial load continuously in the field. The voltage magnitude alters the balance of the five loss mechanisms for a particular load; one can adjust the supply voltage to improve the motor efficiency for a continuous constant load operation. The increase in losses under unbalanced voltages leads to the necessity of derating the motor. The most significant factor which influences the efficiency of motor is the determination of operational loading.