Induction Motor Equivalent Circuit Including the Stray Load Losses in the Machine Power Balance

The stray load losses, also known as additional losses, represent a not negligible term in the power balance of industrial induction motors and, unfortunately, these losses can not be directly measured with targeted experimental tests. As a consequence, the rated machine efficiency has to be evaluated in accordance to international standards [1, 2] that report several procedures to estimate the stray load losses. In this paper, the stray load losses due to the airgap spatial harmonics effects on the rotor squirrel cage are considered and a simplified induction motor equivalent circuit able to include them in the machine power balance is reported. In the proposed model, an additional resistance appears in series at the stator impedance. This additional parameter has been defined on the basis of power balance considerations applied to the induction motor classical theory. The complete theoretical analysis together the guidelines to measure the equivalent circuit parameters are reported in detail. Finally, the proposed approach has been experimentally verified using five total enclosed fan cooled (TEFC) induction motors.