Variation of additional losses at no-load and full-load for a wide range of rated power induction motors

This paper presents a comparison of losses at no load and full load of six different induction motors which rated powers are between 3 kW to 280 kW. The results show that additional core losses are strongly dependent on the load of the motor. The reason is the saturation of rotor tips which leads to an increase of the effective slot openings and the strengthening of harmonic by the induced currents in rotor cage. That leads to significant increase in additional core losses concomitant with motor load increases. For loss calculations, field-circuit and analytical methods are used. Using the FEM is presented an in-depth analysis of the phenomena at no-load and load indicating the reasons for increasing losses at the load conditions. Additionally, improved analytical method was employed to calculate the curves of stray losses versus load power. The results for no-load losses, stray losses and efficiency are compared with the measurements and good agreement was obtained for both field-circuit and analytical method. The change of stray losses versus output power was compared with formula proposed by IEC 60034-2-1 standard. The assigned values of stray load loss were found to be generally lower than average test result for motor with rated power below 18.5 kW and generally higher for motors with bigger rated power. In situations where a fixed value of stray load losses may be required the obtained curves can be used.