Prevention of harmonic torques in squirrel cage induction motors by means of soft ferrite magnetic wedges

Abstract

It is well known that the harmonic torques in squirrel cage induction motors are caused by the space harmonic waves in air-gap flux distribution. The soft ferrite magnetic wedges inserted into the stator slot openings reduce the ripple of air-gap permeance. Therefore, this wedges reduce the magnitude of harmonic torques. A trial was performed on prevention of harmonic torques in squirrel cage induction motors, by inserting a soft ferrite magnetic wedges into its stator slot openings. Results obtained were as follows; (1) The soft ferrite magnetic wedges made it possible to decrease by about 40% the 17th and 19th asynchronous torques. (2) The magnitude of 17th and 19th asynchronous torque on the induction motor with the soft ferrite magnetic wedges and the unskewed rotor is approximately equivalent to that on the induction motor with the rotor skewed through 0.4 stator slot pitch. (3) The soft ferrite magnetic wedges were incapable of decreasing the 5th asynchronous and synchronous torque. The authors suggest that the utilization of soft ferrite magnetic wedges will become important to prevent the harmonic torques in induction motors.