Analysis of core losses in induction machine due to skewing

Abstract

This thesis deals with the analysis of losses due to skewing in induction machines.Normally skewing is done in induction machines to reduce noise, torque ripples and selective harmonics on the rotor cage of the machine. Skewing also can increase the effect of inter-bar currents in the machine. Due to die casting of rotor bars in the rotor at high temperatures, a very low resistance path develops between rotor bars. This resistance is called inter-bar resistance. Skewing is believed to promote these inter-bar currents and increase the stray losses happening in an induction machine.

The work was carried out using both measurements as well as simulations using finite element analysis. Two rotors were available at the Laboratory, one a skewed and another non-skewed rotor for a 37 kW induction machine. Locked rotor measurements were done by using both the rotors and the losses were calculated. Comprehensive measurements were done by changing different parameters such as frequency, torque, flux and current. A magnetic equivalent circuit and a multislice FEM model were also used to validate the results from the skewed machine. A small inter-bar resistance measurement set-up was also made. In the end all the data from both measurements and simulations were analyzed and conclusions were drawn.