Equivalent circuits for cage induction motors with inter-bar currents

Abstract

It is shown that generalised harmonic analysis for cage induction motors can be adapted to allow for the presence of inter-bar currents. For a balanced three-phase machine, this leads to a simple modification to the conventional per-phase equivalent circuit. This equivalent circuit is used to explore the effect that inter-bar currents can have on the performance of a typical 7.5 kW 4-pole motor. It is found that, at starting, the torque may be significantly increased in a machine with an uninsulated cage. It is demonstrated that differences in inter-bar resistance from motor to motor may be one of the causes of the variations in locked rotor torque that are observed in some designs of machine. The full-load performance of the machine is considered. It is shown that an uninsulated cage promotes the flow of harmonic rotor currents and that these have the effect of reducing full-load efficiency.