Inter-bar currents in the rotor of large three-phase cage induction motors: Analysis and detection

The analysis and detection of inter-bar currents flowing in the rotor of large skewed cage induction machines are presented. A modeling methodology based on finite-elements and circuit equations is used to calculate these currents. Depending on the number of transversal inter-bar paths the losses produced by the currents flowing through them can be very significant. The effects of these currents on the armature current harmonic spectrum is presented and analyzed. The average torque versus speed curve show these effects in the region of acceleration, while the average torque in the operating region is not affected. At low speeds the losses due to inter-bar currents are even larger than the shaft power while they are only slightly larger than 2.5% of the apparent power at the machine terminals. Being a phenomenon that can be present immediately after manufacturing due to failure in the fabrication process or can appear due to aging it is not easily early detected. In spite of possibly of causing serious rotor damage, its effects are not so evident over its rated torque and power as will be shown.