Improvement of Indirect Rotor Flux Orientation Control of Induction Motors with Skewed Rotor Bars

This paper focuses on a simple method of achieving optimal transient torque responses in indirect rotor flux orientation (IRFO) control of induction motors having skewed rotor bars. When the rotor bars are skewed, the q-axis stator current component produces its own field which is in the direction of the rotor field due the d-axis stator current component in one-half rotor, but in opposition to the rotor field in the other half. The distortion due to iron saturation causes the loss of both rotor flux, and, consequently, torque when an abrupt load is applied and/or a rapid speed change is required. The magnetizing rotor current has to be increased to compensate the flux loss and to improve the drive dynamics. The proposed method consists in a simple correction to the standard slip frequency calculator of the IRFO control scheme. Experimental results are presented to show the loss of flux and torque due to the skew effect and the effectiveness of the corrected slip frequency calculator.