A Design Approach to the Analysis of Rotor Slip Dominance in Torque Production during Low Slip Motoring.

Abstract

The study beams more light on torque production at low slip frequency, where the effect of rotor resistance clearly overrides that of the rotor leakage reactance. It is widely admitted that the cross-sectional area (CSA) of the rotor bar has the major design influence on the rotor resistance (R2 ) at this slip region. Besides, the low slip approximation of the developed torque equation tends to reveal that at a given constant voltage and frequency, the only parameter by which the squirrel cage induction motor (SCIM) designer could significantly influence torque development for a given load torque, is the R2 . But it is also widely agreed that torque is operationally dependent on slip, and that the slip at which a given torque is produced rises with R2 as the slope of the torque speed curve decreases; and this seems to imply the flow of more current at a given operating point. Further, it is also true that this current rise ought to be limited by this increase in R2 from the design or redesign of the rotor bar. There seems to be a subtle conflict between the individual influence of rotor slip and R2 as far as torque is concerned. However, this study shows that the superior rate of change of slip, relative to that of R2 in their respective responses to a design modification in bar CSA, largely supports the fact that slip will always dominate in influence on torque at the high-speed motoring mode of the SCIM.