The influence of rotor skew on cage motor losses

The die-cast rotors of small induction motors are commonly skewed to reduce the high-frequency torque ripple and acoustic noise that can arise from slotting effects. In addition, if the rotor is skewed one stator slot, the EMFs induced in the cage due to stator slotting are greatly attenuated. This results in a reduction in the high-frequency rotor Joule losses. Although skewing solves some problems it creates others. It is well known for example that skew promotes inter-bar currents and also causes an axial redistribution of the air-gap flux. Both of these effects increase losses, the former producing extra Joule loss and the latter extra iron loss. World-wide concern for the environment, in particular global-warming accelerated by the emission of gases from power stations, is leading to a growing pressure for higher efficiency electrical machines. It is estimated that the generation of electricity for three-phase induction motors produces 97 million tonnes of CO₂ annually, 17% of the United Kingdom´s total CO₂ emissions. The use of higher-efficiency motors would reduce this figure substantially. The purpose of this paper is to examine the effect that rotor skew has on iron loss in an induction motor, with a view to establishing the basis from which a cost-benefit analysis of rotor skew can be carried forward