Double and Triple Squirrel Cages for Polyphase Induction Motors [includes discussion]

This paper presents a lumped circuit method of calculating the performance of multiple squirrel-cage motors. For convenience, the squirrel cage is represented by three parallel circuits, in place of the two circuits conventionally employed. In this way, allowance is made for eddy currents within the separate bars, as well as the division of currents between bars, with a minimum of mathematical analysis. The method permits convenient allowance for saturation effects, which change the reactances of the various circuit elements over the speed and current range of the motor. As an example, the method is applied to calculating the performance of a simple rectangular bar. The results are shown to compare favorably with those given by the conventional skin effect theory. As another example, the method is used to calculate the speed torque and current curves of a 4-pole 75-horsepower motor, with a cast aluminum double squirrel-cage winding.