Dead Points in Squirrel-Cage Motors

Certain squirrel-cage motors are known to have a wide variation in starting torque depending on the angular position of the rotpor. In extreme cases the torque may be actually negative at certain positions making the motor entirely useless. Various names such as ``dead points,'' ``cogging,'' and ``locking'' torque have been used to describe this condition but no detailed study of its mechanism appears to be available. An analysis of the causes of such behavior is presented here as one of several phenomena not covered by classical theory since they arise from the nonsinusoidal nature of the air-gap field. It is shown (1) that only with certain definite relations of slot numbers can dead points exist at all, and (2) that, given one of these vulnerable slot combinations, the amplitude of the torque variation moves over an extremely wide range as the coil span is changed. The effect of coil span is not merely that due to the numerical change of harmonic chord factor but is shown to be the result of changes of direction of the various component forces. Test results confirming the analysis are included. It now becomes possible to correlate a great many apparently divergent results and to avoid troublesome combinations in future designs.