Slip-Ring P. A. M. Induction Motors for Two Synchronous Speeds

A great deal has already been published about change-speed squirrel-cage induction motors using the principle of pole-amplitude modulation (P.A.M.), but it was doubtful, in the earlier stages, whether the same techniques could be applied to slip-ring machines; and whether, even if they could, there would be many useful applications. In fact, it has now been found that the latest P.A.M. designs are applicable to slip-ring machines, and a number of large slip-ring P.A.M. motors are already in service. This paper discusses those special features of slip-ring P.A.M. motors which are not possessed by squirrel-cage P.A.M. motors, and also some of the ways in which these machines can be applied.