Abstract :
It has already been shown how an induction motor with a single winding of normal type can be made to give efficient and economic operation at two alternative speeds, using the new method of polyphase pole-amplitude modulation. Various detailed improvements in this method have been devised since it was first developed; but both the original method and the modifications hitherto disclosed are only directly applicable to pole-combinations in which neither pole-number is a multiple of three, or to multiple repetitions of such pole-combinations. The paper discusses further theoretical developments which enable the same principles to be applied for any speed and pole ratio between unity and 1.5; and it describes theory and design methods for 6/8-pole and 6/4-pole machines, as typical examples. For all speed ratios, the winding can be made of coils which are all identical, undivided and of standard type. Six control leads and six terminals only are required in every case, and the control gear is therefore extremely simple, and can be standardized for all speed ratios. The method is so general that, 70 years after its invention, the standard squirrel-cage induction motor has, in effect, ceased to be a single-speed machine, and has become a 2-speed machine at a trivial extra manufacturing cost.
