Speed-changing induction motors: reduction of pole number by sinusoidal pole-amplitude modulation

Previous papers on pole-amplitude modulation have described methods of increasing the pole number and reducing the speed, by modulation. The reciprocal possibility has already been pointed out; and it has now been found that, under some conditions, it is better to use modulation to reduce the pole number and increase the speed. The particular form of modulation used in the examples described here gives exceptionally good performance. By way of example, test results are recorded for a small laboratory machine, and for a very large machine made by an industrial company, both for 10/8 poles, and both being very successful. The general conclusions are drawn that, for very many purposes, a 10/8-pole motor is to be preferred to an 8/10-pole one; and that modulation to reduce the pole number of an electrical machine has important industrial possibilities.