Introductory notes to a lecture on ¿Variable-speed alternating-current motors without commutators¿

After a short general discussion, a new method of polechahging, involving the use of a number of phases greater than that in the line, is explained, together with means for simplifying the connections:¿ (A) By the use of mutually reversed coils. (B) By a star-mesh connection. The phase transformer needed to produce the increased number of phases is next described, and it is shown that the current in the windings intermediate between the three-phase tapping points is reduced to a very low value as in the rotary converter, making the apparatus small and inexpensive. The switchgear employed is next discussed, and it is pointed out that the methods described render the squirrel-cage motor adaptable to almost all purposes, since they enable it to give good starting torque with low current. Machines with slip-ring characteristics are also described and test-results discussed. A description is given of the cascade motor, and its unique characteristic of giving gradual adjustment between speeds is explained. Two-, three- and four-speed motors having this characteristic are referred to. Some tests of large rolling-mill and other motors are given and finally a description of the application of the cascade motor as a short-circuit motor machine with slipring characteristics, and as a unity-power-factor synchronous machine of greater simplicity and less cost than any other type.