Squirrel-cage induction motors

Abstract
In this paper the principal features of an electric motor are discussed and the characteristics of the squirrel-cage machine are considered in relation to them. The disadvantages of the squirrel-cage motor are then examined and their importance on certain types of drive is considered. These disadvantages are mainly connected with the poor starting performance of the motor, and various methods of over coming the starting difficulties are dealt with. A centrifugal clutch is described which enables a motor with a star-delta startor to start heavy loads with comparatively small starting currents. The characteristics of high-resistance rotors for both continuously-rated and short-rated motors are discussed, and various other devices for the improvement of starting performance are reviewed. The advantages of the double-squirrel-cage rotor over many of the alternative proposals are then described.A close examination of the starting currents drawn by squirrel-cage motors under various conditions of starting is made and the subject is considered from the point of view of the supply engineer. Records of the current peaks on various feeders are given, the characteristics of every motor on each feeder being known. Additional records show the effect of the diversity factor for these current peaks on the current of the transformer supplying several feeders, and on a complete substation. Finally, the regulations of certain supply undertakings for the installation of squirrel-cage motors are examined and the following general regulation is evolved:-The starting current of any of the induction motors in an installation, when expressed in terms of full-load current, must not exceed ¾+total installed h.p./4×motor h.p.

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