Low-Inertia Induction Motors

This paper describes the design and performance of a new type of induction motor capable of extremely fast starting and reversing. Several experimental 3-phase 60-cycle motors are described, with the best unit having a linear-torque speed characteristic, a maximum horsepower (hp) of 0.175, a starting time of 0.015 second, and a reversal time of 0.026 second. These times compare to 0.039 and 0.053 second respectively for an experimental low-inertia 3-phase 60-cycle motor of conventional construction but designed for fast response, and to 0.21 and 0.38 second respectively for a commercial 1/4-hp 3-phase 60-cycle motor. The motor dimensions are shown in Fig. 1. This performance is achieved by substituting for the conventional design of a squirrel-cage rotor a new 2-part rotor consisting of an outer low-inertia hollow rotor constructed in the form of a thin magnesium sleeve that surrounds the iron core needed to complete the magnetic circuit efficiently. In effect this arrangement serves to separate the major torque-producing part of the rotor, embodied in the magnesium sleeve, from the large inertia-contributing part, the iron core, by assembling the core as a free-running part on the magnesium sleeve shaft. Bars of soft magnetic material are inserted in the magnesium sleeve so that the effective air gap between the stator and the inner core is comparable to the air gap of a conventional squirrel-cage rotor.