The logmotor—a cylindrical brushless variable-speed induction motor

Abstract

In a conventional induction motor the synchronous speed at the rotor surface is set by the phase difference between currents in adjacent slots and the spacing of the slots. The synchronous speed may be increased by changing the phase of the current in each slot progressively so as effectively to stretch the poles. The paper describes a transformer arrangement giving a multi-phase output, which can provide such a `phase-stretching? system to be used to supply the stator of a squirrel-cage induction motor. The transformer is similar to a conventional phase-shifting transformer, except that the pitch of each coil of the primary is proportional to the log of its distance from a fixed point in the system. The primary and secondary of the transformer are analogous to the C and D scales of a slide-rule, and movement of the primary relative to the secondary results in a variation of the phase increment between adjacent secondary coils. The secondary is connected to the stator winding of the motor in such a way as to provide a uniform-velocity field in the latter, the velocity being varied by adjusting the position of the primary of the transformer. The method of construction of an experimental machine is described and test results over a speed range of 4:1 are included.