A.R.W. Broadway’s research while affiliated with University of Bristol and other places

A novel type of a.c. machine is described which has an asynchronous mode of operation but which does not rely on induced currents or losses in the rotor. It has a polyphase stator winding and a laminated rotor with `directional¿ magnetic properties, and thus closely resembles a reluctance motor. In contrast to the reluctance motor, however, two rotating main fields are produced, and the machine is self starting (without a cage winding) and runs up to a speed which is inversely proportional to the sum of the two pole numbers. It may also be run synchronously by doubly feeding the stator winding with alternating and direct current. The machine is basically a low-speed device. On theoretical grounds, it was expected to have an asynchronous performance comparable with that of a conventional squirrel-cage induction motor, but this has not been borne out in practice. It is shown, however, that there is scope for further development by making use of an axially laminated form of rotor construction.

Comments are presented on a paper by A. R. W. Broadway and M. R. Weatherley (see Engineering Index 1969 p 846). A reply by the original authors is included.

The self-cascaded machine is a single-unit version of two separate induction machines connected in cascade. In construction, however, it closely resembles a conventional induction motor. It may be run asynchronously, with resistance control if required, or synchronously without any external connections to the rotor winding. As a motor it is particularly suited to low-speed duties. With its simple and robust form of rotor construction, and the absence of slip rings or rotating diodes, it is also ideally suited for operation at high speeds as a high-frequency brushless alternator. A general description of the machine, and the underlying principles involved, are presented. A theoretical treatment is included based on doubly stator-fed operation of the machine. General performance equations are derived, and equivalent static networks are obtained for steady-state operating conditions.

The principle of pole-amplitude modulation is described in relation to pole-changing concentric windings for single-phase motors. It is shown that satisfactory performance can be achieved at two speeds, using single `main¿ and `auxiliary¿ windings which are similar to those commonly employed in single-speed fractional-horsepower motors. Speed selection is effected by means of a simple switch, and, in some cases, only three control leads are needed. The geometrical properties of pole-changing windings are discussed, and, by way of example, windings for 6/8 poles and 10/6 poles are described in detail. Test results of a typical 6/8-pole motor are included, for four different starting and running conditions, together with the appropriate switching arrangements.

The paper describes the application of the principle of pole-amplitude modulation (p.a.m.) to the well known induction frequency changer. By using a single secondary winding responsive to two fields simultaneously, the need for two separate machines is eliminated. The driving motor and the `excited¿ machine are combined in a single frame of conventional construction. A number of allied developments are described, including a single-unit cascade connection to give a step down in frequency, a single-unit machine with synchronous drive and a method of resistance control for variable-frequency output.