Pole-change windings for linear induction motors

Pole-change techniques are potentially valuable for the speed control of linear induction motors. However, they are more difficult to apply in the linear case because of the nonuniform nature of the airgap flux. This precludes the use of parallel paths which are normally employed to reduce the complexity of the switching. The paper describes a new winding connection technique, and shows how it can reduce the number of switch contacts required. The method requires the series connection of alternate coils to form one winding; the remaining coils form a second. Parallel connections are then made between these two windings. An analysis is presented that gives the currents in any general set of serially connected coil groups connected to independent voltage sources. From this, the complete electrical and mechanical performance of the machine can be calculated. The technique can deal with any asymmetric windings having parallel paths, and, while it is ideally suited to the machines under consideration, it has other potential applications. The work is supported by a selection of experimental results that were taken from a pole-change linear motor driving a disc secondary.