Linear induction motors with modular winding primaries and wound rotor secondaries

Linear induction motors commonly use double layer windings, which produce good sinusoidal travelling fields, but have relatively bulky end windings and use either half-filled slots or overhanging coil sides at the ends of the machine. Long stator systems are difficult since it is not possible to butt stator modules against each other. Arguably the simplest and most cost effective winding uses modular construction where the coils are planar and do not overlap. Here the end winding is compact and stator sections can be butted together. However modular windings do not produce high quality travelling fields. Two space harmonics of closely the same magnitude are produced that travel in opposite directions, giving induced currents and opposing forces with little net force in plate rotors. The difficulty can be resolved if a wound secondary with a double layer winding is used instead of a plate. Here a substantial induced emf and current is produced only by the field for which the secondary is wound, so that force is produced only in one direction. The use and properties of modular windings for short rotor machines are explored using finite element analysis and the results are validated by practical tests. It is concluded that inexpensive modular windings can be used with wound secondaries to good effect particularly in long stator situations; for example, for electromagnetic launch and urban transport systems.