No abstract

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.

When considering common methods of calculating the end turn reactance of concentrated modular windings, significant effects are seen due to the presence of core iron that are not accounted for in many methods. Experimental study and 3-D Finite Element work is carried out to accurately model end turn reactance. A method of accurately predicting end turn reactance with significantly reduced solving and modeling time is developed and tested against experimental results. A parameterized equation is then developed allowing the calculation of end turn reactance for any concentrated modular winding configuration within the prescribed limits. Finally, a method of reducing end turn reactance through the use of concentric coils is developed.

Linear Induction Motors commonly have the coils of each phase connected in series. The effect of this connection type is that equal currents flow throughout the phase winding, whilst the voltage across coil groups connected in series may vary significantly. As flux is proportional to voltage, the air gap flux may also significantly vary over the length of the machine. This can be an issue particularly where a conductive plate is traveling between a series of discontinuous machines at high speed, for example in electromagnetic launch systems. The plate entry into each discontinuous machine can cause significant drops in flux with a consequent large variation in thrust. With a machine using phase groups connected in parallel, the voltage is now forced whilst the current is variant. This has the effect of maintaining the air gap flux on entry and so producing a more consistent force profile, a significant benefit for high speed launch applications.

Linear synchronous motors can be used for large scale electromagnetic launch. The machines are short time rated and can therefore use substantial values of stator current loading. This leads to high values of armature flux and the risk of demagnetising the excitation magnets. In order to mitigate this risk by reducing the armature flux, quadrature axis flux barriers can be positioned in the permanent magnet backing iron. These reduce the armature field but leave the permanent magnet excitation field largely unchanged. In addition to improving the force due to the larger usable stator current loading, the barriers are beneficial in reducing the synchronous reactance and hence improving the power factor and reducing the Volt Amps per Newton of useful force (VA/N) of a linear motor. The paper explores the use of these barriers for a typical configuration and shows the performance improvements that can be obtained.