Tom Cox

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.

End Turn Leakage Reactance of Concentrated Modular Winding Stators

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.

Superconducting Electromagnetic Launch System for Civil Aircraft

This paper considers the feasibility of different superconducting technologies for electromagnetic launch (EML) to assist civil aircraft takeoff. EML has the potential of reducing the required runway length by increasing aircraft acceleration. Expensive airport extensions to face constant air traffic growth could be avoided by allowing large aircraft to operate from short runways at small airports. The new system positively affects total aircraft noise and exhaust emissions near airports and improves overall aircraft efficiency through reducing engine design constraints. Superconducting linear synchronous motors (SCLSMs) can be exploited to deliver the required takeoff thrust with electromagnetic performance that cannot be easily achieved by conventional electrical machines. The sizing procedure of an SCLSM able to launch A320 in weight is presented. Electromagnetic and thermal aspects of the machine are taken into account, including the modeling of ac losses in superconductors and thermal insulation. The metallic high-temperature superconductor (HTS) magnesium diboride (MgB2) is used and operated at 20 K, the boiling temperature of liquid hydrogen. With modern manufacturing technology, multifilament MgB2 wires appear to be the most cost-effective solution for this application. Finally, the impact of the cryocooler efficiency on the machine performance is evaluated.

Multi layer planar concentrated windings

Planar non-overlapping concentrated windings are simple to wind and robust in operation. Since the coils may be preformed before stator construction they yield a high slot packing factor. However all the forms of the windings produce backward going fields, which can detract from their performance when used in induction machines. A novel system has recently been developed to cancel the backward going fields and produce good performance from these simple windings. Starting with this system the paper develops a number of single-sided machines using multi layered planar coils and analyzes their performance. Machines using these windings are apt for higher voltages and are efficient to construct, with savings in both labor and material costs. The winding layouts for various forms of multi layer planar machine have been outlined and the good performance of these machines has been established compared to conventional two layer windings by both 3D finite element analysis and experimental methods.

Parallel and flux forced windings in discontinuous machines

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.

Comparison between plate and wound secondaries for linear induction motors with concentrated winding primaries

Concentrated windings that use planar non overlapping coils are simple in construction with comparatively narrow end regions. This leads to linear induction motor stators that can be butted together in sections to form long stator machines for urban transport and electromagnetic launch systems. The windings however produce two components of travelling field moving in opposite directions. The response of the plate rotor to the windings is therefore poor since opposing forces will be produced by the two fields. The use of a wound secondary rather than a plate dramatically improves the response. Here substantial emfs, currents and forces are produced only by the pole number for which the secondary is wound. The wound rotor therefore selects one of the fields produced by the concentrated winding for operation. The new principle is explored by the use of Finite Element Analysis and practical tests and it is shown that good force profiles and efficiencies are possible from concentrated stator wound rotor machines.

IGBT-SiC dual fed open end winding PMSM drive

This paper proposes a dual fed common dc link inverter Open End Winding-Permanent Magnet Synchronous Motor (OEW-PMSM) Drive. In order to increase the system efficiency a dual technology converter is used, with one inverter composed of standard IGBT devices and the other composed of fast switching Silicon Carbide (SiC) devices. The common dc link OEW configuration allows the zero-sequence current (ZSC) to flow freely, and the low time constants of the zero-circuit can lead to high zero sequence current flow, with associated losses and stress on the power devices. To avoid this, the zero-sequence voltage produced by the switching combinations adopted to synthetize the control signals needs to be instantaneously eliminated. A novel modulation for dual converter configurations is proposed to eliminate the zero-sequence voltage(ZSV).

A New Method for Determining the Magnetic Properties of Solid Materials Employed in Unconventional Magnetic Circuits

To evaluate the quality of magnetic materials used in electrical machines, accurate material characterization is required. For common, solid (nonlaminated) ferromagnetic materials, characterization procedures such as the toroidal ring sample test method are capable of mapping electromagnetic properties with reasonable accuracy. This is true when the investigation is for solid materials to be used in conventional magnetic circuits, i.e., where the flux paths and induced eddy currents follow the more common “radial” characteristics, as in a standard rotating machine. When solid ferromagnetic materials are employed in unconventional machine structures, such as for transverse flux machines or tubular linear machines, classical methods are not capable of achieving an accurate representation of the flux conditions in the machine, thus resulting in inaccurate characterization data that usually underestimate the total loss prediction. In this paper, a new testing method is proposed to impose the correct flux conditions for solid materials (used in tubular linear machines) and accurately map the eddy current losses in the solid parts. The proposed method uses a simple experimental test setup to characterize the power loss of solid, ferromagnetic material. The basic experimental results from the new setup are compared to results from three-dimensional finite element analysis.

Vehicular suspension and propulsion using double sided linear induction machines

This paper presents a new method of combined electromagnetic levitation and propulsion using a double sided pair of linear induction machines and a simple conductive sheet secondary. If the supply phase angle of one primary is modified with respect to that of the other, a controllable lift force can be developed on the conductive secondary and its load at any velocity or when stationary. Further, a resolution force is developed tending to drive the secondary into the center of the air gap, meaning that the system is inherently self-stabilizing without complex position feedback or control. This effect is studied and predicted using finite element analysis and then measured and confirmed using an experimental rig.

Comparative study and optimal design of alternative PM configuration transverse flux linear machine

This paper presents the comparative study and optimal design of a transverse flux linear machine with different PM configurations, viz. surface-mounted and consequent-pole, in which the consequent-pole version is firstly proposed. Firstly, the effect of variation of the main design parameters on both topologies are studied. Then, the multi-objective optimization method based on genetic algorithm combined with response surface methodology (RSM) is adopted to realize the optimal design of these two topologies and Pareto front solutions will be obtained. Finally, the characteristics of these two topologies are analyzed and compared, with particular regard to the advantages and disadvantages of the consequent pole topology.