Abstract
This document reviews the current state of the art in the linear machine technology. First, the recent advancements in linear induction, switched reluctance and permanent magnet machines are presented. The ladder slit secondary configuration is identified as an interesting configuration for linear induction machines. In the case of switched reluctance machines, the mutually-coupled configuration has been found to equate the thrust capability of conventional permanent magnet machines. The capabilities of the so called linear primary permanent magnet, viz. switched-flux, flux-reversal, doubly-salient and vernier machines are presented afterwards. A guide of different options to enhance several characteristics of linear machines is also listed. A qualitative comparison of the capabilities of linear primary permanent magnet machines is given later, where linear vernier and switched-flux machines are identified as the most interesting configurations for long stroke applications. In order to demonstrate the validity of the presented comparison, three machines are selected from the literature, and their capabilities are compared under the same conditions to a conventional linear permanent magnet machine. It is found that the flux-reversal machines suffer from a very poor power factor, whereas the thrust capability of both vernier and switched-flux machines is confirmed. However, the overload capability of these machines is found to be substantially lower than the one from the conventional machine. Finally, some different research topics are identified and suggested for each type of machine.
Highlights
Linear Machines (LMs) are electromechanical devices that, the same way as the Rotary Machines (RMs), convert electrical energy into mechanical energy and vice versa
The power factor of Linear Primary Permanent Magnet Machine (LPPM) is very low when compared to the conventional machine, especially in the case of the Linear Flux-Reversal Permanent Magnet Machines (LFRPMs) and the Linear Vernier Permanent Magnet Machines (LVPMs)
As a direct comparison of the thrust capability and power factor between Linear Switched-Flux Permanent Magnet Machines (LSFPMs), LFRPMs, LVPMs and conventional Linear Permanent Magnet Synchronous Machines (LPMSMs) has not been found in the literature, there are some interesting facts that can be concluded from the performance comparison
Summary
Linear Machines (LMs) are electromechanical devices that, the same way as the Rotary Machines (RMs), convert electrical energy into mechanical energy and vice versa. As the magnetic circuit has a start and an end in the edges of LMs, a new phenomenon called end effect appears in these machines (see Fig. 2 and Fig. 3) This effect produces some undesirable behaviours in linear motors, and that is why it has been subject of research for a long time [6]. It happens because the flux escapes from the ends of the machine, reducing the flux density in the air-gap of these regions. When comparing the thrust of the selected machines, it is found that a poor overload capability may limit the applications of some of the machines with a high rated thrust density.
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