Paper 6: Application of the Linear Induction Motor to High-Speed Transport

Abstract

High-speed ground transport (150 m/s) to be effective over limited distances also requires high acceleration. Were the power required to overcome air resistance and achieve adequate acceleration provided by conventional means, bulk and mass would be excessive. The linear induction motor offers considerable advantage at high speeds, it is not limited by centrifugal force and is independent of adhesion. A high overhead rating is possible as heat is left behind in the reaction rail. Output of a double-sided machine may be expected to reach 7 lb/in2 (44 500 N/m2) of effective pole face and the system is equally effective in braking. The principle of operation is described. To be efficient the current which is ‘in phase’ with the applied voltage must be made as large as possible compared with the magnetizing current, operation must be at low values of slip and the surface speed of the travelling field should be high. Thus a quantity known as the ‘goodness’ of a machine may be described by the expression where μ0 is permeability of free space; ρr is surface resistivity of rotor; p, pole pitch; g, total effective air gap in the magnetic circuit; and Vs is surface speed of travelling field = synchronous speed of vehicle. Because of the simplicity of the construction, g is necessarily larger than would be the case for a conventional machine and ρr is less favourable. However, these can be compensated for by increasing p with a suitable adjustment of frequency and the effect of the term Vs is such that the linear motor becomes better the higher the speed at which it is required to operate. Tests on a linear motor at Gorton are described and some of the mechanical problems of applying linear propulsion to railways are discussed.