Linear induction motors for low-speed and standstill application

Abstract

SynopsisDevices which are required to produce force with virtually no movement have zero power output. The effectiveness of such devices cannot therefore be assessed in terms of their efficiency or. power/weight ratio. Quantities which are important in the design of force-producing machines or 'actuators' are the force/input, force/weight and force/cost ratios and similar quantities. Reliability is also often a potent factor. The paper examines the use of linear motors as actuators under these criteria and formulates design procedure. The optimisation of the force/input ratio for a machine whose speed is not zero, but low in comparison to that of conventional machines, is carried out with the aid of a computer program. Considerable simplification of manufacture is achieved by the use of the tubular or axial-flux form of construction. Forces in linear motors of both transverse and axial-flux types are calculated, taking end effects into account. The design of an existing commercially produced tubular actuator is discussed. List of principal symbolsa = rotor length B c = peak core flux density* B g = peak airgap flux density* B t -peak rotor leakage flux density* / = supply frequency F = force g = effective airgap G = goodness factor J m = peak stator-magnetising-current loading* J r = peak rotor current loading* J s = peak stator current loading* l r = rotor leakage inductance per square p = pole pitch P r = rotor power input s = stator length v s -synchronous velocity Z = rotor-core radius /x 0 -permeability of free space p r = rotor surface resistivity p s = stator surface resistivity a = slip T, = rotor leakage time constant co = angular supply frequency * Lower-case letters indicate instantaneous values IntroductionIn an earlier paper 1 it was shown that induction machines of high efficiency can only be designed when the surface speed of the rotor is high. Every electrical machine consists of a magnetic circuit linked with one or more electric circuits, and the relationship between the conductivity of copper and the permeability of free space determines the law relating surface speed and goodness of machine. Applications for linear motors therefore appear to be severely limited by the fact that there is no convenient linear equivalent of the gear, for many of the linear motions required by. industry are limited to speeds below lOft/s. There has been, however, a growing awareness among engineers, particularly in the last two decades, that efficiency and power factor are not the only criteria by which a machine can be judged, and that power/weight ratio and cost are often of greater significance