GB978011A - Improvements in electric motors - Google Patents

Abstract

978,011. Control of A.C. motors; electric motors. E. F. D. WEBB. May 2, 1961 [May 2 1960; May 16, 1960], Nos. 15361/60 and 17292/60. Headings H2A and H2J. A reversible servomotor comprises a double polyphase induction motor unit in a common casing 1, and with star-connected wound rotors 7, 8 on a common shaft 6, which also carries a rotor rectifier network on a plate 23 and a rotor impedance 37. There is a rectifier bridge for each phase, corresponding windings 11, 17 of each rotor being connected across opposite terminals of each (Fig. 3). As shown, the rotor impedance comprises a single resistor 37, but alternatively there may be an inductor across each bridge, with centre tappings star connected (Fig. 6, not shown). As a further alternative an additional rectifier bridge is connected in place of resistor 37, its output being connected through resistors to the star point of each rotor. The stator windings are arranged to produce opposite rotation and are supplied through controllers 46, 47 from a common source. The controllers may be saturable reactors, induction regulators, gas-filled relays, grid-controlled rectifiers, or silicon or germanium rectifiers, and are adjusted, e.g. by an error signal produced in a valve positioning servo system, so that a greater voltage is applied to the stator corresponding to the desired direction of rotation than to the other. The motor receiving the lower voltage is thus driven by the other and its rotor is effectively open-circuited by the rectifiers. To produce a high torque at low speeds without increasing rotor impedance, each rotor winding may be sub-divided and arranged so that, to a voltage developed in one phase is vectorially added two smaller voltages, e.g. 58% of the first, developed in the other two phases, the polarity of one being reversed; it is shown that this produces a more favourable rotor phase angle. To produce axial movement of the output shaft, to operate a valve, the rotors may be mounted on it by an Acme thread. An electromagnetic brake, held off by the supply voltage, may be provided. According to the first Provisional Specification a single squirrel-cage motor unit produces axial movement of its output shaft against a spring, by a recirculating ball arrangement. The rotor rotates until its torque balances the spring pressure and hence the shaft comes to rest at a position determined by the applied voltage, which can be varied in any of the ways described in the Complete Specification. Two such units may be mounted in line to produce motion in either direction. Specification 978,012 is referred to.