Patent GB-1387871-A (GB)
Title: Electrical motors having open magnetic circuits
IPC: H02K17/12
Filed: 1972-02-07
Abstract: 1387871 Control of AC motors; asynchronous electric motors ANVAR AGENCE NATIONALE DE VALORISATION DE LA RECHERCHE 7 Feb 1972 [8 Feb 1971] 5657/72 Headings H2A and H2J An asynchronous motor of the type having an open magnetic circuit, for example a linear induction motor or a rotating motor having a stator physically or magnetically extending over an arc of a circle, is arranged to run at a variable velocity by forming its stator as an inductor with at least one polyphase main winding and two single phase supplementary windings disposed respectively at or beyond opposite ends of the main winding, the supplementary windings being wound around a magnetic core of the inductor, and supplying the supplementary windings with single phase current of substantially the same frequency as one phase of the main winding supply and a variable phase relationship with respect to said one phase whereby to vary the motor velocity. Each supplementary winding produces a localized pulsating field in the motor air gap and it is stated that each particular value of phase difference between the main and supplementary winding currents reflects a particular harmonic component frequency in the air gap field and damps or absorbs the other components, so that the frequency of the reflected component predominates in the air gap to determine the motor velocity. Further supplementary windings may be provided at points along the inductor between the end windings to produce further harmonic component reflection at these points. There may be two main windings, one of which is designed to produce a constant velocity and the other a variable velocity by means of its end windings, motor operation being controlled by energizing either one or both of the main windings. A similar effect may be obtained by using a single main winding with two end windings and alternative connections for variable or constant velocity. A larger number of harmonic components may be reflected by using two wound inductors having different numbers of poles, a constant velocity winding being provided if desired. Another embodiment may comprise two main windings of different pole pitch, one winding having supplementary end windings and the other having its pole pitch calculated so that it is effective only during velocity regulation, when it improves the magnetic field configuration produced by the other main winding and amplifies the action of the end windings of this main winding. During constant velocity operation the two main windings may be used to impart a desired shape, for example pure sinusoidal, to the motor magnetic field. Fig. 1 shows the inductor 1 of a linear induction motor having a main winding (not shown) supplied from a transformer 2 via a variable autotransformer 3 and switches (unreferenced). The supplementary end windings 6, 7 are fed with a single phases supply via a conventional phase changer 4, e.g. a rotating field transformer, a variable autotransformer 5 and switches (unreferenced). A phase meter 8 enables the phase of the supply to windings 6, 7 to be varied relative to the phase U of the transformer 2. Phase changer 4 may be fed from a separate auxiliary supply. Two different winding arrangements are described for a three-pole, three-phase linear motor,. Figs. 2 and 3 (not shown) and two examples are given of suitable values of inductor dimensions and number, winding arrangement, operating voltage, frequency, current and power factor and phase shift, the rotor being constitued by an endless strip of conductive material.