GB625030A - Improvements in means for rendering an electro responsive device responsive to load on an induction motor - Google Patents

Abstract

625,030. Control of A.C. motors. IGRANIC ELECTRIC CO., Ltd. May 17, 1946, No. 14966. Convention date, May 19, 1945. [Class 38(iii)] Means for rendering an electro-responsive device, for example a relay, responsive to the load on an induction motor, comprises saturable reactor means subjected to the rotor current of said motor for varying saturation thereof, and affording, as a function of such varying saturation, a varying impedance in a control circuit of said device, said saturable reactor means comprising means substantially suppressing undesired harmonic fluxes in said saturable reactor means, and/or undesired negative sequence saturating fluxes set up when the motor primary is subjected to unbalanced voltages. The suppression of undesired fluxes may be such that the saturable reactor means simulates the impedancevarying action of a saturable reactor supplied with direct saturating current. A relay switch 12 is adapted to operate at a pre-determined value of the rotor current of a three-phase induction motor M, in order, for example, to control transfer ,between sets of motor windings providing different speeds. The rotor' circuit comprises saturable reactors 5, 6, and 7 with variable impedance windings 5b, 6b and 7b which are connected in a single-phase circuit comprising a full-wave rectifier 11 from which the winding of the normally-closed relay is supplied. The reactors are provided with saturating windings 5a, 6a, and 7a, each of which is connected to a rotor phase winding in series with one of the star-connected resistors R1, R2, and R3. The first and other odd harmonic fluxes, except the third and its multiples which are neutralized by means of additional reactors 8, 9, and 10, are suppressed by providing like reactors in each rotor phase. No even harmonics are present unless the reactor cores possess residual magnetism. The third harmonic and its multiples are suppressed by the provisions of impedance coils 8b, 9b and 10b, which are 180 degrees out of phase with-coils 5b, 6b and 7b to which they are connected in series, saturating coils 8a, 9a and 10a being connected in series with coils 5a 6a, and 7a. Undesired negative sequence saturating fluxes, which are set up when the motor stator windings are supplied with unbalanced voltages, are suppressed by the provision of coils 5c-10c, pairs of series-connected coils being interconnected with deltaconnected resistors r<SP>1</SP>, r<SP>2</SP>, and r<SP>3</SP>. In a modification, Fig. 2 (not shown), only three reactors, each having two saturating-coils, are connected in the motor rotor circuit. These coils function selectively for high- and low-speed operation. Each reactor has also two impedance coils.