GB773866A - Improvements in control systems for induction motors of the capacitor type - Google Patents

Abstract

773,866. Control of A.C. motors. LEWUS, A J. Dec. 9, 1955, No. 35457/55. Class 38(3) A single-phase induction motor of the dual operating voltage type has a squirrel cage, rotor and main stator windings 12, 13, which are connected in parallel with each other, for low voltage operation, and in quadrature with an auxiliary stator winding 14. The windings 12, 13, are connected in series with the primary windings 19, 20, of a saturable transformer having a secondary winding 21 for supplying the auxiliary winding 14 through a capacitor 22. The primary winding sections are connected in parallel relation with respect to each other for low voltage operation. The starting current of the motor saturates the core of the transformer whereby peaked voltages are induced in the secondary winding and the auxiliary winding 14 takes a high current. The secondary circuit including the elements 14, 21, 22, is substantially series resonant during saturation of the transformer core so that the capacitor 22 effects substantial de-phasing of the starting current traversing the auxiliary winding 14 with respect to the primary current, so as to produce a high starting torque. As the motor accelerates the transformer core becomes unsaturated, the secondary voltage sinusoidal in character, and the current in the winding 14 is reduced. At the same time, the secondary circuit becomes well removed from series resonance. The saturation of the transformer core is maintained from standstill to at least half full-speed of the motor.. When the motor is cone nected to a high-voltage supply the primary circuit includes the windings 12, 13, 19, 20, in series relation, the secondary circuit remaining unchanged and its characteristics, being identical with those, for the low-voltage connection. The transformer may comprise a three-legged core member with an armature spaced therefrom by an air gap, the centre leg of the core carrying. the three windings 19, 20 and 21, Fig. 3 (not shown). A switching system may be provided having a single control member for changing-over the connections of the windings 12, 13, 19 and 20 to adapt the motor to the different values of the supply voltage, Fig. 4 (not shown).