GB1489997A - Speed control of squirrel-cage motors - Google Patents

Abstract

1489997 Control of A.C. motors GRUNDFOS AS 20 Mareh 1975 [25 March 1974] 11702/75 Heading H2J A three-phase squirrel cage motor has a set of main stator windings H1, H2, H3 whose ends are connected by terminals 4, 5, 6 to a three-phase supply RST and by terminals 1, 2, 3 to the terminals 10, 11, 12 of a set of additional stator windings T1, T2, T3 having terminals 7, 8, 9 at their free ends, the terminals 1 ... 12 being interconnectible by removable links 2, 3 whereby the windings H1, H2, H3, T1, T2, T3 may be sequentially connected in five different arrangements to give progressively increasing motor speeds, the first arrangement consisting of a star connected to the supply RST and having a pair of series-connected main and additional windings in each limb, Fig. 1 (not shown); the second of a delta connection of windings T1, T2, T3 connected to the supply RST through windings H1, H2, H3, Fig. 2 (not shown); the third of a star of all six windings with only windings H1, H2, H3 connected to the supply, Fig. 3 (not shown); the fourth of a delta connected to the supply and having a pair of series-connected main and additional windings in each limb, Fig. 4 (not shown); and the fifth of a delta connection of windings H1, H2, H3 connected to the supply, with the free ends of windings T1, T2, T3 left unconnected, Fig. 5 (not shown). By providing the motor rotor with an external resistance, e.g. a starting resistance, each arrangement of windings may provide an approximately linear torque-speed characteristic, the intersections of these characteristics with the load characteristic of a pump or ventilator driven by the motor producing five sequentially increasing speeds corresponding to the five arrangements, Fig. 7 (not shown). The windings in each set are of equal impedance and, by making the impedance of each additional winding less than twice that of a main winding, the fourth winding arrangement carries more current than the third to increase the torque for a given speed of the motor.