Patent GB-900196-A (GB)
Title: Improvements in electric control devices
IPC: H02H7/09
Filed: 1960-02-12
Abstract: 900,196. Control of A.C. motors. BROOKE, W., and HOWE, H. Feb. 12, 1960, No. 5011/60. Class 38 (3). [Also in Group XXXIX] A starter and protection equipment for a single or three-phase motor load comprises a transductor of the kind described in Specification 864,714, the transductor control windings being connected through normally-open "start" contacts to the supply side of the transductor and through normally-closed " stop " contacts to the load side, both circuits being arranged either in series aiding relationship or in parallel and including respective full-wave rectifiers. As shown, in Fig. 1, a single-phase supply L1, N is applied over isolating contacts V1, V2 and the main windings PA1-2, PB1-2 of a transductor C to a motor load M, the impedance of the main windings being lowered to permit the motor to start and run by energizing control windings NA1-2, NB1-2. When a start contact K is closed, this control energization is effected from the supply side of the transductor by way of a transformer E and fullwave rectifier F. As the motor speeds up the voltage on the load side of the transductor increases and the energization is substantially augmented by the series-applied output from a transformer G and full-wave rectifier G. When the motor is up to speed the start contact K is opened and energization from the load side is maintained over a shunt path including a rectifier u. The energization is interrupted completely when " stop " contacts J are opened, and due to the rise in impedance of the transductor and the reduction of voltage on the load side the motor cannot be restarted except by closing the " start " contacts. Overload protection is provided by a relay I the contacts 43, 44 of which interrupt the energization when opened. An auxiliary contact V4 which opens before the contacts V1, V2 is included on the isolating sintel and is connected in the energization circuits, the earlier opening of this auxiliary contact causing the excitation to be interrupted and the impedance of the transductor to rise to a high value before the main supply is ruptured. Inching of the motor may be effected by moving switch Y so that the load side rectifier is shunted by an adjustable resistor Rp. In this position the load side voltage is unable to take over the transductor excitation, and movement of the motor is then under the complete control of the " start " contacts. In a modification, Fig. 2, the control windings at terminals 6, 8 and 7, 9 are energized by parallel connected supply side and load side transformer and rectifier circuits E, F and G, H. A further modification, Fig. 3 (not shown), involves the use of a separate transductor in each line wire, the respective control windings being connected in series. In a three-phase system, Fig. 4, a separate transductor C, Ca, Cb is provided in each line wire, and the supply side and load side energizations are provided by respective three-phase transformers and rectifiers E, Fa and G, Ha. These energizations may act in series aiding relationship in the series-connected control winding circuits as shown or in parallel, Fig. 5 (not shown). It is stated that the arrangement has the inherent property of providing protection against single-phase operation of a three-phase motor due to an opencircuited line wire and against short-circuit faults on the load side, since reduction of voltage in one or more phases of transformer G reduces the transductor excitation. Modifications include obtaining the supply side energization from across the main windings, either directly from terminals 4, 5 or indirectly from terminals 10, 11 of a transformer coupled winding, Figs. 1 and 4 and Figs. 6 to 9 (not shown).