Patent GB-660463-A (GB)
Title: Improvements in and relating to electromagnetic motor control devices
IPC: H02P1/42
Filed: 1949-01-07
Abstract: 660,463. Control of A.C. motors, electromagnetic switches. BRITISH THOMSONHOUSTON CO., Ltd. Jan. 7, 1949. [Jan. 15, 1948]. No. 514/49. Classes 38 (iii) and 38 (v). A control device for an electric motor having starting and running windings adapted to be energised from a supply circuit comprises a pivoted armature, switching means biased to a closed position associated with the armature, a. plurality of magnet coils at least one of which is series coupled to the running winding and another series coupled to the starting winding, the magnet coils being so arranged with respect to the unpivoted end of the armature as to cause the magnet coils, in response to the current flowing in the windings, to actuate the armature to cause the switching means to disconnect the starting winding of the motor. A split-phase motor has a starting winding 12 and a running winding 11 and is controlled by means of an armature 17 which is under the opposing influence of electromagnetic coils 15, 16, respectively energisable in accordance with the currents in windings 11, 12, and the running winding alone. The armature is adapted to operate contacts 22, 23, and is biased by spring 24 to a position in which the contacts are closed. The air-gap between armature 17 and core 14 is adjusted by screw 19. When switch 21 is closed to start the motor, the winding 16 is energised so as to produce sufficient flux in the narrow air-gap between members 14, to hold the armature against screw 19, the contacts 22, 23 remaining closed. The characteristic of the motor is such that, as the speed increases, the current in winding 16 falls more rapidly than that in the winding 15. At a pre-determined critical speed the pull of the coil 15 overbalances that of coil 16 and the contacts 22, 23, open to de-energise the starting winding. The coil 15 now carries only the current of the running winding but the armature is held in the running-position 21 until the switch is opened owing to the small air-gap between the members 13, 17. The modification of Fig. 4 relates to the control of a motor of which the line and running winding currents decrease at substantially the same rate with increase of speed. The core 13 has opposing coils 25, 27, the latter being connected in series with the windings 16 and 11. The coil 25 is connected through contacts 22, 23, to the starting winding 12. The resultant flux produced in the core 13 varies with the relative changes of phase of the currents in the starting and running windings. Initially, a high resultant flux is produced in the core 13 but the air-gap between the numbers 14, 17, is small and the contacts 22, 23, remain closed. As the motor speed increases, the flux in core 14 decreases slowly, and that of the member 13 at a quicker rate until it reaches substantially for when there is phase coincidence in coils 25, 27. Thereafter, the resultant flux in core 13 increases rapidly, and, at the appropriate speed, the armature 17 is moved to open contacts 22, 23. A shading coil 26 may be included to shift the phase of the flux produced by coil 25 in such a manner as to enable the selection of a wider range of tripping-out speeds of the motor starting winding.