Patent GB-662703-A (GB)
Title: Improved control device for electric motors
IPC: H02P1/44
Filed: 1949-07-02
Abstract: 662,703. Control of A.C. motors; switches partly electromagnetic; automatic circuitbreakers. GENERAL MOTORS CORPORATION. July 2, 1949. [July 20, 1948] No. 17536/49. Classes 38 (iii) and 38 (v). A control device for an electric motor having main and phase windings comprises a bimetal strip arranged to be heated simultaneously with energisation of the phase winding to move the strip from a starting to a running position, a switch controlled by the movement of the strip from one position to another for controlling the energisation of the phase winding, and an electromagnetic device connected in circuit with the motor for controlling the return of the switch means from the running position to the starting position. An electric motor 20 has running winding 22 and starting winding 24 which may be connected in series with a capacitor 32. A control device for the motor comprises a U-shaped bimetallic member combining starter and overload strips 46, 66, associated with compensator strips 48, 76, and toggle springs 50, 78, to produce snap-action circuit closure and opening. When control switch 28 is open the starting strip 46 will normally be biased to the open position against stop 68. Upon closure of the main switch a heavy current passes through main winding 22 and electromagnet 58 which attracts armature 54: and moves the strip 46 to the closed position to start the motor. The current passing through the starting winding 24 heats the starter strip 46 which tends to move to the open position against the combined force of the toggle spring 50 and the electromagnet 58. The opposing force of the electromagnet 58 increases with the starting load of the motor, and, since the starting winding current does not change greatly until the motor approaches its normal running speed, the starting period is prolonged as the load increases. When the motor approaches its operative speed, the starting winding current increases rapidly to open the starting contacts 36, 47. The toggle spring 78 is arranged to prevent excessive rise of temperature of the motor in the event of a prolonged overload during which the overload strip 66 may be successively tripped and re-set. In a modification, Figs. 12, 13 (not shown), the starter strip is urged to the closed position when cool, the armature of the electromagnet having a hook-shaped end adapted to be engaged by a latch member carried on the movable portion of the starter strip. When the main switch is closed, the electromagnet is energised, its armature lifted, and the latch member is released to allow closure of the starting strip under the force of its toggle spring. When the starter strip moves to the open position the latch member raises the hooked end of the armature, which has fallen due to decrease of current passing through the coil of the electromagnet, and the starter strip is held in the open position during the running period. Figs. 14, 15 (not shown), illustrate another form of the hook-shaped armature of the electromagnet. A soft ironarmature may be fastened to the movable portion of the starter strip, the electromagnet not having sufficient force to pull the strip to the open position but being capable of holding it in that position after the starting period, Figs. 16, 17 (not shown). The electromagnet may be de-energised when the starting contacts are closed, Fig. 18 (not shown), and energised to retain the starter strip when the contacts are opened. Fig. 19 (not shown).