Patent GB-620890-A (GB)
Title: Improved control device for electric motors
IPC: H01H61/00
Filed: 1947-01-29
Abstract: 620,890. Control of A.C. motors; thermal switches. GENERAL MOTORS CORPORATION. Jan. 29, 1947, No. 2766. Convention date. March 28, 1946. [Classes 38(iii) and 38(v)] A control device for electric motors having main and starter windings comprises overload and starter bimetal strips operable to open and close their respective switch contacts and a third bimetal strip adapted to move the starter bimetal strip and close the starter switch contacts prior to the full energization of the motor and subsequently to be moved, consequent on a change of heating, away from the switchclosure position to an ineffective position in which it is heated in accordance with the current-flow in the main winding. As shown in Fig. 1 an electric motor 20 has a main winding 22 and a starting winding 24 which may be connected through a condenser 32 to a contact 36. Upon opening of a main switch, after the motor has been running, a bimetallic strip 54 will cool and bow towards a starter bimetallic strip 46 and move it to a closed position. Upon reclosure of the main switch both the starting and main motor windings are energized, current flowing through the starting winding, the strip 46, and from one end to the other of the strip 54, this strip being V-shaped in side elevation. Current also passes through the main motor winding and a heater 64 to the strip 54, both circuits being completed through an overload bimetallic strip 66. The strip 54 is rapidly heated and moves quickly to the position shown. The strip 46 is heated at a slower rate and moves to open the circuit of the starting winding, and to discontinue its self-heating, at the appropriate time. The strip 46 is held in the open position by a toggle spring 50. As soon as contacts 58 and 60 meet, the heater 64 and one leg of the strip 54 are shunted by the other leg of the strip, so reducing its heating rate. The contacts 58 and 60 may be opened periodically during the running period but the increased heating of the strip 54 caused thereby returns it to the contact-closed position. The strips 46 and 66 co-operate with bimetallic strips 48 and 76 respectively which compensate for ambient temperature changes. Toggle springs 50 and 78 provide for snap-action of the starter and overload bimetallic strips. In the arrangement shown in Fig. 8, upon closure of a main switch, current passes through a main motor winding 122, through both legs, one of which is shown at 157, forming a portion of a bimetallic strip 155, and an overload bimetallic strip 166. The rapid heating of the strip 155, caused by the heavy initial current, will cause it to bow towards a starter bimetallic strip 146 which is snapped into a closed position. A notch 175 limits the movement of the member 155. Closure of the starter strip completes a circuit to a motor starting winding 124 and the overload strip 166. During the starting period the current through the strip 155 is reduced sufficiently to allow it to take up a central position in the notch 175, the starter strip 146 resting against a stop 179 during the remainder of the running period. If the strip 155 is subjected to prolonged heavy currents its opposite ends will bow in different directions, since it is folded at an intermediate position therebetween, and the starter strip is not affected thereby.