Patent GB-613237-A (GB)
Title: Improved overload control device for electric motors
IPC: B44D3/24
Filed: 1946-06-12
Abstract: 613,237. Temperature-responsive actuating-elements. CONSTANTINE, W. W. (General Motors Corporation). June 12, 1946, No. 17575. [A Specification was laid open to inspection under Sect. 91 of the Acts, Dec. 17, 1946.] [Class 64(ii)] [Also in Group XXXVII] Relates to control devices for electric motors in which the starting load upon the motor increases with the ambient temperature and provides means for increasing the starting period of the motor with increasing temperature. According to the invention, such a control device comprises a bi-metallic strip, each end being turned round on itself in the plane of the strip and given a further lateral bend out of that plane to provide means for attaching each strip to its anchorage and means for applying adjustable column-loading to each strip. An electric motor 20 having a main winding 22 and a phase winding 24 drives a refrigerator compressor 25 withdrawing evaporated refrigerant from an evaporator 27 and forcing the refrigerant into a condenser 29 where it is liquified and returned to the evaporator, which cools the medium within a compartment 31. An electric supply conductor is connected to the junction of the motor windings 22 and 24, the circuit being completed by a conductor 28 connected through a control switch 33 to the starting and overload control 30. A starting capacitor 23 is connected to the phase winding 24. The conductor 28 is connected through the switch 33 to a bracket 32, to which is welded one end of a coiled electric heater 36, the other end being supported by a stationary contact 40 in contact with a movable contact 42 on a column-loaded bi-metallic strip 44, which is the actuating element of the overload control. The ends of the strip 44 are bent through 180 degrees in the plane of the strip, the tips being further bent at right angles out of the plane of the strip and welded to a bracket 50 on the mounting base 34. Heating causes the strip 44 to flex away from the stationary contact 40. A screw 52 is adjusted to alter the load on the strip 44 and so vary the closing temperature for the overload contacts. Supported at one end by the main bracket 50 is a starter bi-metallic strip 60 shaped like the overload strip and provided with an adjuster screw 68. The other end of the strip is welded to a bracket 66. On the starter strip 60 is a double-ended contact 70 pressing on a starter contact member 78 when the strip is cool. The initial dimensions, type and curvature of the strip may be varied to make it trip as required. A conductor 82 connects the starter contact 78 to the capacitor 23. An electric heater 56 is connected to the bracket 66 at one end and, at the other, to a back contact member 74 connected to a conductor 88 of the main winding circuit. A compensating bi-metallic strip 121 fastened to the starter strip 60 has projections 123 bearing against the starter strip, and tends, when cool, to bow the starter strip out of engagement with the contact 78. On closing the switch 33, current flows through the bracket 32 and through the heater 36 to the contact 40 where it divides to flow in opposite directions through the overload strip 44 to the ends of the main bracket. From the main bracket 50 current flows through the starter strip 60 to the double contact 70, where it divides, part flowing to the phase winding circuit 24, the remainder through the upper portion of the strip 60 to the bracket 66 and thence through the heater 56 to the back contact member 74. Upon starting the motor, the bi-metallic strips 44 and 60 are at room temperature. As the current flows the temperature of the starter strip 60 rises until the strip moves away from the starting contact 78 and into engagement with the back contact 74. The phase winding 24 is then disconnected from the power source and shunts the heater 56 and upper half of the strip 60. The main winding current through the lower half of the strip 60 to the back contact 74 is sufficient to prevent rapid cooling of the strip 60 so that it returns gradually to the starting contact 78. By continuous make-and-break of the back contact the temperature of the strip is kept substantially constant. If the current is excessive, this make-and-break will continue until the overload strip 44 is heated to its tripping point. By adjusting the setscrew 52 it can be arranged that the overload contacts reclose after the starting strip 60 is in its initial position.