Adjustable solid state overload relay

Claim 1

In a control device for controlling the operation of an electric motor or the like, the combination with a current supply source, of main conductors connecting the current source with the motor, means including a contactor for controlling flow of said current to the motor, means for actuating the contactor including a relay coil, an electric circuit wherein said relay coil forms an element of the same, means for sensing the current flowing to the motor through said main conductors and which form the core of respective transformers having secondary windings, condUctors connecting the secondary windings in parallel in said electric circuit, rectifying means in each secondary winding circuit for rectifying the alternating current voltages which are induced when current flows in the main conductors to the motor, thereby producing a rectified direct current voltage proportional thereto, a capacitor in said electric circuit connected in parallel with the secondary winding circuits and across which the direct current voltage is applied, a normally open switch element in connected relation with the capacitor, said normally open switch element being sensitive to voltage whereby it is caused to close when the said capacitor becomes charged to a predetermined maximum value, whereby said closing of the switch element causes the capacitor to discharge through the said switch element and through the relay coil, an arrangement of resistance elements having connected relation in parallel with the secondary winding circuits, certain of said resistance elements having minimum and maximum calibration settings respectively and all of said elements having a voltage divider in associated relation therewith whereby the resistance value thereof can be varied to thereby vary the magnitude of the current flowing in the main conductors to the motor such as will cause the said switch element to close, wherein the resistance elements and the voltage divider in associated relation therewith each comprise an adjustable potentiometer and which are located in advance of the capacitor and also the normally open switch element, whereby adjustment of the potentiometers will cause the said switch element to close on different values of current flowing in the main conductors to the motor, and wherein said potentiometers have a circuit connection such that the trip current will vary linearly with adjustment of the potentiometers while the time constant for charging the capacitor will remain substantially constant.