GB638105A - Improvements in current limiter for electrically driven machine tools - Google Patents

Abstract

638,105. Portable drilling-machines. DUDLEY, F. E. Oct. 24, 1947, No. 28498. Convention date, Oct. 31, 1945. [Class 83 (iii)] [Also in Group XXXVII] A machine-tool driven by an electric motor is provided with a current or torque limiting device comprising means wherein a voltage proportional to the motor current and a constant direct voltage are compared, the motor being de-energized in response to a predetermined difference between the voltages. A machine-tool 10, e.g. driving an expander for use with condenser tubes, is provided with a normally open switch 86 attached thereto, the collar 26 moving to the left, when work is commenced, to close the switch to apply a constant positive potential to the anode of a gas-filled triode 66 through the operating coil of relay 68. When the torque, and hence the current in the motor, exceeds a pre-arranged amount, the positive voltage peaks produced in the secondary winding 42 exceed the constant reference voltage, determined by a potentiometer 46, causing the valve 58 to conduct, thus driving the grid of the valve 66 positive so that it conducts continuously to operate relay 68 to de-energize the motor. Initial retraction of the machine-tool 10 allows the collar 26 to move to the right to open switch 86, thus re-energizing the motor, reversing means being included with the tool so that during retraction the tool rotates in the reverse direction. The value of torque at which the motor is de-energized is adjustable by means of the potentiometer 46, and the effect of transient conditions in the motor current is obviated by the filter circuit 62, 64. To maintain the same operating characteristics should the valve 66 be renewed a testing arrangement is provided by moving a switch 50 to its lower position to allow a prearranged current to flow through the valve 58, determined by the resistor 80, so that the bias on the grid of the valve 66 assumes a standard value, the resistor 72 then being adjusted until the relay 68 just operates. A more positive indication of the relay operation is assured by the use of a capacitor 78 which alternately charges and then discharges through the relay operating coil when the valve 66 conducts, thus giving the relay a vibratory movement. The system may operate with either A.C. or D.C.