GB1251722A - - Google Patents

Abstract

1,251,722. Control of D.C. motors; protective arrangements. LANSING BAGNALL Ltd. 12 Feb., 1970 [28 Feb., 1969], No. 10857/69. Headings H2J and H2K. A control circuit for a D.C. shunt motor of the type wherein a controllable rectifier between the field winding of the motor and a D.C. source is made intermittently conductive to supply pulses of current, is provided with a safety circuit to de-energize the field winding should the rectifier conduct for longer than a predetermined time. In the described embodiment the shunt field winding 17 of a D.C. motor 13 is controlled by a thyristor 18 which is made intermittently conductive by a conventional pulse supply circuit 24 and commutating circuit 19 ... 22. Winding 17 is supplied from a D.C. source 10 through a contactor 16 whose control coil 16a is energized through a circuit comprising normally-closed switches 26, 27 and a transistor 39 whose conductive state is determined by the voltage at a point 33, which voltage is dependent on the relative on/off periods of the thyristor 18. Normally, these periods cause charge and discharge of a capacitor 32 to provide a voltage at point 33 sufficient to break down a Zener diode 34 and so cause transistors 36, 39 to conduct and maintain the contactor 16 closed. If the period of conduction of the thyristor 18 is prolonged, the voltage at point 33 falls below the Zener breakdown voltage to cut off transistors 36, 39, de-energize the coil 16a and open contactor 16. The safety circuit may be overridden by the circuit 41, 42, closure of whose switch 41 maintains the capacitor 32 charged and so maintains the voltage at point 33 above the Zener breakdown voltage. The armature control circuit 14 may comprise starting resistors and switches or may comprise a controlled rectifier and safety arrangement generally similar to that provided for the field winding. Switch 27 is operated by the hand or foot brake of a vehicle driven by the motor.