Patent GB-1177071-A (GB)
Title: Improvements relating to D.C. Electric Motor Control Systems
Inventor: SIMCOCK JOHN HENRY
IPC: H02P3/12
Filed: 1965-10-14
Abstract: 1,177,071. Control of D.C. motors. CABLEFORM Ltd. 5 Jan., 1967 [14 Oct , 1965; 26 July, 1966; 17 Aug., 1966], Nos. 43608/65, 33672/66 and 36922/66. Heading H2J. A load circuit includes a battery, a series motor having armature 20 and field winding 21, and a main thyristor 12, whilst a braking circuit includes means for by-passing the armature from the load circuit and connecting it in parallel with resistor 75 for braking. Both the resistor and armature may be included in the part of the load circuit that is by-passed, contacts 9/1 short-circuiting the resistor during driving operation. The armature and resistor may be shunted by a diode to pass the armature current during braking. The main thyristor 12 is shunted by a commutating circuit comprising capacitor 15 in series with thyristor 16. The capacitor is charged through thyristor 18 during pulses of load current, and is discharged to divert the load current from the main thyristor to terminate such pulses. The thyristors are fired in succession to vary the duty factor. The braking contactor 9 is responsive jointly to reversal of the armature voltage and to actuation of a manually-operated contact. A transistor conducts when the armature voltage reverses to energize relay 1 having contacts 1/4 in series with the coil of contactor 9. The forward and reverse relays 3, 4, associated with field winding 21 have a preselector circuit including contacts 10/1 of relay 10 and a manual switch 50/1, 50/2. A holding circuit includes foot pedal switch 46/1, 46/2. If the reversing switch is shifted, the relay 10 which also controls the main thyristor, is not actuated until the pedal is released, whereby the reversing contactors cannot operate when substantial load current is flowing. As soon as key switch 35 is closed and the pedal depressed to energize relay 10, the contactor 9 is energized to close contact 9/1 and short-circuit resistor 75. If the motor is plugged by means of the reversing switch, the contactor 9 closes contacts 9/2. Alternatively, braking without plugging is effected by actuation of a manual switch, the contactor 9 being de-energized to by-pass the resistor 75 and armature 20 from the load circuit. The pulse generator which controls the thyristors is arranged to reduce the field current during braking while leaving it dependent upon the position of a pedal associated with a potentiometer. The tapped voltage is supplied to a controlling transistor for driving, but is attenuated by a potential divider for braking. A brake pedal may be arranged, after initial movement, to engage an accelerator pedal for increasing the mark-space ratio of the current passing through thyristor 12, and hence increase the braking torque. In another arrangement an accelerator pedal has an additional button which is optionally actuated to establish the braking circuit. The direction of rotation of the motor may be detected not by the armature polarity, but by an eddy current drag device or a D.C. generator driven by the motor when establishing the braking circuit to effect plugging. The accelerator pedal controls a potentiometer 45 whose output passes through a transistor 90 and delay network 104, 105, to a pair of Schmitt trigger transistors 101, 102. These control a switching transistor 103 so that the voltage applied to the trigger circuit varies between limits set by the potentiometer 45. The outputs of the trigger circuit are fed through transistors 111, 112, and 113, 114, to diodes 115, 116, to feed the firing circuits of the thyristor 16 and thyristors 12, 18, respectively. When the load current exceeds a predetermined value, transistor 132 cuts off and transistor 90 is controlled to limit the drive from the pulse generator.