Patent GB-1193871-A (GB)
Title: Electrical Control Systems for the Drive Motor of an Industrial Truck
Inventor: TEDD DAVID CLIFFORD
IPC: H02P3/10
Filed: 1967-01-19
Abstract: 1,193,871. Control of D.C. motors. EATON YALE & TOWNE Inc. 4 Jan., 1968 [19 Jan., 1967], No. 2894/67. Heading H2J. A motor 14 which drives an industrial truck is provided with pulses of constant width, the speed of the motor being controlled by varying the time between the pulses. The width of the pulses is reduced automatically for restricting the motor torque during plugging. Before one pair of the field-reversing contactors 18, 19, is closed, there is no charge on capacitor 45 so that transistors 41, T3, 49, are conductive to supply gate current to rectifier 25 which does not conduct at this stage. As soon as the contactors close, capacitor 24 is charged, and capacitor 45 is charged to switch-off transistors 41, T3, 49, the rectifier 25 receiving gatecurrent through resistor 29. When the voltage across capacitor 54 exceeds a predetermined value, transistors 55, 56, conduct to render transistors 58, 61, conductive. A microswitch 65 closes when the throttle pedal is depressed beyond a certain amount so that rectifier 26 is switched-on. At the same time, the rectifier 27 is switched-on by means of the transistor 64. As soon as the rectifier 26 conducts, the capacitor 54 discharges to interrupt the gate-current to the rectifiers 26, 27, but before this current ceases, the motor is supplied through rectifier 26. Capacitor 24 discharges through rectifier 26, inductor 28, and rectifier 27. After the gatecurrents have stopped, the motor continues to be energized through the rectifier 26, and the capacitor 24 continues to transfer its charge until the rectifier 27 becomes non-conductive, the capacitor being left with a reverse charge. When the rectifier 26 conducts, capacitor 45 discharges and, after a fixed period, the transistor 41 conducts to apply a pulse to the gate of the rectifier 25. Capacitor 24 discharges to switch-off the rectifier 26, the capacitor recharging through the rectifier 25 whereby the cycle continues as before. The speed of the motor is varied by adjusting the time taken by the capacitor 54 to charge sufficiently to switch-on the transistor 55. When the throttle pedal is not depressed, potentiometer 77 is adjusted so that the collector voltage of transistor 73 is at a maximum value, the transistors 84, 87, being both off and the charging current of the capacitor 54 being limited by resistor 53. As the pedal is depressed, the capacitor 54 is additionally charged through the transistor 87 to increase the speed of the motor. Capacitor 75 controls the rate of acceleration. When the pedal is fully depressed, diode 82 conducts to increase the base current of transistor 73, so decreasing the charging-time of capacitor 75 to permit a greater rate of acceleration. A switch 83 is closed when a brake pedal is depressed to reduce the speed irrespective of the position of the throttle pedal. When the truck is plugged, diode D1 becomes reverse-biased and transistor T1 conducts whereby transistor T2 short-circuits the capacitor 75. The collector voltage of the transistor 73 is now such as to reduce the motor power to a low value by increasing the period between pulses. Release of the throttle pedal closes switch S1 and transistor T4 is turned-on to short-circuit the resistor 39. The transistor 41 is switched-on by discharge of the capacitor 45 at an earlier instant so that the pulse width is reduced.