Patent GB-1456588-A (GB)
Title: Variable duty cycle electric traction motor control
IPC: H02P3/08
Filed: 1974-04-05
Abstract: 1456588 DC motor control ALLIS-CHALMERS CORP 5 April 1974 [9 April 1973] 15252/74 Heading H2J Motor control apparatus comprises a reversible series wound D.C. motor A and a power source BATT, the two being connected by a transistor power switch PS via forward or reverse contactors F, R, a source of control pulses to turn the power switch on-and-off, a regulating circuit which selectively varies the duty cycle of the control pulses and a diode D2 in shunt with the armature, wherein an armature current sensor RH operates when a predetermined current flows through the diode D2 and a direction change sensing circuit-operates in response to contactor change-over, the. apparatus further comprising a frequency changing circuit which reduces the frequency of the control pulses during a plugging operation to eliminate jolts of the driven vehicle owing to "hard" plugging, the charging circuit only being operable when both the armature current sensor and the direction change circuit operate. Trigger pulses at a constant frequency, e.g. 300 Hz, are derived from the oscillator and each pulse sets a bistable gate circuit in box PWM to start a control pulse, and also activates a variable delay circuit VDS which, under the control of an accelerator pedal on the vehicle FP produces a further pulse at a later time to re-set the bistable gate and terminate the control pulse. A manual switch DS on the vehicle may be moved e.g. from contact DS1 via the neutral position shown, to contact DS2 which reverses the supply to the field FLD and armature A of the motor. In this event, transistor Q19 will be turned off at the time switch DS is in its neutral position and transistors Q21 and Q20 will also be turned off if the pedal FP is relaxed. Prior to this operation, capacitor C17 has its left-hand plate held at + 5 volts and its right-hand plate clamped to earth through transistor Q20. The switching off of Q20 withdraws this clamping and the switching off of Q19 allows the capacitor to discharge via positive terminal of the battery BATT. When the switch DS is returned to the other terminal and the pedal FP is depressed transistors Q 19 -Q 21  become conductive again and a charging current flows into C17 which turns on transistors Q 17  and Q 18 . If the armature current sensor sensor (reed relay RR, RR1) has also responded, a logic signal on line 40 will latch the transistors Q 17 - Q 18  in the on state and this will turn off transistor Q 1  on the oscillator. This increases the charging time of capacitor C2 and thereby reduces the oscillator frequency to the range 8-30 Hz depending upon the setting of potentiometer PLUG POT. The reduced frequency and thereby the reduced mark-space ratio of the field current makes for smooth plugging. Switch LS operated by foot pedal FP may be replaced by a transistor (Q25) responsive to the position of switch DS to be "off" when it is moved to the neutral position and "on" in the forward or reverse position, Fig. 2 (not shown). The switch PS and its control members are the subject of Specifications 1,448,496 and 1,448,497.