Patent GB-1263261-A (GB)
Title: Improvements in electrically operated drive systems
Inventor: GURWICZ DAVID
IPC: H02P5/68
Filed: 1968-02-06
Abstract: 1,263,261. S.C.R. circuits. SEVCON ENG. Ltd., and RANSOMES SIMS & JEFFERIES Ltd. 5 Feb., 1969 [6 Feb., 1968], No. 5924/68. Heading H3T. [Also in Division H2] A traction system comprises series motors 1, 2 supplied from battery 3 through thyristors 4, 5 whose gates are connected through pulse delay circuits 8, 9 to a master oscillator 10. A speed controller 11 comprising an accelerator pedal varies the output pulses from the oscillators, whilst a pulse width regulator 12 actuates a commutating circuit 13 to switch off the main thyristors. A potentiometer 14 which is operated by a steering wheel changes the delay of either circuit 8 or 9 whilst the delay of the other circuit remains constant. Depression of the pedal increases the frequency of pulses supplied to the main thyristors whilst rotation of the steering wheel increases the delay of pulses supplied to one of the thyristors. The circuit 13 commutates the thyristors simultaneously so that a delay caused by circuit 8 or 9 reduces the mean current fed to the motor which drives the inside wheel when the vehicle moves in a curved path. The frequency control effected by the oscillator 10 may be replaced by variable pulse width control. Closure of key switch 23 charges capacitor 18 which is reverse charged when a pulse from the master oscillator triggers thyristor 22. Thyristors 4, 5, are commutated by firing thyristor 17 from circuit 12. The capacitor 18 is discharged through the battery and then forward charged in excess of the battery voltage through the motors, diodes 15, 19, inductor 16 and thyristor 17. The latter then becomes non-conductive and the cycle is repeated by firing the thyristor 22 and the main thyristors.  Referring to the master oscillator, the potentiometer 11 controls the rate of charge of a  capacitor forming part of a relaxation oscillator associated with a transformer for triggering the thyristor 22, as well as being connected to the circuits 8 and 9. This capacitor is not charged until the capacitor 18 is fully forward charged, Fig. 3 (not shown). Referring to the pulse width regulator, when thyristor 22 is fired to reverse the charge on capacitor 18, a pair of transistors are made non- conductive to allow charging of a timing capacitor until a further transistor conducts to discharge the capacitor. A pulse is then applied through a transformer to trigger the thyristor 17, Fig. 4 (not shown).  Each delay circuit comprises a multi-vibrator which is triggered by the master oscillator and which is controlled by the charging of a capacitor associated with a timing resistor. The main thyristor is fired a predetermined period after the arrival of the initiation pulse. The timing resistor has its effective impedance varied by a shunt transistor associated with a non-linear network and controlled by the steering potentiometer. The pulse delay effected by the circuit 8 or 9 is thus a non-linear function of the steering angle, Fig. 5 (not shown). In modifications, the motors may drive the wheels of fork lift truck and an hydraulic pump for auxiliary services of the vehicle respectively, or the motors may be coupled to tracks on opposite sides of a vehicle. Field control instead of armature control may be provided. To avoid overloading the main thyristors 4, 5, the current therethrough is limited. Pick-up coils are disposed adjacent the thyristors and connected in phase opposition to the input of a full wave rectifier the output of which is connected in series with a further coil responsive to the battery current. These coils are associated with a current limit detector for reducing the frequency of the master oscillator when the output of the detector exceeds a predetermined value, Fig. 6 (not shown). Further commutating arrangements for the thyristors 4, 5, are described in Specification 1,263,262.