Patent GB-1434933-A (GB)
Title: Thyristor chopper circuits
IPC: H02M3/137
Filed: 1972-07-06
Abstract: 1434933 Automatic control of current C A V Ltd 27 June 1973 [6 July 1972] 31593/72 Heading G3R [Also in Divisions H2 and H3] A thyristor chopper circuit includes a first thyristor through which current can flow to a load when the thyristor is switched on, a saturating transformer including a primary winding in series with the thyristor and a secondary winding, a commutating capacitor, a second thyristor through which the commutating capacitor can be charged from the secondary winding during the period when the first thyristor is switched on, means responsive to saturation of the transformer for turning off the first thyristor using the charge on the commutating capacitor, a firing circuit for supplying a firing pulse to the first thyristor a predetermined time after the first thyristor has been turned off and circuit means responsive to the time taken for the transformer to saturate and acting in the event that the time taken is less than a predetermined value, to extend said predetermined time whereby the mean value of the current flowing in the first thyristor will be limited to a  predetermined level. In the described embodiment, the traction motor 11 of a road vehicle is supplied from a battery 12 through a saturating transformer 16 and a first thyristor 18 which is periodically fired at a frequency determined by the setting of a control means 45 linked to the accelerator pedal, this setting determining the current passed by a transistor 43 and hence the charging time of a capacitor 47 which, when at a predetermined voltage, fires a uni-junction transistor 51 to turn on the thyristor 18 and a second thyristor 23. The firing of transistor 51 also momentarily turns on a transistor 38 to discharge a capacitor 37, which charges during each conductive period of the thyristor 18 during which a transistor 34 is turned on. The thyristor 18 is turned off when the transformer 16 saturates and thus the capacitor 37 is charged for a time inversely related to the current through members 11, 18, which current is normally equal to or below a predetermined level so that the capacitor 37 is charged to a voltage greater than that required to fire transistor 51. This voltage is retained during charging of the capacitor 47 but does not affect the latter operation since a diode 48 is reverse biased. However, if the current exceeds the predetermined level, the capacitor 37 does not reach the firing voltage of  transistor 51 and is charged with the capacitor 47 by the transistor 43 through the diode 48. Thus the firing of transistor 51 and thyristor 18 is  delayed to increase the off period of the latter and so reduce the mean current until it is equal to or below the predetermined level. The motor is thus supplied with a mean value of current whose level is determined by the off-time of thyristor 18. A resistor 41 senses the voltage of battery 12 and automatically reduces the predetermined level when the battery voltage is high. When the transformer 16 saturates, the voltage developed in winding 22 turns on a transistor 28 which applies a firing pulse (of length determined by a capacitor 30) to a thyristor 19, whose conduction causes discharge of a commutating capacitor 21 to turn off thyristor 18, the capacitor 21 then recharging through thyristor 19 until it turns the latter off. When thyristors 18, 23 are turned on, the voltage induced in winding 22 reverses the charge on capacitor 21 through thyristor 23 until it turns the latter off, leaving it ready for a further commutation of the thyristor 18, which commutation also causes turn-off of a normally-conducting transistor 49.