Patent GB-1513799-A (GB)
Title: Four-quadrant static converter
IPC: H02P7/03
Filed: 1975-07-31
Abstract: 1513799 Converting; control of DC motors LICENTIA PATENT VERWALTUNGS GmbH 31 July 1975 [6 Aug 1974] 32147/75 Headings H2F and H2J A four-quadrant static converted comprises a bridge circuit, each limb of which contains a bidirectional thyristor, a control device and multiple pulse generator for producing control pulses, wherein the control device includes switch means for reversing the current without mechanical interruption of the converter circuit by switching respective sets of control pulses to respective sets of thyristors each thyristor set corresponding to a respective polarity of the AC input to the converter, to obtain one direction of current and then switching to interchange the sets of pulses and thyristors to obtain reverse current. In Figure 1, a DC motor M is supplied from a bridge comprising bidirectional thyristors 11-14 connected to one phase RS of a three phase supply and switched in synchronism therewith. Control pulses to fire the thyristors 11-14 are obtained from a control device 2 consisting of a two-pulse generator 21 feeding the primary windings 221, 222 of two pulse transformers, whose secondary windings 221<SP>1</SP>, 221<SP>11</SP>, 222<SP>1</SP>, 222<SP>11</SP> produce pulses T1-T4 which are applied to the thyristors 11-14 through the contacts 231, 232, 233, 234 of two relays, whose coils 2412, 2434 are energised or de-energised by operation of a switch 25<SP>1</SP> controlled by a command stage 25 of a programme transmitter. With the switch 25<SP>1</SP> in the position shown, the relay contacts route the pulses T1, T4 to thyristors 11, 14 and the pulses T2, T3 to thyristors 12, 13 in respective positive and negative half cycles of the AC supply, whereby the motor rotates in a clockwise direction. Changeover of switch 25<SP>1</SP> changes over the moving contacts of the relays to route the pulses T1, T4 to thyristors 12, 13 and pulses T2, T3 to thyristors 11, 14, whereby the bridge output is reversed to drive the motor anti-clockwise. The two relays may be replaced by one having a single coil controlling eight contacts or by electronic switch means. The bridge may have six arms each with a bidirectional thyristor for operation on a three phase supply. Operation of the single phase embodiment of Figure 1 is discussed for a firing angle of 90‹, Figure 2a (not shown), for variation of this angle over a range to vary the voltage applied to the motor, and for variation of this angle to operate the bridge as mainscontrolled inverter whereby the motor is regeneratively braked. To prevent mal-operation during the latter condition, due to one set of thyristors (e.g. 12, 13) being fired before the other set 14, 11 have quenched, firing of the thyristors 12, 13 is delayed by a limit g corresponding to the angle # of current zero, Fig. 2c (not shown) and firing of the thyristors 11, 14 is prevented after a second limig g<SP>1</SP> to ensure that they are quenched by the limit g, the period g<SP>1</SP> g, Fig. 2a (not shown) defining a safety interval in the form of a short dead time. The state of the thyristors is monitored to control switch over of the control pulses and so obtain the required dead time.