Patent GB-952127-A (GB)
Title: Improvements in and relating to the control of electric converting apparatus
Inventor: AINSWORTH JOHN DESMOND
IPC: H02M1/08
Filed: 1959-01-09
Abstract: 952,127. Converting. ENGLISH ELECTRIC CO. Ltd. April 7, 1960 [Jan. 9, 1959]. No. 876/59. Heading H2F. In a converter for both rectifying and inverting which comprises controllable rectifying devices, the control circuit for each phase comprises two pulse generating arrangements I and II, each providing a pulse every cycle and the earlier pulse being applied to the control electrode to effect conduction. The circuit I provides a pulse whose leading edge (representing the firing instant) may occur in an extensive phase range from- 30 degrees to 210 degrees and is suitable for rectifying and inversion. The pulse of circuit II is in the inverting range only and occurs at the appropriate limiting safety angle. Fig. 2 represents the circuit for controlling the phase R of the three-phase supply R, W, B; (waveform a, Fig. 3). Voltages derived from phases B and W are added to give waveform b which is appl'ed to a squarer, e.g. a Schmitt trigger circuit, giving a waveform c. A similar waveform d is obtained from phase B. These are applied to an " and " gate 13 giving an output e which is applied to a gate 14 to control the charging of a capacitor 15 to give an output j. The level of the waveform is varied by the addition of a control voltage V c , the resultant being squared to give a square pulse output whose leading edge is varied between- 30 degrees and 210 degrees by the voltage V C  and whose trailing edge is fixed at 210 degrees by applying the pulse and waveform e to a gate 19, thus giving a final output waveform g. In the inverter control circuit II a waveform m is derived from the addition of four voltages, the anode-to-cathode voltage h, a sinusoidal voltage j representative of the alternating current in the converter system, a direct voltage proportional to the total D.C. input and the time differential of the D.C. This resultant is squared (n), inverted and applied to a gate 23. Waveform t derived from phases R, W and B is also applied to gate 23 and the output u has trailing edges at 210 degrees due to t and a leading edge dependent on the termination of pulses n and is indicative of the firing point. The termination of pulse n is dependent on voltages in such a way that increasing n retards the firing point and increasing any of the others retards it. The D.C. component is derived from a shunt resistor, and the D.C. timedifferential component from a quasi differentiator or a D.C. reactor winding and is connected with diodes so that falling current values give a zero time differential component. The outputs g and u are applied to an " or " gate so that the earlier is applied to the control grid. The system may be modified and applied to a double three-phase system (Fig. 4, not shown), and control of the output may be effected by suppressing the firing signal to each half of the double three-phase system in alternate cycles. The grid pulses may be short or may be broad (120 degrees) and provided by an arrangement disclosed (Fig. 7, not shown). The rectifying devices may be mercury arc converters, and the active elements in the control circuit may be transistors.