Patent GB-981597-A (GB)
Title: Improved converter circuits
IPC: H02M1/20
Filed: 1961-05-11
Abstract: 981,597. Converting; control of D.C. motors. SOC. GENERALE DE CONSTRUCTIONS ELECTRIQUES ET MECANIQUES ALSTHOM. May 11, 1961 [May 12, 1960], No. 17252/61. Headings H2F and H2J. In a converting arrangement operable in either direction between an A.C. and a D.C. network and suitable for supplying a rolling mill or traction D.C. motor, a bridge circuit 2 comprises two groups of controllable rectifiers 4e (rectifiers 3a, 3b, 3c) and 4s (rectifiers 3x, 3y, 3z) and the firing control of the groups is effected in such a manner that reactive current in the A.C. network is minimized as the output voltage is varied. It is known that for a bridge in which all the rectifiers operate at natural commutation times reactive currents are a minimum and similarly these currents are also a minimum when all rectifiers operate with maximum inverter delay angle. Hence in effecting voltage variation when the bridge is operating as a rectifier the group 4e is maintained to operate at natural commutation times whilst the D.C. output voltage is decreased only by delaying the firing angle of the rectifiers in group 4s. In Figs. 2A, 3A, 4A the dashed lines represent the voltage at 5c (natural commutation of 3a, 3b, 3c) and the full lines represent the voltage at 5s, the firing angle of 3x, 3y, 3z being delayed by increasing angles # 1 , # 2 , # 3  to reduce the output voltage which is given by the difference between the full and dashed lines. This output voltage is shown as full lines in Figs. 2B, 2C, 2D. To obtain zero D.C. output the angle # 1  is increased to a maximum (maximum inverter angle) and the firing of the rectifiers 3a, 3b, 3c is delayed by a small amount from the natural commutation point to give symmetry (Fig. 5A, not shown). On progressing further through inverter operation the group 4s is maintained at maximum inverter angle and the voltage control is effected by the firing of the group 4e. A further bridge may be provided in parallel with the bridge 2 (Fig. 6, not shown) and by reversing the operation of the groups an output (dashed in Fig. 2B) is obtained, the overall D.C. output being the mean of these two. Various further elaborations comprising bridges in series and parallel are disclosed to minimize harmonics. Asymmetrical operation i.e. where the conduction periods are longer and shorter in successive cycles is also disclosed (Fig. 8, 9 and 10, not shown). From full voltage V down to ¥ full voltage (Fig. 8) group 4e undergoes natural commutation (dashed line, Fig. 8A) and the rectifiers in 4s commutate naturally on one cycle (t 31 ) and after a delay angle # 5  in the next (heavy line, Fig. 8A) the output voltage being the difference between these lines. When # 5  equals 120 degrees the rectifiers in 4s conduct only one cycle in two and further voltage reduction is effected (¥V to ¢V) by delaying alternate commutations of the rectifiers in group 4e by # 6  (Fig. 9A) and when S 6  = 120 degrees 4e conducts in alternate cycles only giving one half maximum output voltage. Fig. 10 discloses voltage outputs less than this. Fig. 11 shows combination of bridges to eliminate harmonics whose outputs are shown by the heavy and dashed lines in Figs. 9B, 10B. In a further modification, Fig. 14, an additional pair of rectifiers 3n 3p are connected across the bridge with their junction connected to the neutral point. In moving from maximum rectification to maximum inversion one or other of the groups 174e, 174s is operated at maximum, rectifying, zero, or maximum inverting control whilst the other effects the voltage change and the operation is depicted in Fig. 15 (not shown). Asymmetrical operation is also disclosed (Figs. 16 ... 19, not shown). Specifications 976,803 and 976,804 are referred to.