Patent GB-1255833-A (GB)
Title: Improved thyristor current converter
Inventor: FRANK KJELL
IPC: H02M1/06
Filed: 1969-04-28
Abstract: 1,255,833. Static D.C./A.C. converters; control of A.C. motors. ALLMANNA SVENSKA ELEKTRISKA A.B. 28 April, 1969 [29 April, 1968], No. 21526/69. Headings H2F and H2J. In order to rapidly extinguish the load current of an inverter such as that shown in Fig. la wherein an oscillator 6, Fig. 1b controls the firing pulse frequency of main thyristors in a bridge circuit 2 and the operating rate of an extinction circuit 5 and thus the inverter output frequency, there is provided a control means which, at least for a certain interval, firstly blocks the main thyristors firing pulses and secondly influences a control signal to the oscillator so that the oscillator frequency controlling the extinction circuit thyristors during the interval is higher than the frequency immediately before the start of the interval, the interval length being chosen in relation to the oscillator frequency that all main thyristors are extinguished during the interval. When the inverter is supplying a motor 1 and the direction of rotation thereof is to be reversed, the inverter output frequency is first reduced by a control system (not shown) to a low value e.g. 1Hz, then the control pulses to the main thyristors are blocked and when these are extinguished the phase sequence can be switched, a delay means being provided to ensure that all firing pulses are blocked before switching. Control circuit (Fig. 1b) operation.-The oscillator 6 (e.g. a Unijunction) emits pulses at 6 times the desired inverter output frequency and its frequency is controlled by an amplifier 7 wherein a control voltage f o , corresponding to the output frequency desired, is added to a signal #f which upon operation with unaltered phase sequence, is zero but which deviates from zero in response to a signal A-e.g. for altering the phase sequence of motor 1 and hence its direction. The oscillator output pulses control a bi-stable circuit 8 for control of the extinction thyristors in circuit 5 and in addition control a ring counter 10 having six outputs, via a normally conducting electronic switch 9. The ring counter outputs are connected via a phase sequence determining switch unit 11 to the main thyristors in circuit 2. The unit 11 is controlled by the phase sequence determining signal A having two distinct values supplied through a delay circuit 12 with a delay #2. Signal A is also supplied through front and rear edge deriving circuits 13, 14 to a monostable circuit 15 having a reset time #1 which is normally in "0"-position but is set to "1"-position by an alteration in signal A. This causes the signal #f to be added to f 0  in amplifier 7 and the main thyristor firing pulses to be blocked by switch unit 9. After a period #2 when blocking should be complete the switching unit 11 switches to the opposite phase sequence. The extinguishing circuit now operates with the frequency (f 0  + ##) where ## is 50-100 times # 0 , and extinguishes both halves of the main circuit during the period #1, after which time the monostable 15 returns to its "0"-position, ## becomes zero and switch unit 9 becomes conducting. Control pulses are again supplied to the main thyristors and the inverter operates in the new phase sequence and the frequency f 0  can now be increased to the normal operating value.