Patent GB-1222291-A (GB)
Title: Static frequency converter with voltage control
IPC: H02M5/27
Filed: 1968-04-08
Abstract: 1,222,291. Cycloconverter type static frequency changers; control of A.C. motors. WESTING- HOUSE ELECTRIC CORP. 8 April, 1968 [21 April, 1967], No. 16725/68. Headings H2F and H2J. In a Cycloconverter type frequency changer using static bilateral switches, the switches are artificially commutated and controlled in a cyclic switching pattern, whereby output frequencies can be provided in a range on both sides of the input frequency, and wherein output voltage is controlled independently of the input voltage and output frequency by controlling the conduction angles (dwell times) of the bilateral switches. Each bilateral switch may be a controlled transistor in a diode bridge Fig. 2 (not shown) or a thyristor in a diode bridge, Fig. 15, in which the thyristor is turned both ON and OFF by gate signals from a control circuit, or two parallel back to back thyristors may be used (not illustrated). In a 3-phase to 1-phase embodiment, Fig. 3, a source 34 is connected to a 3-phase bridge type cycloconverter having six conversion phases for successively applying to a load 36, increments of six uniform time spaced inputs from lines A-B, B-C, C-A, C-B, A-C, B-A. Any pair of lines can be connected to the load by turning ON an appropriate pair of switches, a pair being conveniently called a switching unit. Control of frequency.-Assuming that control circuit 38 (Fig. 3) allows switching units A 1 B 2 , A 1 C 2 , B 1 C 2 , B 1 A 2 , C 1 A 2 , C 1 B 2  to conduct in that sequence for a fixed time T, the sequence being repetitive at a predetermined rate R (e.g. determined by a clock generator as in Fig. 7). As shown, Fig. 4, the repetitive pattern extends over a period TP comprising six uniform time frames T 1  to T 6 . Each switch is ON for full frame T as indicated by switching curve SW. A wanted fundamental voltage waveform E F  at a frequency fo is obtained, the output frequency f 0  being equal to the difference between the input frequency f in  and the repetition frequency. Thus the output frequency can be varied by varying the repetition frequency. Control of voltage output.-The average voltage magnitude is controlled by controlling the conduction period or angle t 1  within and relative to the length of its associated time frame T 1 , T 2  &c., for example as shown in Fig. 5. During the remaining period t 2  of a time frame the input line voltage is disconnected from the load and the output terminals are short circuited by series connection of two bilateral switches in any two legs of the bridge e.g. switches A1, A2. By controlling the ratio t 1 : t 2  and maintaining their sum constant (t 1  + t 2  = T) the fundamental component E F  of the output voltage can be continuously varied from a maximum to zero at a range of given frequencies. Control circuitry.-Fig. 7 illustrates a converter 50 for supplying from a 3-phase source 34, a variable frequency variable voltage 3-phase output to a 3-phase motor 40; and comprise 3 converters # 1 , # 2 , # 3  each supplying 1-phase power (120 degrees apart) to a different phase of the motor, each converter being as that in Fig. 3. The turning ON and OFF of the switches in proper sequence by a control circuit 52 (including a timing waveform circuit 54, logic and distributer circuits 56) produces a set of drive waveforms, which are distributed via isolators 94 to 99 to the switches. A clock generator 60 produces trains of timing pulses P o  the frequency of which (and hence the repetition frequency) is adjustable to adjust f 0 . Pulses P 1  are used to initiate conduction of the switch units and pulses P 2 , at the same rate but displaced by time T I  therefrom by a delay unit D 3 , are employed to terminate conduction and to initiate conduction of the shorting switches. By varying the position P 2  to P 1  by means of delay unit D 3 , the ratio t 1 : t 2  is varied. Thus P 0  (and P 1 ) determines the output frequency and P 2  the output voltage magnitude. Instead of short-circuiting the load a reverse voltage may be applied during period t 2  (Figs. 6B, 10, 11, not shown). An arrangement whereby the output voltage is made to vary linearly with output frequency is claimed in Specification 1,222,292 and is described with reference to Figs. 12, 13, 14 herein (not shown), a sawtooth oscillator being used instead of the clock generator of Fig. 7.