Patent GB-1246536-A (GB)
Title: Improvements in the control of electrical frequency converters
Inventor: KRABBE ULRIK
IPC: H02M5/27
Filed: 1969-01-24
Abstract: 1,246,536. Static frequency changers. ALLMANNA SVENSKA ELEKTRISKA A.B. 24 Jan., 1969 [25 Jan., 1968], No. 4046/69. Heading H2F. In a cycloconverter, for converting from a higher to a lower frequency and comprising at least two groups of rectifiers 1, 2 and 3,4 (Fig. 3), which groups supply the positive and negative half waves of the output respectively, there is provided a control arrangement comprising a pilot voltage generator 80 having an output at the required voltage amplitude and frequency of the converter output, and comparison of the pilot voltage with a reference voltage derived from the convertor input determines the required firing angles for the rectifiers so that the converter output will correspond to the pilot voltage waveform. The pilot voltage is integrated during one half cycle of the converter input and the integrated value is stored and used to control a rectifier during the next half cycle. Thus the control means for each rectifier, includes a respective integrating capacitor memory device 10, 20, 30, 40, which is alternately switched to the pilot voltage source 80 by transistors 101, 201, 301, 401 respectively, and to the rectifier gate circuits by transistors 102, 202 &c. Transistors 103, 203 &c. are provided to discharge completely the respective capacitors at the end of the said input half cycles. The switching transistors are controlled by the input A.C. mains applied to a primary 51, of a transformer having secondaries 511 to 516. Operation.-Considering the operation of rectifiers 1, 2 which are controlled to conduct alternately to supply the positive half cycles of the converter output: The reference voltage is produced at secondaries 521, 522 from the A.C. mains input at 52 which is phase shifted by 90 degrees and applied to common primary 523. C 10  is charged to voltage e 10  (Fig. 4c), during the first shown half cycle of the converter input voltage e 5  (Fig. 4a), by the pilot voltage e 80  (Fig. 4b) when Tr 101 conducts. During the second half cycle Tr 101 is blocked and Tr 102 conducts so that the charge e 10  on C 10  is applied in series with the reference voltage e 521  (Fig. 4c) across secondary 521. When the curves of e 10  and e 521  intersect a pulse is produced by transformer 11 to fire rectifier 1 which produces the hatched voltage portion e 1  at the converter output terminals 8, 9. At the end of this second period Tr 103 is turned on to short circuit C 10 . During the third shown half-period rectifier 2 is likewise caused to conduct and provide the voltage portion e 2  at outputs 8, 9, by the turning ON of Tr 202 and turning OFF Tr 201. Also C 10  is charged again via Tr 101, ready for the fourth half cycle period when rectifier 1 is to conduct again. Since the pilot voltage is greater during the third half cycle, the charge in C 10  will be higher than in the first half cycle. Thus rectifier 1 will be fired earlier during the fourth half cycle than in the second, as shown. Thereby the output voltage at 8, 9, with the assistance of chokes 81, 82, 89 is made to follow the pilot voltage curve. During the negative half cycle of the pilot voltage, the rectifiers 3, 4 which have identical control circuitry are controlled as above. Each transistor switch is provided with a reverse parallel connected transistor (not illustrated) so that C 10 , C 20  are reverse charged and rectifiers 1, 2 operate in the invertor mode since their delay angles exceed 90 degrees while rectifiers 3, 4 operate in rectifying mode. In a 3-phase frequency changer (e.g. as Fig. 5, not shown) there are 3 single phase rectifier bridges connected in Delta, the rectifiers in each bridge being controlled in pairs as described with reference to Figs. 3, 4 and the pilot generator delivers 3 phase voltages. Alternatively the rectifiers may be star connected (Fig. 6, not shown).