Patent GB-952945-A (GB)
Title: Improvements in means for producing variable frequency polyphase outputs
Inventor: BOWLER PETER
IPC: H02M5/44
Filed: 1962-01-23
Abstract: 952,945. Converting; control of A.C. motors. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Jan. 9, 1963 [Jan. 23, 1962], No. 2476/62. Headings H2F and H2J. [Also in Division H3] A device for providing a polyphase supply of variable frequency and reversible phase sequence comprises two signal generators, one single phase, one polyphase, one of fixed frequency and the other of frequency variable over a range above and below the fixed frequency, and an output unit for each phase. Each output unit receives signals from the single phase generator and from one phase of the polyphase generator, and provides one phase of the supply, whose frequency is the difference of the frequencies of the two signal generators. Oscillators.-Fig. 2 shows a three-phase oscillator which may be used for both signal generators (only one output being taken in the single phase case). Transistors T1, T2, T3 are connected in a ring, with coupling circuits which introduce phase shift. The collector of transistor T2 is connected through the resistor R2 to the negative pole 13N of the D.C. source 13, and the emitter of transistor T2 is connected through resistor R12 to the conductor 15, and thence through resistor RX to the positive pole 13P. The base is connected through resistor R22 to the positive pole. Zener diodes, such as D2, are connected between the collector of one transistor and the emitter of the following one. Capacitors such as C2 are connected between the collector of each transistor and the neutral pole 13E of the D.C. source 13, and these capacitors together with their associated resistances determine the frequency of the oscillator. The outputs are taken at 11, 12 and 13. For the variable frequency oscillator the capacitor C2 is replaced in each stage by a voltage controlled silicon capacitor C2Y, or a Zener diode, which is connected in series with a fixed capacitor C2X and the series combination is then connected in parallel with another fixed capacitor C24, as shown in Fig. 3. A variable D.C. bias is applied at 7 to determine the capacitance of C2Y, and hence the oscillator frequency. Output units.-Each of the output units, one of which is shown in Fig. 4, comprises a transformer 61 whose primary winding 61P is connected through terminals I1A to the single phase generator, and whose centre tapped secondary winding is connected through diodes D7, D8 to the series chain of resistor R29, potentiometer 30 and resistor R31. One phase of the polyphase generator is connected, at terminals 13A, to earth and to the centre tap 61ST. The series capacitors C8, C9 are connected to the junctions of the diodes and the resistor chain, and their mid-point is earthed. The output is taken between earth and the wiper 67A of the potentiometer 30. Three outputs from three output units constitute a three-phase supply whose frequency is the difference of the frequencies of the two signal generators and whose phase sequence depends on whether the variable frequency is above or below the fixed frequency. This supply may be used directly or may control silicon-controlled rectifiers, for example for supplying motors. If required, buffer amplifiers may be included in the circuit between the signal generators and the output units.