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A direct AC converter for converting a polyphase AC input supply into an output DC or single or polyphase AC supply of amplitude, phase, frequency or power factor which is different from the input supply using a matrix of bidirectional switches having contiguous width modulated conduction periods in

A direct AC converter for converting a polyphase AC input supply into an output DC or single or polyphase AC supply of amplitude, phase, frequency or power factor which is different from the input supply using a matrix of bidirectional switches having contiguous width modulated conduction periods in cycles which synthesize the output supply voltage or voltages from samples of the input supply voltages taken cyclically at a much higher frequency than the supply frequency or frequencies. The switches are operated so that the or each output supply conductor is connected to only one of the input supply conductors at a time. The width modulation may be effected in response to phases of a sinusoidal oscillation to produce a frequency change. The output may have a "negative" frequency so that a phase displacement between current and voltage due to a reactive load appears in the opposite sense in the input supply. Width modulation may alternatively be effected by a combination of two sinusoidal oscillations so that the output frequency has equal positive and negative values to eliminate phase displacements in the input supply due to a reactive load. Motor speed control and generator output frequency control applications are described.

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1. A direct AC converter including input phase conductors for a balanced polyphase AC input voltage system; a plurality of output phase conductors for a polyphase AC output voltage system having at least one characteristic such as frequency, amplitude, phase angle or phase displacement which is diff

1. A direct AC converter including input phase conductors for a balanced polyphase AC input voltage system; a plurality of output phase conductors for a polyphase AC output voltage system having at least one characteristic such as frequency, amplitude, phase angle or phase displacement which is different from that of the input voltage system; a plurality of bidirectional switch means for individually connecting each input conductor to each output phase conductor; means for producing a repeated sequence of mutually abutting width-modulated drive pulses; and means for utilizing said drive pulses to operate said bidirectional switch means at a frequency substantially higher than the frequencies of both the input and output voltage system, to connect, in respect to each output phase conductor, only one input phase conductor at a time to that output conductor and for connecting all of said input phase conductors to said at least one output phase conductor a plurality of times during any cycle of said input system or of said output system. 2. A converter according to claim 1, in which the means for utilizing said drive pulses includes data processing means having a stored program for controlling the said operation of the bidirectional switch means. 3. A converter according to claim 2, including data storage means operatively interconnected with said data processing means for controlling operation of said bidirectional switches, and means for entering input data into said data storage means for adjusting operation of said bidirectional switches under control of said stored program. 4. A converter according to claim 2 or 3, including a plurality of output latches each operatively coupled between said data processing means and an individual one of said switch means, said stored program controlling setting and resetting of the latches of the time required for the corresponding switch means to be closed and opened. 5. A converter according to claim 1 or 2 or 3, wherein said means for producing drive pulses comprises means for producing sinusoidally-width-modulated pulses. 6. A direct AC converter including input phase conductors for a balanced polyphse AC input voltage system; a plurality of output phase conductors for an AC input voltage system having at least one characteristic such as frequency, amplitude, phase angle or phase displacement which is different from that of the input voltage system; a plurality of bidirectional switch means for individually connecting each input phase conductor to each output phase conductor; control means comprising oscillator means for generating equiphase displaced sinusoidal outputs equal in number at least to the number of input phase conductors and for maintaining said equiphase displacement between said sinusoidal outputs, and timing means for receiving said equiphase displaced sinusoidal outputs and producing a repeating sequence of mutually abutting sinusoidally-width-modulated drive pulses; and means for utilizing said drive pulses to operate said bidirectional switch means at a frequency substantially higher than the frequencies of both the input and output systems, to connect, in respect of each said output phase conductor, only one input phase conductor at a given time to that output phase conductor and for connecting all of said input phase conductors to each output phase conductor a plurality of times during any cycle of said input system or of said output system. 7. A converter according to claim 6 wherein for each output phase conductor the drive pulses of the sequence are applied respectively to the switch means which are connected to that output phase conductor, the correspondence between the pulses of the sequence and the input phase conductors being shifted cyclically for the different output phase conductors. 8. A converter according to claim 4 wherein the timing means includes a ring of monostable circuits connected so that the resetting of one monostable circuit sets the next monostable circuit in the ring, each monostable circuit having an input for a width modulating signal which determines the set time for that circuit, and means for deriving said sequence of width-modulated pulses from the monostable circuits. 9. A converter according to claim 8 including means for applying a variable voltage to the monostable circuits thereby to adjust the response of the circuits to the width modulating signals. 10. A converter according to claim 6, including a polyphase sinusoidal waveform generator means for producing the same number of equally phase displaced since waves as there are phases in the input voltage system, and means for applying each sine wave to a modulator means for modulating in width a respective pulse of the sequence. 11. A converter according to claim 10 including means for adjusting the amplitude of the sine waves produced by the generator. 12. A converter according to claim 6, wherein the control means comprises digital differential analyser means for generating the repeating sequence of width modulated pulses. 13. A direct AC converter including input phase conductors for a balanced N-phase AC input voltage system; output phase conductors for a balanced polyphase AC output voltage system having at least one characteristic such as frequency, amplitude, phase angle or phase displacement which is different from that of the input voltage system; a plurality of bidirectional electronic switch means for individually connecting each input phase conductor to each output phase conductor; control means which includes two groups of N frequency controllable sinusoidal oscillator means, each group of N oscillator means for producing N equally phase-displaced sinusoidal outputs, the frequencies of said two groups of oscillator means being, respectively, (i) the desired frequency of said output system less the frequency of said input system with said phase displaced outputs arranged in one sense, and (ii) the sum of the frequencies of said input and output systems but with the phase displaced outputs arranged in a sense opposite from said one sense; means for controlling the amplitudes of the outputs of said groups of oscillator means in response to a common control signal; means for controlling the amplitude of the outputs of one of said groups of oscillator means differentially with respect to the outputs of the other said group of oscillator means; summing means for summing the outputs of the oscillator means of one said group of oscillator means individually with the outputs of the oscillator means of the other said group of oscillator means to produce N 2 summed outputs; means for converting said summed outputs into sequences of abutting sinusoidally-width-modulated drive pulses; and means for utilizing said drive pulses to operate said bidirectional switch means to connect, in respect of each said output phase conductor, only one input phase conductor at a given time to that output phase conductor. 14. A converter according to claim 13, wherein said means for utilizing said drive pulses is operable for connecting all of said input phase conductors to each output phase conductors a plurality of times during each cycle of said input system or of said output system. 15. A converter according to claim 6 or claim 13 or claim 14, wherein each switch means includes a four-diode bridge circuit and a controllable semiconductor element connected across a diagonal of the bridge, the other diagonal of the bridge being connected in a current path joining one of said input phase conductors to one of said output phase conductors. 16. A converter according to claim 15 wherein the controllable semiconductor element is a bipolar transistor. 17. A converter according to claim 15 wherein the controllable semiconductor element is a darlington pair of bipolar transistors. 18. A converter according to claim 16 including diode means connected to reduce the charge storage time of the transistor or transistors when switched off. 19. A converter according to claim 15 wherein the controllable semiconductor device is a power field effect transistor. 20. A converter according to claim 6 or claim 13 or claim 14, wherein each switch means comprises a pair of thyristors connected in anti-parallel. 21. A converter according to claim 15, including means for protecting the controllable semiconductor device from high voltage transients when switched. 22. A converter according to claim 6 or claim 13 or claim 14, wherein each said switch means is characterized by an opening delay, and wherein said switch operating means utilizes said drive pulses to apply closing and opening signals to said switch means; and for each said switch means, means for delaying response of that switch means to a closing signal by an amount corresponding to said open delay but not delaying response of that switch to an open signal. 23. A converter according to claim 6 or claim 14 wherein the sequence of drive pulses is repeated between 1,000 and 100,000 times per second. 24. A direct AC converter including input phase conductors for a balanced polyphase AC input voltage system; at least one output phase conductor for an AC output voltage system having at least one characteristic such as frequency, amplitude, phase angle or phase displacement which is different from that of the input voltage system; a plurality of bidirectional switch means for individually connecting each input conductor to said at least one output phase conductor; control means comprising oscillator means for generating equiphase displaced sinusoidal outputs equal in number at least to the number of input phase conductors and timing means for receiving said equiphase displaced sinusoidal outputs and producing a repeated sequence of mutually abutting sinusoidally-width-modulated drive pulses; and means for utilizing said drive pulses to operate said bidirectional switch means at a frequency substantially higher than the freequencies of both the input and output voltage systems, to connect, in respect of the said at least one output phase conductor, only one input phase conductor at a time to that output phase conductor and for connecting all of said input phase conductors to said at least one output phase conductor a plurality of times during any cycle of said input system or of said output system. 25. A converter according to claim 24 wherein the output system is a polyphase system. 26. An engine speed control system comprising: an engine having a shaft; tachometer means coupled to the shaft for generating a signal indicative of the speed of said shaft; a direct AC converter including input phase conductors for a balanced polyphase A.C. input voltage system a plurality of output phase conductors for a balanced polyphase A.C. output voltage system having at least one characteristic such as frequency, amplitude, phase angle or phase displacement which is different from that of the input voltage system; a plurality of bidirectional switch means for individually connecting each input phase conductor to each output phase conductor; means coupling either said input phase conductors or said output phase conductors to said engine; timing means for producing a repeated sequence of mutually abutting width-modulated drive pulses; means for utilizing said drive pulses to operate said bidirectional switch means at a frequency substantially higher than the frequencies of both the input and output voltage systems, to connect, in respect of each said output phase conductors, only one input phase conductor at a time to that output phase conductor and for connecting all of said input phase conductors to each said output phase conductor a plurality of times during each cycle of said input system or of said output system; control means connected to receive an output signal indicative of shaft speed from said tachometer means and utilizing said tachometer output signal to adjust the width modulated drive pulses produced by said timing means and thereby control the engine speed. 27. A system according to claim 26, wherein said means for producing drive pulses comprises means for producing sinusoidally-width-modulated pulses. 28. A system according to claim 26, wherein the engine is a polyphase motor coupled to said output phase conductors; and wherein said tachometer output signal is utilized for adjusting the drive pulses to control the motor speed. 29. A system according to claim 28, wherein the tachometer output signal is compared with a reference speed signal and is utilized to adjust the drive pulses to maintain a substantially constant motor speed. 30. A system according to claim 28, wherein the tachometer output signal is compared with the A.C. output voltage of said converter and is utilized for adjusting the drive pulses to maintain a substantially constant motor torque. 31. A system according to claim 26, wherein the engine is a generator and said tachometer output signal is utilized for adjusting the engine speed to control the frequency of said A.C. output voltage system. 32. A system according to claim 31, wherein the tachometer output signal is utilized for adjusting the timing pulses to adjust the engine speed to cause the converter to apply to said output phase conductors a constant frequency AC signal despite variations in the generator shaft speed.