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A three-phase resonant cyclo-converter including a closed loop control module for controlling the switching frequency of the cyclo-converter, the closed loop control module including: a voltage signal development module arranged to develop a voltage signal representative of a voltage output waveform

A three-phase resonant cyclo-converter including a closed loop control module for controlling the switching frequency of the cyclo-converter, the closed loop control module including: a voltage signal development module arranged to develop a voltage signal representative of a voltage output waveform of the cyclo-converter, a storage module arranged to accumulate voltage signal values for phase portions of the voltage output waveform, where the voltage signal values are based on a voltage error signal and accumulated historical voltage signal values for the same corresponding phase portions, and a switching frequency control module arranged to develop a switching frequency control signal to control the switching frequency of the cyclo-converter based on the accumulated voltage signal values for corresponding phase portions of the voltage output waveform, and a proportional voltage signal based on a difference between the developed voltage signal and a reference voltage signal.

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1. A three-phase resonant cyclo-converter including a closed loop control module for controlling the switching frequency of the cyclo-converter, the closed loop control module including: a voltage signal development module arranged to develop a voltage signal representative of a voltage output wavef

1. A three-phase resonant cyclo-converter including a closed loop control module for controlling the switching frequency of the cyclo-converter, the closed loop control module including: a voltage signal development module arranged to develop a voltage signal representative of a voltage output waveform of the cyclo-converter;a storage module arranged to accumulate voltage signal values for phase portions of the voltage output waveform, where the voltage signal values are based on a voltage error signal and accumulated historical voltage signal values for the same corresponding phase portions; anda switching frequency control module arranged to develop a switching frequency control signal to control the switching frequency of the cyclo-converter based on the accumulated voltage signal values for corresponding phase portions of the voltage output waveform, and a proportional voltage signal based on a difference between the developed voltage signal and a reference voltage signal. 2. The cyclo-converter of claim 1, wherein the phase portions are within a range from 1 degree to 30 degrees. 3. The cyclo-converter of claim 1, wherein the switching frequency control module is further arranged to adjust a switching frequency period for a phase of an input voltage waveform having the greatest absolute voltage to achieve frequency control. 4. The cyclo-converter of claim 1, wherein the storage module is arranged to develop an integrated signal based on the accumulated voltage signal values for corresponding phase portions, where the switching frequency control signal is developed from the integrated signal. 5. The cyclo-converter of claim 4, wherein each corresponding phase portion is split into a plurality of phase segments, and the voltage signal value is stored in a window of phase segments such that the voltage signal value is stored next to one or more historical voltage signal values in neighboring phase segments within the window. 6. The cyclo-converter of claim 5, wherein an average of selected historical voltage signal values within the window is fed back to the storage module to develop the voltage signal value. 7. The cyclo-converter of claim 6, wherein the selected historical voltage signal values include a first historical voltage signal value positioned in a first phase segment which was previously used to store the voltage signal value and a second historical voltage signal value positioned in a second phase segment which is to be used subsequent to storing the voltage signal value. 8. The cyclo-converter of claim 7, wherein the first historical voltage signal value is given a different weighting to that of the second historical voltage signal value. 9. The cyclo-converter of claim 1, wherein the switching frequency control module further includes a PID control module arranged to develop a switching frequency control signal from proportional, integral and derivative voltage signals. 10. The cyclo-converter of claim 9, wherein the closed loop control module further includes a load current development module arranged to develop the derivative voltage signal based on a load current signal representative of an output load current. 11. The cyclo-converter of claim 9, wherein the PID controller is arranged to develop the proportional voltage signal based on a difference between the developed voltage signal and the reference voltage signal. 12. The cyclo-converter of claim 11, wherein the developed voltage signal is developed from a load voltage measured across a load placed on a secondary side of a transformer in connection with an output of the cyclo-converter. 13. The cyclo-converter of claim 12, wherein the load voltage is measured after rectifier diodes on the output of the cyclo-converter. 14. The cyclo-converter of claim 1 further including: three phase inputs;bidirectional switches between each input and a first output line;capacitors between each input and a second output line; anda controller arranged to control the switching of the bidirectional switches on the basis of the output voltage and/or input voltage. 15. The cyclo-converter of claim 14, wherein the controller is further arranged to control the switching frequency of the bidirectional switches in dependence upon the output of the cyclo-converter. 16. The cyclo-converter of claim 14, wherein the controller is further arranged to control the switching of the bidirectional switches to control the power transfer and power factor of the cyclo-converter in dependence upon the output of the cyclo-converter. 17. The cyclo-converter of claim 14, wherein the controller is further arranged to control a switching sequence of the bidirectional switches to provide control over the power transfer and power factor of the cyclo-converter. 18. A method of controlling a three-phase resonant cyclo-converter, the method including the steps of: developing a voltage signal representative of a voltage output waveform of the cyclo-converter;accumulating voltage signal values for phase portions of the voltage output waveform, where the voltage signal values are based on a voltage error signal and accumulated historical voltage signal values for the same corresponding phase portions; anddeveloping a switching frequency control signal to control the switching frequency of the cyclo-converter based on the accumulated voltage signal values for corresponding phase portions of the voltage output waveform, and a proportional voltage signal based on a difference between the developed voltage signal and a reference voltage signal. 19. The method of claim 18, wherein the phase portions are within a range from 1 degree to 30 degrees. 20. The method of claim 18, further including the step of adjusting a switching frequency period for a phase of an input voltage waveform having the greatest absolute voltage to achieve frequency control. 21. The method of claim 18, further including the step of developing an integrated signal based on the accumulated voltage signal values for corresponding phase portions, where the switching frequency control signal is developed from the integrated signal. 22. The method of claim 21, further including the step of applying a weighting to selected historical voltage signal values, where the weighting is based on the position of the selected historical voltage signal values within the phase portion relative to the position in which a current voltage signal value is stored. 23. The cyclo-converter of claim 21, wherein the developed integrated signal is an average value of stored historical voltage signal values for the corresponding phase portion. 24. The method of claim 21, further including the step of splitting each corresponding phase portion into a plurality of phase segments, wherein the voltage signal value is stored in a window of phase segments such that the voltage signal value is stored next to one or more historical voltage signal values in neighboring phase segments within the window. 25. The method of claim 24, wherein an average of selected historical voltage signal values within the window is used to develop the voltage signal value. 26. The method of claim 25, wherein the selected historical voltage signal values include a first historical voltage signal value positioned in a first phase segment which was previously used to store the voltage signal value and a second historical voltage signal value positioned in a second phase segment which is to be used subsequent to storing the voltage signal value. 27. The method of claim 26, wherein the first historical voltage signal value is given a different weighting to that of the second historical voltage signal value. 28. The method of claim 27, wherein the selected historical voltage signal values include a third historical voltage signal value positioned in a third phase segment adjacent the second phase segment, where the third phase segment is to be used subsequent to storing the voltage signal value in the second phase segment. 29. The method of claim 18, further including the step of controlling the switching of bidirectional switches on the basis of the output voltage and/or input voltage. 30. The method of claim 29, further including the step of controlling the switching frequency of the bidirectional switches in dependence upon the output of the cyclo-converter. 31. The method of claim 29, further including the step of controlling a switching sequence of the bidirectional switches to predominantly switch in order of the phase having the greatest absolute voltage, followed by the phase having the middle absolute voltage followed by the phase having the lowest absolute voltage.