| 摘要 |
The performance of the cycloconverter when used to drive an induction motor is at present limited because of inadequate methods of modulating the thyristor conduction angles. The main problems are: low maximum frequency due to subharmonics; voltage distortion due to discontinuous current and to uncertainty of current cross-over (for non-circulating current mode); and poor power factor on the input. This specification describes a modulation method which overcomes the first two of these problems without sacrificing performance in any area and proposes a new method to greatly improve the input power factor, although at the expense of possible input current subharmonics. A non-limiting application of the method to a 3 pulse, 3 phase, non-circulating current cycloconverter is described. The technique may also be applied to static power frequency changers other than the cycloconverter. In one broad form, there is provided in a static power frequency changer connecting one or more input phases to one or more outputs, said changer comprising one or more electronic switching means comprising a plurality of electronic switches, modulation means to sequentially activate individual switches of said electronic switching means, said electronic switching means connecting an AC voltage supply comprising one or more input phases to an output, so that the output voltage waveform at said output is built up of sections of the input voltage waveforms on said one or more input phases; a method of selecting, for each said output, an instant of switching of the input waveform to be connected to said output, wherein: for each output said instant of switching is chosen so that during a predetermined time interval the average of the difference between the continuous integral of the desired output voltage and an estimate of the continuous integral of the actual output voltage is minimised, said predetermined time interval including said instant of switching to another input waveform. |
