New power convertor technique employing power transistors

A new power convertor, using power transistors in a switching mode as the main power elements, has been introduced. This convertor is capable of conversion from a fixed 3-phase source to variable frequency, controlled output current, such that the operating frequency range is not limited by the supply frequency. The power transformers, inductors and capacitors normally associated with many existing forms of such a convertor are no longer needed. Further advantages of the new convertor are that it is capable of accepting regenerative power, operates without the need for a d.c. link and the input displacement factor is unity, regardless of the output condition.A new mode of operation, that of class ABD, has been introduced, which allows the switching rate of the power transistors to be considerably reduced for a given output. Such a technique reduces switching losses and results in a much less severe operational environment for the power transistors. It also helps to improve the power factor to a value close to unity.The present paper describes a 3-phase to single-phase convertor but the method can be used to provide a fully-regenerative 3-phase to 3-phase static converter, using only 18 power elements and producing a variable frequency, 3-phase controlled output current whose waveform is a very close approximation to a sinusoid.