A direct ac to ac frequency converter suitable for variable speed induction motor drives

The provision of variable frequency and voltage from a fixed frequency and constant voltage supply by the combination of two solid state switching converters is described. No intermediate dc link is employed and it is shown that the output voltage and supply currents contain no harmonic components below 300 Hz in a 6-step system. The converters are force-commutated and bi-directional and therefore capable of direct regeneration to the supply. Frequency is continuously variable from zero to a value which is only limited by the switch and motor losses. The results of a numerical analysis performed on a 3,5 kW, 3-phase induction motor when fed from the output of this converter, are described. It appears that the energy stored in the leakage reactance is usefully added to the output of the machine during the ‘free wheel’ period when the phases are commutated to produce a frequency change.