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SUMMARY This paper deals with the theory of asynchronous motors running at variable frequency. Motor characteristics are found by the help of a computer programme where higher harmonics are neglected, but the influence of saturation is considered. It is shown that it is impossible to get optimum performance if the voltage is a function of the frequency only. A regulator which controls the voltage as a function of both frequency and load to keep constant flux gives torque and current characteristics equal to those of a dc-machine. This paper presents two alternatives for such a regulator. Braking methods are discussed and a regulator principle for a four-quadrant drive without current sensing is shown.