Method for operating a frequency converter with intermediate DC link for supplying a rotating-field machine

摘要

A method for operating a frequency converter of the type having an auxiliary commutating device for commutation at speeds which are lower than a machine-related minimum speed of operation. In a first speed range which begins at speed zero, the thyristors of a three-phase bridge of the inverter are commutated cyclically, as customary, six times per revolution of the machine. In a second speed range which is adjacent to the first speed range and below the machine-related minimum speed of operation, only every second commutation is executed. In embodiments of the invention wherein there is provided a third speed range which is adjacent to the second speed range and extending up to the machine- related minimum speed, only every third commutation is executed. In this manner, an auxiliary commutating device having a predetermined frequency limit can be utilized at speeds corresponding to two or three times the speed at which such an auxiliary commutating device could be used in prior art systems. Only slight drops in mean torque are experienced with the inventive method.