VLT 2800 control principle A frequency converter is an electronic unit which is able to infinitely variably control the rpm of an AC motor. The frequency converter governs the motor speed by converting the regular voltage and frequency from mains, e.g. 400 V / 50 Hz, into variable magnitudes. Today the frequency converter controlled AC motor is a natural part of all types of automated plants. The frequency converter has an inverter control system called VVC (Voltage Vector Control). VVC controls an induction motor by energizing with a variable frequency and a voltage suitable for it. If the motor load changes, so do its energizing and speed. That is why the motor current is measured on an ongoing basis, and a motor model is used to calculate the actual voltage requirement and slip of the motor.