Patent ID: 7294989

Claim:
A method of protecting a frequency converter against overvoltage by controlling an AC motor during braking or during light load, wherein the frequency converter is a voltage-controlled PWM frequency converter operating in single quadrant and comprising at least one uncontrolled rectifier bridge to be connected to a AC supply line, a direct-voltage intermediate circuit, at least one controlled inverter bridge for feeding an AC motor with an alternating voltage of varying magnitude and frequency, and a control unit including angular frequency controller, a scalar controller and a PWM modulator, the method comprising: measuring an angle between the output voltage vector and current vector, in said angular frequency controller, controlling the angle between the output voltage vector and current vector, in said PWM modulator on the basis of the measurement, and controlling the angle between the output voltage vector and current vector, in said scalar unit so that the angle does not exceed a preset threshold value, of approximately π/2.