Patent ID: 7688603

Claim:
A method for monitoring a frequency converter, said frequency converter being a voltage-controlled PWM frequency converter having an uncontrolled main bridge connectable to an alternating-current source (U L1 , U L2 , U L3 ), a direct-voltage intermediate circuit and a controlled load bridge for producing a variable-frequency multi-phase output voltage (U U , U V , U W ), said direct-voltage intermediate circuit of the frequency converter is provided with a small-capacitance direct-voltage capacitor, the main bridge being connected to the load bridge directly without a large-capacitance direct-voltage capacitor functioning as an intermediate energy storage, and in which method the direct voltage of the intermediate voltage circuit is measured, the method comprising the steps of: sizing a capacitance value of the small-capacitance direct-voltage capacitor so that the direct voltage of the intermediate voltage circuit varies strongly and follows a highest main voltage of the source, measuring at least one quantity of the intermediate-circuit direct voltage, and determining on the basis of the aforesaid quantity at least one input voltage value and/or a failure situation of the frequency converter or its load circuit, wherein an a.c. source and the main bridge are three-phase, and the failure situation is a short circuit which is identifiable so that a d.c. level drops to a value less than predetermined value.