Patent ID: 8350506

Claim:
A method to start a frequency converter equipped with a direct current intermediate circuit when a permanent magnet motor whose rotor is rotating at a start-up time is connected to the frequency converter, wherein the frequency converter has a network bridge ( 10 ) and load bridge ( 11 ), and the load bridge ( 11 ) has an upper branch with controllable power semiconductor switches (V 1 -V 3 ) and parallel connected zero diodes (D 1 -D 3 ) a lower branch with controllable power semiconductor switches (V 4 -V 6 ) and parallel connected zero diodes (D 4 -D 6 ), and a direct current intermediate circuit between the network bridge ( 10 ) and load bridge ( 11 ), and the said frequency converter uses a current transducer (R DC ) placed in the direct current intermediate circuit, and an analogue current signal composed by the frequency convertor features samples of measured output currents, and the switches (V 1 -V 3 ) of the upper branch of the frequency converter converter are controlled using a bootstrap method, the method comprising: initiating the start-up of the frequency converter by controlling the controllable power semiconductor switch of the lower branch of one output phase to a conductive state for one or several periods of time.