Frequency converter, which is fitted to supply power between a poly phase alternating electricity source and an electric machine; which frequency converter comprises a network rectifier connected to an alternating electricity source as well as an inverter connected to an electric machine; in which network rectifier and inverter the power supply is implemented with solid-state changeover switches (6,7), whereby each changeover switch consists of a series connection of two controllable IGBT's to each of which a diode is connected anti-parallel, whereby one of those two IGBT's forms a positive changeover contact whereas the other one forms a negative changeover contact of the changeover switch; the network rectifier and the inverter being connected to each other with an intermediate circuit, which intermediate circuit is implemented without a passive storage (9, 10); the positive changeover contact of the network rectifier is controlled to be continuously conductive in the phase of the network rectifier in which the instantaneous value of the phase voltage is greatest, for supplying power between the alternating electricity source and the intermediate circuit; the negative changeover contact of the network rectifier is controlled to be continuously conductive in the phase of the network rectifier in which the instantaneous value of the phase voltage is smallest, for supplying power between the alternating electricity source and the intermediate circuit; whereby the inverter of the frequency converter is fitted to regulate the supply voltage of the electric machine with the inverter control; and in which frequency converter either the positive or negative changeover contact of the changeover switch of the inverter is controlled to be continuously conductive in the phase of the inverter in which the instantaneous value of the supply voltage is greatest in absolute value, whereby the current of the intermediate circuit is determined from the switching state of the changeover switches of the inverter; and whereby at least one changeover switch of the network rectifier is controlled on the basis of the current of the intermediate circuit determined from the switching state of the changeover switches of the aforementioned inverter, characterized in that all the changeover switches of the inverter are controlled into the same switching state at the switching moment of the changeover switch of the network rectifier and on the basis of the switching state of the switches of the inverter, the current of the intermediate circuit is determined to reduce to zero with a certain time delay; and in that the IGBTs of the network rectifier are controlled according to the direction of the current of the direct-current intermediate circuit such that the transistor is controlled to be conductive only when the direction of the current in the positive busbar of the direct-current intermediate circuit is towards the electricity network, and in the negative intermediate circuit busbar away from the electricity network; the frequency converter further comprising a determination for detecting an operational abnormality; and the control of the changeover switch of the inverter is fitted to be disconnected on the basis of a detected operational abnormality.