Method and arrangement in connection with frequency converter

A method and arrangement for determining the effective time of a voltage pulse of phase voltage generated by a frequency converter provided with an intermediate voltage circuit, the voltage pulses of the phase voltage being generated from the upper and lower voltage levels (UDC, 0) of the intermediate voltage circuit, the voltage levels showing a difference in potential (UDC). The method comprises the steps of generating a first reference voltage (Uref1) and a second reference voltage (Uref2); scaling the phase output voltage by means of the scaling coefficient (k); comparing the low-pass filtered phase output voltage with the first reference voltage (Uref1); detecting a time instant (t1) when the low-pass filtered phase output voltage exceeds the first reference voltage (Uref1); comparing the low-pass filtered phase output voltage with the second reference voltage (Uref2); detecting a time instant (t2) when the low-pass filtered phase output voltage is lower than the second reference voltage (Uref2); and calculating the effective time of the voltage pulse on the basis of the time between the first and the second time instants and a constant term.