Measuring average of phase voltage of a frequency converter based on an idealized waveform

A method 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 upper and lower voltage levels of the intermediate voltage circuit, the voltage levels showing a difference in potential,comprising the steps of:

• generating a comparison voltage from the voltage of the intermediate voltage circuit by scaling the voltage of the intermediate voltage circuit to a lower voltage by means of a scaling coefficient;
• generating a first reference voltage and a second reference voltage, the first reference voltage being lower than the second reference voltage, and the second reference voltage being lower than the comparison voltage;
• scaling the phase output voltage by means of the scaling coefficient;
• switching the phase output to the higher voltage level of the intermediate voltage circuit;
• low-pass filtering the scaled phase output voltage to generate a low-pass filtered phase output voltage;
• comparing the low-pass filtered phase output voltage with the first reference voltage when the voltage pulse of the phase output rises from the lower voltage level of the intermediate voltage circuit;
• detecting a first time instant when the low-pass filtered phase output voltage exceeds the first reference voltage;
• switching the phase output to the lower voltage level of the intermediate voltage circuit;
• comparing the low-pass filtered phase output voltage with the second reference voltage when the voltage pulse of the phase output drops from the upper voltage level of the intermediate voltage circuit;
• detecting a second time instant when the low-pass filtered phase output voltage is lower than the second reference voltage;
• 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 the magnitude of which is determined by the magnitudes of the reference voltages, the effective time thus determining the length of an ideal pulse corresponding to the output voltage pulse, having an amplitude corresponding to that of the difference in potential in the intermediate circuit, and having infinitely fast rise and fall times.