Ernesto Wiedenbrug, Ph.D.
The voltage waveform to the left in Fig 1 looks normal, but at another point in time it looks like the waveform to the right of Fig. 1; where the waveform stays constant on the top and the bottom. What is going on? Is this a problem?
VFDs don’t only change the frequency of the voltage, but also the voltage level in proportion with the frequency.
Comparing the frequencies:
Both plots show two complete cycles for the voltage waveforms; the one to the left takes 46.1ms for two cycles – which means 43.4Hz, and the one to the right takes only 41.7ms, which means that the VFD is running at 48Hz. According to the Volts per Hertz control technique , this means that the VFD output voltage needs to be almost 10% higher for the waveform to the right.
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