Danfoss VLT Series Service Manual page 84

With a phase loss, the current waveform of the remaining
phases would take on the appearance shown in Figure 5-11.
Figure 5-11. Input Current Waveform with Phase Loss
Always verify the condition of the input voltage waveform before
forming a conclusion. The current waveform will follow the
voltage waveform. If the voltage waveform is incorrect proceed
to investigate the reason for the AC supply problem. If the
voltage waveform on all three phases is correct but the current
waveform is not then the input rectifier circuit in the drive is
suspect. Perform the static soft charge and rectifier tests and
also the dynamic SCR/Diode module test.
5-18
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