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VFD phase loss fault

Most variable frequency drives determine input phase loss fault by comparing ripple content in DC bus section and comparing it with set values (fault response will be based on setting done in VFD like trip level and time duration) . Whenever one phase is isolated, the VFD will function as 4pulse instead of 6 pulse rectifier and will have high ripples in DC content and more capacitor bank loading.

Some VFDs can be set to indicate a loss of input phase as a fault condition, even if a run command has not been given. Suffice it to say that a traditionally sized drive will fault rather quickly upon loss of input phase, with a run command, since the DC bus voltage would most likely drop below an acceptable level. I have seen an application where a VFD was deliberately oversized, so that it could run the motor at full load with only two input phases. But this was a very special application and is not generally practical.

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