Motor phase to phase short shows GF on VFD

Saturn_Europa

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC, QMED Electrician
My facility uses the cheap automation direct Durapulse GS VFDs. Twice I’ve seen the drives show Ground Fault. Then the motor and the wire meg fine. In both instances I replaced the motor and the Ground Fault on the drive went away.

I’m assuming the motors had a phase to phase short and this is what caused the ground fault alarm on the VFD.

In a different instance. I’ve seen a terminal strip melt due to loose connections and the metal parts of the strip were shorting out 480v leg to leg. This would cause the ground fault meter on the switch board to switch from zero ohms to infinity constantly back and forth.

It’s my understanding that the ground fault meter on the switch board acts similarly to an Megger Insulation Tester. Where it sends a voltage and looks for a milli amp reading that it converts to M Ohms. So it makes sense that a leg to leg short would show up as a ground fault.

Do VFD ground fault indicators work in the same way ?
 
Saturn_Europa said:
It’s my understanding that the ground fault meter on the switch board acts similarly to an Megger Insulation Tester.
Click to expand...
Very very few Ground Fault devices work like this, except for those installed on ungrounded systems.
The majority of GF devices work by measuring the current 'going out and the current coming back' typically by passing all current carrying conductors through a single CT or by summing the outputs of several CTs.
 
jim dungar said:
Very very few Ground Fault devices work like this, except for those installed on ungrounded systems.
The majority of GF devices work by measuring the current 'going out and the current coming back' typically by passing all current carrying conductors through a single CT or by summing the outputs of several CTs.
Click to expand...
Correct. All VFDs I am aware of use a summing method on the 3 outputs (typically using Hall Effect Transducers on the DC side of the transistors) looking for a current imbalance that shows up as non-zero summing of the phase currents. It’s not measuring anything to ground, but it does assume that if the phase currents do not cancel out, that it is likely going to ground.

Most likely the motor had a turn-to-turn short in one of the windings, meaning that winding then had fewer turns, so less resistance and thereby a different amount of current outside of the tolerance band in the VFDs firmware. A Megger looking at phase-to-ground cannot detect that, because the short is not to ground. Very common on VFD driven motors when no attention was paid to whether the motor was capable of being run from a VFD, especially if it was 480V. It’s caused by the reflected wave phenomenon.
 
Saturn_Europa said:
My facility uses the cheap automation direct Durapulse GS VFDs. Twice I’ve seen the drives show Ground Fault. Then the motor and the wire meg fine. In both instances I replaced the motor and the Ground Fault on the drive went away.

I’m assuming the motors had a phase to phase short and this is what caused the ground fault alarm on the VFD.

In a different instance. I’ve seen a terminal strip melt due to loose connections and the metal parts of the strip were shorting out 480v leg to leg. This would cause the ground fault meter on the switch board to switch from zero ohms to infinity constantly back and forth.

It’s my understanding that the ground fault meter on the switch board acts similarly to an Megger Insulation Tester. Where it sends a voltage and looks for a milli amp reading that it converts to M Ohms. So it makes sense that a leg to leg short would show up as a ground fault.

Do VFD ground fault indicators work in the same way ?
Click to expand...
2 ohm phase to phase?
 
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