VFD "ground fault"??

[jahilliard]

Received this email from a fountain contractor? Any feedback as far as a solution for the ground fault issue would really help. I have copied the email I received.

justin----cerus vfd drive --ci-030-p2 the in put on drive is 240 volts 1 phase output is 240 volts 3 phase ---we are running a 15 hp 3 phase 230 volt lake fountain motor with this drive ---the motor rpm is 3450 the maximum amp draw is 43.5 amps under running load we are drawing 38.5 amps . power from main to drive is 150 ft away fountain from control panel is 250ft out in lake.
the unit has been running for 3 months fine ---now it is shutting down on vfd ground fault ---have megged out all wirring and they check out ok ------at firdt we disconnected light ground ran fountain it ran for 5 days tripped on ground fault-----disconnected motor and light ground ran for 5 days ripped on ground fault------disconnected ----set vfd ground fault paramater from 5.0 to 1.5 as per manufacturer...---ran unit for 1 day tripped on ground fault---disconnected all ground sources ran unit ran for 2 days trippd on ground fault----we have tryed everything spoke to manufacturer ( ccant give us answer just guesses )
reading the manual it says vfd should not be grounded into control cabinet ground terminal ???????? do they need to be grounded differently why is it tripping if we have no ground hooked upp---i know that not correct but we are trying to figure this thing out i have another drive comming --- just to see if its the drive----this thing driving me nuts help all cable sizzes are correct size as to voltage drops -----motor ok input voltage 240 ok output voltage 240 3 phase ok ??????????????

 

[ptonsparky]

A VFD does not require an EG conductor to trip on a ground fault. The earth would be serving as the fault current path, albeit a poor one.

 

[kwired]

Is there a line reactor on the output? Sounds like a lot of distance from the drive to the motor, and it is probably suffering from "reflected wave" issues which destroys motor insulation. I have had this before, your motor looks good, tests good, but drive gives you a ground fault. Connect motor directly to line voltage and it will fault and become most obviously damaged. The sensitive monitoring in the VFD just shut it down before current levels become destructive.

Here is a link that can describe it better than I can.

http://abweb.rockwellautomation.com/support/abdrives/documentation/techpapers/ieee/pcic.pdf

 

[Jraef]

Is there a line reactor on the output? Sounds like a lot of distance from the drive to the motor, and it is probably suffering from "reflected wave" issues which destroys motor insulation. I have had this before, your motor looks good, tests good, but drive gives you a ground fault. Connect motor directly to line voltage and it will fault and become most obviously damaged. The sensitive monitoring in the VFD just shut it down before current levels become destructive.

Here is a link that can describe it better than I can.

http://abweb.rockwellautomation.com/support/abdrives/documentation/techpapers/ieee/pcic.pdf

:thumbsup:Like:thumbsup:

GF in a VFD is not like personnel protection GF, it is Arcing GF protection. It is based on a Residual Current Detection method where the current return from all three output phases must match. If you are getting standing wave generation in the output conductors and getting even small "punch throughs" in the insulation at the first turn of the motor windings, the RCD in the drive will see the imbalance immediately. The fact that it seems to be getting progressively worse points in that direction as well. if you meggered at too low of a voltage, it may not have looked bad on your megger. Yet.

 

[kwired]

:thumbsup:Like:thumbsup:

GF in a VFD is not like personnel protection GF, it is Arcing GF protection. It is based on a Residual Current Detection method where the current return from all three output phases must match. If you are getting standing wave generation in the output conductors and getting even small "punch throughs" in the insulation at the first turn of the motor windings, the RCD in the drive will see the imbalance immediately. The fact that it seems to be getting progressively worse points in that direction as well. if you meggered at too low of a voltage, it may not have looked bad on your megger. Yet.

Unfortunately for the OP this motor is probably damaged to the point it needs replaced or rewound, and should have something in place to protect from this on the next one like a line reactor.

 

[Jraef]

Side note: when you are speaking to Cerus, you are not speaking to the mfr. So their tech support is not as good as it should be (in my opinion). These drives are made by LSIS (the old LG out of Korea). But if you call LSIS in the US and tell them you have a problem with a Cerus drive, they tell you that you have to call Cerus, who can't help. Been there, done that, got burned on it myself. Take a look at Benshaw drives, they are also brand labeled from LSIS, but Benshaw tends to have better tech support. You may have to look for the equivalent Benshaw drive series and tell them a little white lie about what you have in order for them to be interested in helping you though.

 

[Jraef]

Unfortunately for the OP this motor is probably damaged to the point it needs replaced or rewound, and should have something in place to protect from this on the next one like a line reactor.

Agreed. In this case the GF indication is more like an "Idiot light" on your car dashboard that is telling you that the engine is overheated. By the time you see it, it's too late.