square d drive question

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steveng

Senior Member
Location
Texas
have a vfd that is tripping on a ground fault,
this is a new drive, less than 8 months installed, got any suggestions on what to check?

this is 460 3ph 20hp ,drive, i dont have the model # of the drive, will post it later, square d with full bypass cabinet

there are no grounds evident when reading L-g on any phases.
i can run the motor bypassing the drive, so i know its something in the drive itself, possibly a parameter set to low?

i have reset it after some time and it has run for a few days, then faults
again on same fault, i have had a tech check it, but he didnt fix it:mad:

any suggestion?
 
You most likely have a lower reading to ground than the drive likes. This usually will not show up with a DMM and will with a megger. If this is true the reading probably is not low enough to cause problems when running off of a motor starter but the drive is highly sensitive and will not like it. I mainly deal with allen-bradley drives but I don't believe there are any settings to adjust ground fault settings. If megger test fails to provide any info try running the drive without the motor connected ruling the drive out as a possible cause.
 
Municipal Job?

Municipal Job?

Municipal job? Perhaps another pointless waste of time, effort, & expense due to the creation of an irrefutably less reliable system requiring added moving parts requiring exponentially more power connections just to create a bypass circuit? Was it invoked by virtue of some freak drive fearful engineering firm’s specification?

OK Apologies, please pardon my attitude, at 20HP I just had to belly ache.

You need to identify the VFD model as Marc states. But don’t be optimistic your VFD will have a parameter allowing you desensitize the GF trip level.

You need to check the additional 15-18 power connections required to do a VFD with bypass carefully if you have not done so already. Another trick – Power only the VFD from the breaker (or fuses), and hook the motor up directly to the VFD and see what happens. Something is probably leaking, isolate what you can.

You don't know "its something in the drive itself"
 
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Hello Steve,

For starters i am allergic to idea of Vsd with bypass contactors.

I would like to have either/or situation, try running the VSd directly connected with motor..., and if it trips again (most likely) ..then do the diagnostic test ( ATV61 series drives have this facility to check thyristors and IGBT)

WHat is the fault code ?

Is it Scf1 or Scf 3

If it is Scf3..try reducing the switching frequency to 4 khz

How far is the motor from the drive?

Cheers.
 
I am not aware of any Square D drive that has ever had adjustable GF parameters.

What else is happening in your facility when the drive trips? Does it happen at the same time of day? Does it happen when a specific different process is on line?
 
thanks

thanks

as soon as i get more info i will pass it on, i think ranch said it well, yep another eng firm, firmly in control, specing stuff that makes $ not sense,
leaves us the mess the deal with after they're gone.

vfd's are not my strong suit, so cyphering thru the troubleshooting will be a little slow.;)

thanks for the suggestions,
 
update on drive

update on drive

SQUARE D DRIVE eflex altivar 61 15kw -20hp
drive specs: class 8839 type: EFDLG4VY
mod: A07B08B09J09

application: drives a 25 hp oahu motor, this motor /blower supplies fresh air supply to 6 air handler units on the 2nd and 1st floors below with drives also.

when testing the motor windings,
L-G ,the megger was inhibited by a 80 vdc
reading on the fluke 1587,

there is a ct sensor / used by the bas/on L3 at the main drive contactor
could this cause the 80vdc?

i put the drive in test mode and no faults occurred
when i reset the drive and started it, it faulted immediately on accel. ground fault short circuit.

steve
 
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Why was a 20 hp drive used for 25 hp motor? (A usual pitfall for bypass contactor application)

Hope you tested the thyristors/IGBt using the drive menu > diagnostics..

what is the fault code?

What is the existing switching frequency? if higher than 4 khz reduce it to 4khz...

cheers.
 
steveng said:
there is a ct sensor / used by the bas/on L3 at the main drive contactor could this cause the 80vdc?
Click to expand...

With a bypass circuit, you must have an overload relay external to the drive for protection when the VFD is bypassed. Could that be what you are referring to as a "ct sensor"

I trust you have tried connecting your motor direct to the T1/2/3 VFD terminals. That the problem still exists, you need to disconnect and megger your motor correctly, double plus a thousand, round it up, 2kV is good.
 
steveng said:
there is a ct sensor / used by the bas/on L3 at the main drive contactor
could this cause the 80vdc?

i put the drive in test mode and no faults occurred
when i reset the drive and started it, it faulted immediately on accel. ground fault short circuit.
Click to expand...

You building automation Systems (BAS) is monitoring a single phase of the drive - it assumes that the motor draws balanced current.

You drive a self test mode as well as a auto-tune mode have you down both of them? during an auto tune the drive performs a motor test which compares its calculated "motor impedance" parameters with the actual ones of the motor. If the auto tune is not successful you have another indication that the motor is faulty.

The bypass circuitry of this package is not the cause of a motor GF fault, as there is a transfer contactor which prevents the motor from being connected to the VFD and Bypass at the same time.

Have you tried using a different motor?
 
Most VFDs use a simple "residual current" method of detecting a ground fault. It compares the currents in all 3 legs and looks for the sum to be zero (or something close to it, within it's stated tolerance). If your CT sensor for the BAS is on the load side of the VFD, it is possible that the added impedance created on the motor conductor by that CT on only one leg is causing an imbalance in current that the VFD is interpreting as a ground fault. It may not be much, but it may interact with other naturally occurring imbalances in the motor and circuit to push it over the edge.

Step one, remove that CT and see if the problem goes away. Don't just disconnect the secondary (but you knew that), you need to re-route the motor conductor to NOT pass through the CT.

If that solves the problem, you need to find a better way of getting the motor current info to the BAS. A CT on the output of a VFD is virtually useless anyway, and I have seen a lot of Mechanical Contractors or BAS people who have no clue that this is the case. That VFD should have an analog output that can be used to send motor current to the BAS. It may take an adapter of some kind, i.e. the VFD puts out a 0-10VDC signal and the BAS needs 4-20ma, but it would be a lot more accurate.

The new problem would be, it will NOT then show the current when in Bypass, which is likely the reason why they decided to do it that way in the first place. I have had this exact argument with BAS people several times, they don't want to hear that by having a VFD, they need TWO inputs for motor current. One compromise I have settled on is to monitor current on the LINE side of the VFD, ahead of the switching device that splits the power to the VFD and the bypass. The current is not the motor current, but often times the BAS is only concerned with generalities, not accuracy.
 
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Jraef said:
Most VFDs use a simple "residual current" method of detecting a ground fault. It compares the currents in all 3 legs and looks for the sum to be zero (or something close to it, within it's stated tolerance). If your CT sensor for the BAS is on the load side of the VFD, it is possible that the added impedance created on the motor conductor by that CT on only one leg is causing an imbalance in current that the VFD is interpreting as a ground fault. It may not be much, but it may interact with other naturally occurring imbalances in the motor and circuit to push it over the edge.

Step one, remove that CT and see if the problem goes away. Don't just disconnect the secondary (but you knew that), you need to re-route the motor conductor to NOT pass through the CT.

If that solves the problem, you need to find a better way of getting the motor current info to the BAS. A CT on the output of a VFD is virtually useless anyway, and I have seen a lot of Mechanical Contractors or BAS people who have no clue that this is the case. That VFD should have an analog output that can be used to send motor current to the BAS. It may take an adapter of some kind, i.e. the VFD puts out a 0-10VDC signal and the BAS needs 4-20ma, but it would be a lot more accurate.

The new problem would be, it will NOT then show the current when in Bypass, which is likely the reason why they decided to do it that way in the first place. I have had this exact argument with BAS people several times, they don't want to hear that by having a VFD, they need TWO inputs for motor current. One compromise I have settled on is to monitor current on the LINE side of the VFD, ahead of the switching device that splits the power to the VFD and the bypass. The current is not the motor current, but often times the BAS is only concerned with generalities, not accuracy.
Click to expand...

thanks for the help, jraef, you must be a expert level tech, do you need a job?:grin:

this equipment is under warrranty, when the gc turned over the bldg to us, most of the hvac/elec,equipment had no real problems, however, several are starting to become more obvious with no reliable warranty service after the sale.imho

i will post a follow up, thanks for all the suggestions and support.

steve
 
steveng said:
thanks for the help, jraef, you must be a expert level tech, do you need a job?:grin:
Click to expand...
Thanks, got one (for the moment anyway). But in my office, I am right next to our company's Building Automation Systems Division and I hear these guys talking about these kinds of problems all day long over the cubicle walls. When they finally come over to ask, I tell them what to do, they fix it, it works, then they do it again next time... sigh. :roll: The problem I see is that BAS people tend to be MEs and focus on the mechanical issues They recognize the benefits of using VFDs, however they don't hire EEs to make sure it is done right. If they do hire an EE, it tends to be a "bit head" type who is concerned with twiddling bits and bytes in the BAS system, but as no experience in power systems.
 
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Jraef said:
Most VFDs use a simple "residual current" method of detecting a ground fault. It compares the currents in all 3 legs and looks for the sum to be zero (or something close to it, within it's stated tolerance). If your CT sensor for the BAS is on the load side of the VFD, it is possible that the added impedance created on the motor conductor by that CT on only one leg is causing an imbalance in current that the VFD is interpreting as a ground fault. It may not be much, but it may interact with other naturally occurring imbalances in the motor and circuit to push it over the edge.

Step one, remove that CT and see if the problem goes away. Don't just disconnect the secondary (but you knew that), you need to re-route the motor conductor to NOT pass through the CT.

If that solves the problem, you need to find a better way of getting the motor current info to the BAS. A CT on the output of a VFD is virtually useless anyway, and I have seen a lot of Mechanical Contractors or BAS people who have no clue that this is the case. That VFD should have an analog output that can be used to send motor current to the BAS. It may take an adapter of some kind, i.e. the VFD puts out a 0-10VDC signal and the BAS needs 4-20ma, but it would be a lot more accurate.

The new problem would be, it will NOT then show the current when in Bypass, which is likely the reason why they decided to do it that way in the first place. I have had this exact argument with BAS people several times, they don't want to hear that by having a VFD, they need TWO inputs for motor current. One compromise I have settled on is to monitor current on the LINE side of the VFD, ahead of the switching device that splits the power to the VFD and the bypass. The current is not the motor current, but often times the BAS is only concerned with generalities, not accuracy.
Click to expand...

Grounded or resistance grounded power supply?

Do you have any power correction or harmonic mitigation gizmos on the line side of the ASD?

Control transformer wired on the wrong side of the bypass contactor.

The overload relay on the bypass maybe introducing an suprious impedance.

Is the bypass automatic at full speed? Does the bypass contactor close before the isolation contactor opns on the line side of the ASD, or do you even have an isolation contactor. (I hate bypass.) Do you have an output isolation contactor?
 
follow up report

follow up report

steveng said:
thanks for the help, jraef, you must be a expert level tech, do you need a job?:grin:

this equipment is under warrranty, when the gc turned over the bldg to us, most of the hvac/elec,equipment had no real problems, however, several are starting to become more obvious with no reliable warranty service after the sale.imho

i will post a follow up, thanks for all the suggestions and support.

steve
Click to expand...

it turns out that the motor was grounding at the terminal box ,due to condensation entering the make up box, thru the conduit, this is an oahu unit
which brings in hot moist outside air into the bldg, across a 55 degree chill water coil, the conduits feeding the motor is straight down into the box,

the motor is wasted, under warranty, 25 hp 480 3ph the vfd is ok.

the time it took to solve this problem under warranty is something that
the gc should be concerned about.:mad:

has anyone seen this before?
 
Good find! I feel your pain I work in a food manufacturing facility and everynight we have wet clean up with some powerful chemicals. We use all allen-bradley but I know for a fact they dont like even a little water. Keeping it simple saves alot of headaches.
 
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