VFD tripping on instantaneous overcurrent

[philly]

I have a Powerflex700S 480V drive running a 350hp motor connected to a process fan. I appears that every once in a while (mostly at night for some reason) the drive trips on an instantaneous trip.

When looking at this paramater it states that this trip occurs when the motor current reaches 214% of full load current. Full load current is 392A so the drive must see 838A in order to trip on this fault.

Our DCS does not show any increased current or any trend upward in the current before it trips. Even if there was an upward trend due to process conditons or something the drive would hit its "overload" or "overcurrent' fault before the instantaneous, is that correct?

I have put a power meter on the drive output to watch and see if I can recordd any transients, since our DCS may not be quick enough. My question is what on a fan can cause the current to be driven so high so quickly that it causes an instantaneous fault? The only thing I can think of is some of intermittent cable or motor fault? The system is ungrounded (zig-zag xfmr for ground detection) so this pretty much rules out any ground fault current.

Is it possible that the drive is being fooled into thinking there is this much current for some reason? Noise, harmonics, etc..?

 

[petersonra]

It's a good bet your DCS won't be able to record the instantaneous current that is tripping the drive out. The drive shuts off very fast, like in microseconds.

Is this happening on startup? If so, it's possible the fan is moving when the VFD tries to start, and that can create a very high current. Usually the drive itself can do DC injection braking to keep the fan motionless when it is off.

If it is not something happening on startup, I would be looking at what else is going on at the same time. It's not likely that the motor current would spike fast enough to trip on instant OL.

Think of the instant settings as the magnetic part of a thermal magnetic breaker, and the overload setting as the thermal side. One is instantaneous and the other is an I^2T calculation.

ETA:
Does this thing have an encoder on it?

 

[mhi-us]

You indicate the problem is intermittent. You may want to try replacing the CT control card. While, my experience is with ABB (SAMI and MegaStar) units, I do believe the control concepts are transferable.

 

[masterinbama]

Is this happening on startup? If so, it's possible the fan is moving when the VFD tries to start, and that can create a very high current. Usually the drive itself can do DC injection braking to keep the fan motionless when it is off.ETA:
Does this thing have an encoder on it?

I agree with peter. I had 2 100 HP air handlers on vfd's in a hospital surgery suite. When the power failed these motors would still be spooling down when the emergency generator picked up the non critical gear. I installed some time delay relays that also energized the braking feature upon transfer of power, both from normal to emergency but the other way too. Solved the problem.

 

[philly]

If it is not something happening on startup, I would be looking at what else is going on at the same time. It's not likely that the motor current would spike fast enough to trip on instant OL.

No this is not happening on startup but rather while the motor is running. The motor will run for several hours at steady current under full load, and then all of the sudden appears to trip. I will look at process conditions to see what is happening at the same time, but I really cant see any process condition that could happen this quickly that would cause this amount of current without first causing an overload condition. With a fan I think the only things that can happen are a damper opening somewhere allowing more airflow to move for a given fan speed, or the air becoming more dense with dust or some other material. Even in both these cases however I would not expect this to happen this quickly.

ETA:
Does this thing have an encoder on it?

No this motor does not have an encoder on it.


If there is a short circuit on the motor (L-L) will the motor trip on an instantaneous as such. Is the drive quick enough to prevent damage? Can the drive interrupt fault currents at the motor or is it the primary protection (breaker, fuses, etc..) on the drive that have to interrupt fault current on load side of drive?

Although in this case gnd fault current shouldn't be an issue, typically if there is a ground fault will the drive detect this and display it as such or will it simply show this as an instantaneous overcurrent condition?

 

[Jraef]

Most high end drives have short circuit protection for the motor built-in. But short circuits in the windings tend to not be fleeting, they either happen or not. Things I have seen in the field that have caused phantom SC trips though:

• Water in the feeder conduit; uncommon, but possible. Usually the damage is worse though.
• Bad connection at the pecker head, i.e. Kearney bolts vibrating against each other and wearing through the insulating tape.
• Intermittent short duration power loss, i.e. a quick dip. This is the thing I have seen more often that anything else with this trip. Check to see if the drive is configured for "Flying Restart", a feature that detects the decaying field so that the drive output can be made to match it when power is restored and re-accelerate to the desired speed; it helps to prevent that very thing. masterinbama's fix is what you do for a drive that isn't capable of flying restart, but I'd be surprised if the Power Flex was not.

 

[Jraef]

Looking into it, the PowerFlex is capable of Flying Restart. But you had better take a look at this! Pay particular attention to the bottom paragraph of page 3:
Frame 12 Drives Experienced Unstable Operation and Trip on IOC, Over Speed, or Over Load Faults

You may just need a firmware upgrade.

 

[petersonra]

Most high end drives have short circuit protection for the motor built-in. But short circuits in the windings tend to not be fleeting, they either happen or not. Things I have seen in the field that have caused phantom SC trips though:

• 
  Intermittent short duration power loss, i.e. a quick dip. This is the thing I have seen more often that anything else with this trip. Check to see if the drive is configured for "Flying Restart", a feature that detects the decaying field so that the drive output can be made to match it when power is restored and re-accelerate to the desired speed; it helps to prevent that very thing. masterinbama's fix is what you do for a drive that isn't capable of flying restart, but I'd be surprised if the Power Flex was not.

Some drives call this "ride though" capability, as in riding through a short duration power loss.

 

[petersonra]

If there is a short circuit on the motor (L-L) will the motor trip on an instantaneous as such. Is the drive quick enough to prevent damage? Can the drive interrupt fault currents at the motor or is it the primary protection (breaker, fuses, etc..) on the drive that have to interrupt fault current on load side of drive?

Although in this case gnd fault current shouldn't be an issue, typically if there is a ground fault will the drive detect this and display it as such or will it simply show this as an instantaneous overcurrent condition?

Most drives these days have ground fault protection built in.

Drives can typically shut themselves off fast enough to avoid damge if a short is seen downstream. The semiconductors can switch off very fast.

 

[philly]

This problem has again resurfaced after some time. We ran fine for a while without these issues but they seem to be appearing again.

Is it possible that we are overloading the fan motor? Maybe by moving to much air through it?

If there is an overload condition would we expect to see an overload fault from the dirve before we saw an instantaneous fault or can an overload happen quick enough to just send the drive into an instantaneous fault? I would expect that we would see an overload fault first but am not sure.

 

[petersonra]

This problem has again resurfaced after some time. We ran fine for a while without these issues but they seem to be appearing again.

Is it possible that we are overloading the fan motor? Maybe by moving to much air through it?

If there is an overload condition would we expect to see an overload fault from the dirve before we saw an instantaneous fault or can an overload happen quick enough to just send the drive into an instantaneous fault? I would expect that we would see an overload fault first but am not sure.

I don't think any process upset could happen fast enough to cause an instantaneous trip. It would trip on overload.

Did you update the firmware?

If you updated the firmware and it is still doing it, my guess is you have some kind of intermittant short somewhere.

You might want to megger the wiring out to the motor, and the motor itself to see if that yields any useful information.

But really, the next step after that is to rip out the drive and put the spare in and send it back to AB to have them look at it.

 

[Electric-Light]

Could you connect a data logger and monitor the control input, as well as the mtoor current and see whats going on and go from there?


If you suspect ground fault, you can use something like a Fluke 189 which has data logging designed to respond to sudden change. Use a clamp-meter probe and wrap around ALL three conductors. Under normal operation, the current should be zero, but if there is a fault to ground, clamp meter will register it and 189 will report those as an event. Make sure you have your % change threshold setup right.

 

[philly]

I don't think any process upset could happen fast enough to cause an instantaneous trip. It would trip on overload.

Did you update the firmware?

If you updated the firmware and it is still doing it, my guess is you have some kind of intermittant short somewhere.

You might want to megger the wiring out to the motor, and the motor itself to see if that yields any useful information.

But really, the next step after that is to rip out the drive and put the spare in and send it back to AB to have them look at it.

I will look into updating the firmware.

The drive is on an ungrounded system so this should rule out any ground fault current contribution. However I wonder if an intermitent ground on an ungrounded system could be causing some issue?

Even if somehow the fan rotor was suddenly locked could the motor draw enough current quickly to cause an instantaneous trip our woud it cause an overload trip first? How quick does the change have to be for the instantenous trip to occur?

We have current trend for the motor and looking at the trend we don not see any upward change in current. The motor seems to be running at normal current before it suddenly drops out.

 

[IslandSparky]

I would bet you have capacitor switching triggering it. Try disconnecting your power factor correction.

Do you have a line reactor on the drive?

 

[philly]

We were able to look at this drive and motor today.

We checked the firmware version of our drive to find that we had a firmware revision that was later than the version 3.05 which had the corrections by Rockwell.

We also verified that the flying start was indeed enabled.

We went through the circuit and meggered both the cables and the motor at the same time and found the readings to be o.k. We checked both L-G and L-L and everything looked o.k.

There have been reports however of this motor getting "extremely hot" while in operation as reported by the field personel. When looking at the circuit we found a junction box where the feeder circuits had been reduced in sized and utilized a split bolt connection to conneced the reduced sized cables to the larger ones. We found a couple of these split bolt connections to be loose and went ahead and tighened them all the way. The junction box is located about 10ft or so from the motor.

I know that with a regular across the line starter high impedance connections can lead to unbalanced voltage and therefore unbalanced currents in the motor (causing heating) however with a drive, if the VFD smart enough to recognized the unbalanced voltage or unbalanced current conditions in the motor and make adjustments to its output to correct these.

I verified that there are line reactors on the input to the drive and there are no power factor capacitors on the system. Could the ungrounded system be leading to issues on the drive?

I guess it is possible that we have an intermittend L-L fault (since L-G fault wont cause current flow) somewhere on the cables or in the motor. If we had this L-L fault in the motor would this be something that would be intermitent and come and go or would the initial fault although cleared by the drive be damaging enough to cause permenant damage to the stator. I would think that a fault of this magnitude would cause permenant damage to the motor and not necessarily something that would come and go as the motor heated up.

 

[mxslick]

We were able to look at this drive and motor today.

There have been reports however of this motor getting "extremely hot" while in operation as reported by the field personel. When looking at the circuit we found a junction box where the feeder circuits had been reduced in sized and utilized a split bolt connection to conneced the reduced sized cables to the larger ones. We found a couple of these split bolt connections to be loose and went ahead and tighened them all the way. The junction box is located about 10ft or so from the motor.

I guess it is possible that we have an intermittend L-L fault (since L-G fault wont cause current flow) somewhere on the cables or in the motor. If we had this L-L fault in the motor would this be something that would be intermitent and come and go or would the initial fault although cleared by the drive be damaging enough to cause permenant damage to the stator. I would think that a fault of this magnitude would cause permenant damage to the motor and not necessarily something that would come and go as the motor heated up.

I suggest you replace the motor soon. I'm willing to bet it has a winding failure that happens when the motor gets hot enough and it will finally burn out or short out completely. I have had that happen with film projector motors made by a certain maker.

 

[petersonra]

I suggest you replace the motor soon. I'm willing to bet it has a winding failure that happens when the motor gets hot enough and it will finally burn out or short out completely. I have had that happen with film projector motors made by a certain maker.

The new information of the hot motor makes me also suspect the motor is failing. Perhaps it would be best to put the spare motor in and send the old one old to be rewound.

 

[robbietan]

I have a Powerflex700S 480V drive running a 350hp motor connected to a process fan. I appears that every once in a while (mostly at night for some reason) the drive trips on an instantaneous trip.

When looking at this paramater it states that this trip occurs when the motor current reaches 214% of full load current. Full load current is 392A so the drive must see 838A in order to trip on this fault.

Our DCS does not show any increased current or any trend upward in the current before it trips. Even if there was an upward trend due to process conditons or something the drive would hit its "overload" or "overcurrent' fault before the instantaneous, is that correct?

I have put a power meter on the drive output to watch and see if I can recordd any transients, since our DCS may not be quick enough. My question is what on a fan can cause the current to be driven so high so quickly that it causes an instantaneous fault? The only thing I can think of is some of intermittent cable or motor fault? The system is ungrounded (zig-zag xfmr for ground detection) so this pretty much rules out any ground fault current.

Is it possible that the drive is being fooled into thinking there is this much current for some reason? Noise, harmonics, etc..?

we have experienced the same thing in one of our customers. it would seem that their motor is not built to handle voltage from a drive. during cold nights, the drives would spin the fan motor slower since the cooling is not needed. however, the fan motor heats up, as its own cooling fan cannot adequately cool the motor itself. we recommended that either an outside source of cooling for the fan motor or have the fan motor be replaced by one that is designed to run from a drive

 

[philly]

This motor still continues to get hot even after we fixed some loose connections.

We want to determine exactly what temperature is too hot to decide weather the motor is actually too hot when operating or it just feels hot to us.

The motor insulation class is Class F which I believe is good for about 155degC (Motor is inverter duty rated). So if we do an IR scan on this motor what temperature should we say is too hot for operation. Somewhere around 145C or so?

The motor has temp switches imbedded in the motor which shut down the motor if they reach their designed temp, however unfortunately we dont know what the switches are rated for. Does anyone know what they may be rated for on a class F motor?

I am looking into if this motor is running at lower speeds leading to the heating issue.

 

[petersonra]

This motor still continues to get hot even after we fixed some loose connections.

We want to determine exactly what temperature is too hot to decide weather the motor is actually too hot when operating or it just feels hot to us.

The motor insulation class is Class F which I believe is good for about 155degC (Motor is inverter duty rated). So if we do an IR scan on this motor what temperature should we say is too hot for operation. Somewhere around 145C or so?

It will be much cooler on the outside where the IR scan is then inside the motor where it matters.

The motor has temp switches imbedded in the motor which shut down the motor if they reach their designed temp, however unfortunately we dont know what the switches are rated for. Does anyone know what they may be rated for on a class F motor?

I don't think it matters. They will trip when they are supposed to and that should be wired back into the VFD to shutoff the VFD.

I am looking into if this motor is running at lower speeds leading to the heating issue.

Most drives can be set to have a minimum speed. If you really have a motor overheating problem due to it running at low speed there are two answers. One is an external cooling fan for the motor, and the other is to set the drive to have some minimum speed.

In any case, if the heat issue really is an issue, and it has damaged the winding leading to intermittant shorts, it is going to have to be rewound or just replaced before it fails completely.