Transient Voltages Causing Drives to Trip on Overvoltage
Transient Voltages Causing Drives to Trip on Overvoltage
(OP)
I have 2 480V Siemens MM440 VFD's which I have discussed on this forumn previously. I have an issue with both of these drives tripping on a DC Bus Overvoltage when these drives are not running but rather are just sitting idle. The only thing that can cause these drives to trip on overvoltage while sitting idle would be a voltage spike or fluculation on the line side of the drive. I'm assuming that a voltage spike of a certain amount will cause the DC bus link to go high and thus trip the drive on an overvoltage condition.
I have set up a power monitor to try and capture some of these events. However when I see that drive is tripped and then check power meter I see nothing on the power meter. I suspect that since the power meter (Fluke 1735) is only capable of capturing events longer than a half cycle (8.3ms) then it is possible the voltage spikes are happening too quickly for the meter to capture. I have done some research and saw that these voltage spikes are usually very quick and in the order of .5m - 2ms which would be much too quick to capture wih my meter. Would a voltage spike this quick be enough to cause the DC bus to go high and trip the drive?
I would suspect that if these drives were tripping on an overvoltage condition as a result of a transient then I would see other drives througout the plant on the same distribution system trip as well for the same reason. I do not see this happening. Of course this is assuming the transient is coming from the utiltiy or somwhere up in the plant. These drives are by far located at the furtherst point on the distribution system. Does this furthest distance have anything to do with only these drives tripping?





RE: Transient Voltages Causing Drives to Trip on Overvoltage
Our VFDs are Reliance.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
"mellanled" = DC link
"nät" = mains voltage
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Transient Voltages Causing Drives to Trip on Overvoltage
A couple of things:
1) When the VFd is not running the DC bus voltage will be high anyway and so any instability has even less tolerance before it trips. The trip voltage on the DC bus should be (depending on the rating) between 820 and 840Vdc. If you are on the end of line and therefore a weak supply, and maybe you have AC line reacotrs fitted then this can have the effect of adding a high level of resonance onto the DC bus causing increased ripple. The overvoltage DC bus trip circuit takes the top of one ripple, plus any transient (big or small) and can trip the drive.
2) I think the latest FW version of the MM440 should have a facility to ignore transients when the drive is not running. I understand this was discussed. The reality is that fast transients are virtually impossible to protect against with a VFD and so if you get a shoot through, it would have already passed through the rectifier before it is picked up. So if it is high energy then it would damage prior to any protection in SW picking it up. MOV's on the input would have little chance to protect this as well.
On this basis, it was looked at to simple ignore fast transients when the drive is not running but this was a decision pending.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Any chance of the braking resistor being used to pull the DC bus down before it rises to a dangerous level, or are the transients you're thinking of too fast and too energetic to allow this method to be used? I'm not sure how fast the code controlling this function executes, and whether it could be made to execute more quickly. In the case of the example shown by Gunnar it looks plausible provided the brake resistor can dissipate the energy faster than the the transient can inject it.
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If we learn from our mistakes I'm getting a great education!
RE: Transient Voltages Causing Drives to Trip on Overvoltage
The transient shown was a result of around 40 Mvar PF correction capacitors being connected to a 130 kV grid. Lots of power behind that. I think that a braking resistor stands very little chance against that.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Choose a resistor that has the lowest resistance for the rating of the chopper circuit. Choose the resistor wattage at the low end of the scale to save on costs. A resistor can be expected to handle 10 times its rated wattage for a short period if it is fully cold at the beginning of the event.
While the points mentioned above about pf caps and the expectations about high energy in the pulse are all valid, they are speculations and if may well be that the brake snubber system will end the faulting. In most cases, it has done that for me.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
To ScottyUKs point, it would seem to me that there must be some sort of operating code in the drive's mP that is telling it to trip on high DC bus. So if you inserted an interrupt right before it to fire the braking transistor first, the only remaining delay would be the firing time of the transistor. Wouldn't that work? I'm not saying it would change the nature of the transient, but maybe change the nature of the drive's RESPONSE to the transient. Essentially a shunt resistor?
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Transient Voltages Causing Drives to Trip on Overvoltage
Or, insert a line filter followed by MOVs. The filter could even be too small to support any form or running operation. Have a shunt contactor short the line filter as the drive starts.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Transient Voltages Causing Drives to Trip on Overvoltage
I would say no to the brake resistor. The speed transients come through would never be picked up by the brake chopper circuit and then diverted to the brake resistor.
I'm not sure there would be any point either. Once it gets to the DC section of the VFD then high energy transients would have already done the damage. MOV's and maybe line reactors would take some steam out of the transient but these passive solutions are for slow acting surges.
I have also, as with jraef, seen the main cause to be PFC's switching on and off and causing these transients.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
RE: Transient Voltages Causing Drives to Trip on Overvoltage
I agree with you about the issue of damage to the DC bus components being unavoidable if the energy in the transient is high enough. But if the drive is tripping on OVDC then it is obviously surviving the transients, yet the energy buildup is causing the nuisance trip. Having the energy shunted to the resistors could at least avoid the nuisances.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Transient Voltages Causing Drives to Trip on Overvoltage
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RE: Transient Voltages Causing Drives to Trip on Overvoltage
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If we learn from our mistakes I'm getting a great education!
RE: Transient Voltages Causing Drives to Trip on Overvoltage
It would be a lot of money in brake resistors for the odd transient that passess through.
It could also be one of those insects that get left in the firmware and the reason to check parameter r0018 to ensure you have the latest version. But I cannot say too much, especially as jraef has divulged my mispelt name..
RE: Transient Voltages Causing Drives to Trip on Overvoltage
RE: Transient Voltages Causing Drives to Trip on Overvoltage
If I recll correctly, I believe I heard or saw somewhere that the braking choppers were only active when the drive is in the run mode. If that is the case, then the chopper whould not even be active when the drive was sitting idle, and therefore the braking resistor would do nothing to help with these line disturbances while sitting idle.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Do you think a logical first step would be to get ahold of a high speed meter to try and capture these transients. Once we know the speed of the transient we can the use this information to size or specify reactors and or supressors?
Any truth the the drives being at the end of the line, as to why we only see this problemm on these particular drives?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Looking at Gunnar's curve, I see no reason why the brake chopper could not turn on quicker than the rising DC buss voltage. The IGBT's in a VFD are capable of switching much quicker than the DC buss is rising in that graph.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
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RE: Transient Voltages Causing Drives to Trip on Overvoltage
The AC line reactors will slow down the charging rate for the DC Bus capacitors and so the transient must be there for longer to boost the DC Bus.
If the transient is long enough for the dc Bus voltage to be elevated to trip level with the line reactors in series, there is more chance of other devices being used to get rid of the energy.
I have also seen this effect where the transient was lifting the three phase network above ground, (not phase to phase) and the output was effectively coupled to ground by the capacitance in the cable and the motor, and the output stage was acting as a rectifier. In this case, the problem was isolated to a faulty earth neutral bonding on the supply transformer and when this was sorted the problem went away. The stimulus for the transient was... HV power factor switching once again.
Transient monitoring between the input phases showed no result, but transient monitoring between an incoming phase and earth showed a very high transient with energy.
The problem with very fast transients is that the voltage monitoring circuit can be fooled into thinking that the average DC Bus has risen when infact there is just a transient superimposed and that is very different.
Best regards,
Mark Empson
L M Photonics Ltd
RE: Transient Voltages Causing Drives to Trip on Overvoltage
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Oldironman
That is a very interesting solution that you mentioned regarding tying the DC bus's together. Can you explain more how if either set of bus caps had spare capacity it would ride through the transient? Are the bus DC capacitors the limiting factor to these transients and DC overvoltage conditions? Why is that so?
I'm assuming that we are talking about the capacitor on the front end of the DC bus used to smooth the ripple from the rectifier?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Without knowing the magnitude or duration of the transient, is there a method for determining reactors or suppressors on line side of drive?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Attached are the part numbers from Siemens to order the line commutating choke. (AC Line Reactor to most people)
Check the size of MM4 you have and pick the option. (you will find this part number of the label on top or side.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Thanks for the information on the line reactor.
I know typically reactors are used as a current limiting device and others above have mentioned that the reactor will limit the dc bus capacitor charging rate from a result of the surge.
Does this reactor in this situation work by limiting the current to the drive associated with this fast transient or does it somehow work as a filter as well for attenuating the transient voltage magnitude and duration. I want to learn more on how this reactor works in this application. Any information I can find anywhere?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
So, the reactor presents a higher impedance to the transient (10 to 30 times higher) then it does for 60Hz line power. If you insert a fairly high impedance between the transient and the DC buss capacitors then the it will take longer for the transient to charge the capacitors. Hopefully, the transient has passed before the buss capacitors are charged much higher.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
LionelHutz
The frequency of the large voltage spike in waveform that Gunnar posted looks to be about 1.5ms which would equate to about 10 times line frequency.
So then this 10 time line frequency will cause the impedance of the reactor to increase by 10 times line impedence and will present a higher impedence to the transient thus limiting the current that can pass through to charge the capacitors?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
RE: Transient Voltages Causing Drives to Trip on Overvoltage
More than 10 years ago I performed some investigations how drives could be protected against such kind of events.
The basic outcome was that such an event -at least as defined in the standard- injects an amount of energy into the drive that is so high that you cannot cope against it with varistors or with storage in the DC-link.
But I assume activating the brake should work as long as there is some impedance in the AC-line to limit the current.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
I'm still trying to look for a solution to this issue. The other day the two drives tripped at exactly the same time, which almost definitely tells me that there is something on the line side of the drive causing this.
The last two times this happened, it seemed to happen as other equipment was shutting down. There is no PFC switching on this system, but is it at all possible that other motors or other pieces of equipment shutting down could cause some sort of voltage transient that would effect the drive like such?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
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RE: Transient Voltages Causing Drives to Trip on Overvoltage
RE: Transient Voltages Causing Drives to Trip on Overvoltage
It was happening at 3.40am on a saturday morning when most loads were off, the voltage was higher, and a PFC cap was switching on/off.
The problem with the drive was that it was not running. The trip state was evident in the morning when the VFD was required to run but as the fault was still active the VFD would not start as it could not clear the fault. A change of logic then cleared the fault and even though the drive would still trip, it would restart anyway and clear the fault on the drive.
Not actually solving the problem but actually drive tripping was not the problem. As I have spent the last 22 years telling people: when a drive trips, it is doing it's job. "Don't shoot the messenger!"
RE: Transient Voltages Causing Drives to Trip on Overvoltage
LionelHutz
Like I mentioned in an earlier post I had place a fluke 1735 meter on the line side of the drive hoping to capture any transients on the system that may have caused the drive to trip. After looking through this post and seeing that most of these transients are typically much quicker than 1/2 cycle (too quick for the fluke)I realized that I would not be able to capture any with this meter. I am now working with the utility and trying to see if they have a high speed type meter that they can lend me to try to capture these events. I am waiting to hear back from them.
The funny thing is, for the 3-4 weeks for which I had the Fluke meter hooked up to the line side of the drive there were no reported incidents of this drive tripping. After realizing the meter would not capture transients I removed the meter, and since then there have been several trips both at night and during the day. I found this to be strange.
This may be a crazy question, but is there any reason why having the fluke meter connected to the line side of the drive may have prevented it from tripping? Maybe by presenting an high impedence or something? I'm sure this is purely coincidal but I figured it would be worth mentioning.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Or, has this been nothing but conversation up to this point?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
I spoke with Siemens about brake resistors and I was informed that the chopper circuit was only active when the drive was in the "run" mode so therefore the brake resistors would not do anything to eliminate this issue while the drive is sitting idle.
I have not yet installed line reactors. I wanted to see for myself that there was indeed a transient on the line that was causing these issues. I have been waiting on a response from the utiltiy trying to borrow one of their high speed meters which they have used out here in the past.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Transient Voltages Causing Drives to Trip on Overvoltage
Keith Cress
kcress - http://www.flaminsystems.com
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Jraef
I should have explained my meter setup better, but I did not have the current probes hooked up both rather only the three voltage probes on the line side of the drive. I figured I was only looking to capture transients on the incoming voltage supply so I was just monitoring the voltage and was not concerned with the current.
My apologies for not explaining better.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Coincidence perhaps?
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Transient Voltages Causing Drives to Trip on Overvoltage
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If we learn from our mistakes I'm getting a great education!
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Sounds like a little inductance on the input might solve the problem. Fit a line reactor, it's a lot cheaper than leaving your fluke connected.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
O.k. sounds like all roads point to the line reactor and it may be best just to fit a line reactor rather than try to capture any transients.
I know you typically fit these reactors to 3-5% of the circuit impedence but I would like to become a little more educated on fitting these reactors. Can anyone suggest anywhere where I can read up on this application?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
I understand how line reactors can protect the drive by creating a high impedence for high freuency transients and thus limit the current to the DC bus.
However can the reactor actually attenuate the magnitude of the voltage transients itself to prevent it from damaging components in the drive. I would think that with these high frequency transients the impedence of the reactor goes to a high value and alot of the transient voltge is therefore dropped across this impedenc thus diminishing the votlage that would appear at the terminals of the drive.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
RE: Transient Voltages Causing Drives to Trip on Overvoltage
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If we learn from our mistakes I'm getting a great education!
RE: Transient Voltages Causing Drives to Trip on Overvoltage
good point. In this drive they are bloody difficult to take out.
rockman
As the drives are not actually being damaged currently, they are tripping on overvoltage, the transient may not have that much energy to cause real damage, just trips. The added inductance may just eliminate the trips.
You are right in your thoughts about high frequency transients and line reactors though-sometimes they may just pass straight through.
The overvoltage circuit is on the DC link of the VFD so measuring means the transient has already passed through the rectifier. hence the comment I made a while back that trying to solve the problem of "what to do with this voltage once the VFD has measured it" may be seen as a little too late if it is high energy.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
They recomended using a TVSS from a company called Rayvoss whith whom they claim they work closely with on these type of issues. I've attached a brochure for one of the type of models they recommended.
Has anyone ever heard of this company or their modules? I know many of you have said that reactors would be a viable solution, but do you agree with Siemens on their recommendation?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
http://w
another says line reactor
http://www.mtecorp.com/lineload.html
You can dig thru most vfd manufacturers and they say line reactors on input for nuiscense? (cant spell) overvoltage tripping.
To me there selling the next new toy. maybe it works better, probably pay premium on the product line i am sure.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
MTE recommends the products that they make too... there's a pattern emerging here!
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If we learn from our mistakes I'm getting a great education!
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Yes, Rayvoss are a well established company and their products are good. A TVSS would typically be recommended where high energy surges are a possibility, like lightening.
From your description, and the fact that the Siemens MM4's rectifier is still intact, I don't think you are getting high energy surges but low energy fast transients.
I'm sure the Rayvoss would be ok but it could be seen as 'belts and braces' i.e. overkill. Your choice, your money I guess
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Now, after saying that you would use a 277V arrestor for the 480V system. It appears this arrestor has a let-thoutgh voltage greater than 1000V during a fault, which, to me, appears too high to keep the drive from tripping on over voltage. However, if Siemens has a history of success with these then you have to decide if their claimed successes mean you have success.
Every drive we sell has a braking resistor installed and we also push for a line reactor on every drive. We really don't have to deal with very many overvoltage trip issues. The last one was creating a huge puzzle until I had them turn the resistor switch back on. It was working and suddenly stopped and no-one could figure out why. On power down, the electrician had turned off every disconnect switch he could reach.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Siemens is recommending some particular manufacturers product.
Other VFD manufacturers still recommend line reactors on line input transisents.
I guess the real question you have to ask can you second source this product? Yes, it has great features, but to be tied to them at the hip is risking realibility on a brand new product, can you get replacement parts in time, and at a reasonable price?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
ABB solved the problem by using a relay to connect the 5kV DC bus through discharge resistors when the drive is on idle/standby mode. The relay disconnects the discharge resistor when the drive starts.
The discharge resistor is cooled by the drive's cooling water system.
I think without the discharge resistors, the DC link voltage is being built up accumulatively by the spikes or surges in the line. When the drive is running, there will not be any high voltage built up in the DC link because the inverter is drawing the current off from the DC bus to the load (motor).
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Probably wouldn't hurt to call Siemens back and press them for an explanation on their choice.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
In total I had (3) drives that were having these overvoltage issues. Drives were a 5hp, 10hp, and 70hp.
I purchased a line reactor specified by Siemens for the 70hp drive and this did not solve the transient problem. This reactor however was only a 2% reactor.
For the two smaller drives I purchased 5% reactors and installed them on these drives and they have not tripped since after having been tripped every morning.
Do you think that increasing to a 5% reactor on the 70hp drive will be better than the 2% and help eliminate the problem?
Is it true that it is the impedance matching of the drive to the source (transformer) that leads these transients to be of more effect in some cases? Thus installing the imput impedance to help match more to the source?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Neil
RE: Transient Voltages Causing Drives to Trip on Overvoltage
RE: Transient Voltages Causing Drives to Trip on Overvoltage
For the 70hp drive I am going to get a 5% reactor and also a TVSS to hopefully eliminate this issue.
MAGTIGER
I am curious about how to determine the impedance ratings you discussed above. For instance I looked at a 2hp VFD that had an input rating of 3.8A at 480V. On the line side of this drive I noticed a reactor with the following info:
L= 12mH
If = 4 Amps
Ith = 6 Amps
I was told that this is a 5% impedance reactor. How do you determine from this information what % impedance this presents in the circuit? Can you simply take the drive rated voltage and divide it by the current rating to come up with a rough aproximation of the drive impedance and then use 2*w*L for the reactor to figure out its impedance and compare the two?
Can you explain about the different % impedances and their current values on different drive sizes as you mentioned above?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Why they are rated that way baffles me but that's the way it is. Wouldn't it make more sense if they were rated in microhenries and amps.
I've never been able to get a better explanation than the above and have, after all these years, stopped asking.
I suggest you take it and run with it too!!
RE: Transient Voltages Causing Drives to Trip on Overvoltage
You are right, somthings are better as just being accepted however I cannot help but be curious :)
I found the attached article somewhat interesting, as it goes into a small calculation about how these reactor values are calculated.
The article also goes on to say how the effect on the drive is a function of source impedance and transient magnigude. I guess the more source impedance the lesser effect the magnitude will have on the drive?
The article also says that a reactor may not have any effect when the drive is sitting idle. I have also heard this from others, simply due to the fact that current is not flowing when idle. However it sounds like others have said that even without current flow the drive trips as a function of the current resuting from the transient voltage so I would not think it mattered in this regard wheather or not the drive was pulling current. The current surge that would charge the bus cap would be dampaned by the inductance of the reactor.
I could see however that an idle drive would not have a load contributing to reducing the bus voltage at all which would not help things.
I do plan on taking all advice above and installing both 5% reactor and TVSS however I cant help but be curious to learn certain details. I appreciate all of the help with this.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Forgot Attachment. Here it is.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
I have a great deal of experience with the MM series drives
And there is a setting or two in the setup that may help with this problem. However line reactors are possibly the best. In fact if you look in the install instructions you will see that above a certain KVA feed that line reactors are required.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
I've seen various AC inverter drives that are susceptible to the 'notching' DC drives produce, and which fault on DC bus overvoltage due to them.
A thought - it probably won't help, but is fairly cheap to implement - try adding suitably sized ferrite toroids around each AC line wire. If the observation about drives not tripping when an AC clamp was used wasn't a coincidence then this might do the trick.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
I am curious about how to determine the impedance ratings you discussed above. For instance I looked at a 2hp VFD that had an input rating of 3.8A at 480V. On the line side of this drive I noticed a reactor with the following info:
L= 12mH
If = 4 Amps
Ith = 6 Amps
I was told that this is a 5% impedance reactor. How do you determine from this information what % impedance this presents in the circuit? Can you simply take the drive rated voltage and divide it by the current rating to come up with a rough aproximation of the drive impedance and then use 2*w*L for the reactor to figure out its impedance and compare the two?
Can you explain about the different % impedances and their current values on different drive sizes as you mentioned above?
Well, it depends on your understanding of the terminology. If and Ith are not industry accepted terminology but may be understood to mean fundamental amps (If) and thermal amps (Ith). Which one applies to the 3.8 amps of drive rating. Take your pick. Neither results in 5% Z.
.012 * 2 * pi * 60 * 1.73 * 4/480 = 1.6%
.012 * 2 * pi * 60 * 1.73 * 6/480 = 3.2%
Hopefully you didn't pay for a 5% reactor.
In most cases 3% is enough anyway. But for your drive this reactor looks like about 1.5%, marginal.
Sizing reactors on percent impedance is nothing more than a convenient way for buyers to compare price in a standard format. I agree that Engineers would probably be more comfortable just using inductance but then every reactor rating would have a different L. With %Z, the buyer just buys reactors on amps and for volume contracts, typically would buy all 3% or 5% reactors. It's part of the comoditization of the product.
Neil
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Thanks for the information. I am having a little trouble following your calculation to arrive at the % impedances.
It looks like you are using:
L *2*pi*f*1.73*A / V
So it looks like you are taking the overall impedance of the reactor times the current and dividing it by the voltage? Is this correct for determining %Z? I dont quite follow it.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
then multiply the X ohms times amps to get the voltage dropped across the coil (phase). then times 1.73 to get the line to line voltage drop. Finally take the line to line voltage drop just calculated and divide by the system rated voltage (typically 480). This gives the per unit voltage drop which is by definition, the impedance (of course we are ignorring the very small effect of resistance). Now multiply times 100 to get %impedance.
Neil
RE: Transient Voltages Causing Drives to Trip on Overvoltage
Ah ha! Makes perfect sense now. Thanks for clearing this up!
RE: Transient Voltages Causing Drives to Trip on Overvoltage
RE: Transient Voltages Causing Drives to Trip on Overvoltage
I have been troubleshooting power quality porblems for 27 years. We first used the BMI PQ monitors and then switched to the RPM 1650s. RPM was bought by Fluke and their latest version is the 1750s and 1760s. We still like the 1650s best.
Our experience is that most drive trips on overvoltage are caps switching. The facility may have switching caps for power factor correction or the utility will have voltage support capacitors.
The utilities will normally switch the capacitors in the 6AM region and then at 10-11PM. All depends on the local grid and loads.
The switching causes ringing on the line, which can cause an overvoltage trip. They usually last about 1/4 cycle and do not have any voltage rise, so they don't trigger a transient capture on a PQ meter. They can trigger a waveshape fault.
Usually line reactors are the best bet.
As far as MOVs, the normal failure mode is to fail short and explode. If there is limited fault current avaiable, they smoke and smolder. If there is sufficent fault current available they will short and then blow open. If the problem really is voltage spikes MOVs are fast enough to protect you.
Make sure that you get TVSS with multiple, individually fused MOVs. This way, a single MOV failing will not take out the TVSS main overcurrent protection and leave you unprotected. Also, UL has just come out with a new standard UL1449-3rd Edition. Make sure that the TVSS (UL now calls them Surge Protection Devices SPDs)has a UL1449-3rd edition label from UL or a Nationally Recognized Test Lab (NRTL).
We have had limited success in applying TVSS for overvoltage drive trips.
We have had a lot of success applying high-end TVSS protecting drives from catastrophic utility hits.
Fraser Jim
RE: Transient Voltages Causing Drives to Trip on Overvoltage
RE: Transient Voltages Causing Drives to Trip on Overvoltage
So for example for a given VFD (lets just say 75hp) and the same utility transient, would the transient magnitude seen at the drive be different if the drive was connected to a 1000kVA transformer as opposed to a 1500kVA transformer?
If source impedance does have an effect, then for the same transient on lets say a 1000kVA transformer will there be any differnce of the magnitude seen at the drive on a 10hp and a 75hp power drive side by side? In other words does impedance mataching have an effect?
RE: Transient Voltages Causing Drives to Trip on Overvoltage
So, the source impedance will have an effect and the size of the drive will have an effect on the same source.
Looking at it another way, the 5% reactor sized for a 75hp drive would not have much effect if it was installed on a 10hp drive.
Look up 3-phase rectifiers and get a better feeling for how the caps charge then consider what happens when you place an impedance in the AC lines feeding the rectifier.
RE: Transient Voltages Causing Drives to Trip on Overvoltage
It is possible that the panel serving the VFDs is experiencing momentary voltage sag (within 2-3cycles let's say), that would cause a havoc, and the way the VFD display/parameters are setup they will not necessary indicate the actual cause of problem.
You will hunt it down, just make sure your fluke setup is done properly. I suggest connecting phase-to-neutral (set the low threshold at 255volts hight to 305volts) so you can see each phase seperate. If supplied with 3-phase delta connection w/ground, you have no other choice but to go Delta (try to set the low at 456-460volts and high to 508volts or above, to 528volts dependig on your situation)
Make sure your pre-trigger and post triggers are set properly so you can see what VFD is experiencing prior to the shudown event, etc. Small problems can cause big head-aches.
Based on experience I'd say incoming voltage is a good place to start your investigation to identify the cause. I assume you have checked both the system grounding & equipment grounding, and they all verified/checked OK.
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RE: Transient Voltages Causing Drives to Trip on Overvoltage