# PF40 blowing line fuses. Any ideas?



## JRaef (Mar 23, 2009)

Most likely you have a large transformer feeding this system and are experiencing grid switching transients. The SPD might help a little by clipping the tops of the spikes, but the real answer would be a line reactor ahead of the drive.

Diode bridges are "dumb animals" in that they just passively rectify the AC to DC, then the caps charge up on the DC bus, and the inverter section draws energy out of the DC bus. When there is a severe line transient from a grid switch, it can have a ringing effect, so aside from the SPIKE of voltage, there is a subsequent DROP in voltage for a few cycles. If the voltage drops below the forward conduction voltage of the diode, the diode no longer conducts. But if the inverter is still sucking power off of the caps, the DC bus is depleted. Caps tend to recharge immediately, so when one diode no longer conducts, but the caps are depleted, they demand the NEXT diode in line to make up for it, at a rate commensurate of the available fault current of the line source.

We did an experiment with a 25HP drive fed by a 1000kVA transformer. The drive is rated 40A, the diodes are actually rated for 60A. But during a grid transient, the diode bridge momentarily drew 805A! It was only for a cycle or two, but that stresses fuses (not to mention the diodes and caps). We put in a cheap simple 3% line reactor and the inductive time constant added by that reactor slowed down the rate of rise of the current to peak at only 55A, albeit for a slightly longer time (in terms of numbers of cycles). THAT'S the real benefit of a line reactor. The rule of thumb is that if your source transformer is more than 10X the kVA of the VFD, the drive will experience damaging current spikes on ringing grid voltage transients and you should use a line reactor. One line reactor per drive is fine, but you get essentially the same effect from using one reactor ahead of multiple drives, which can cut down the cost.

What may have happened to you is that repeated instances of this had eventually damaged one of your diodes and / or capacitors so that what was left was pulling even MORE current every time there was a transient. That might explain why the old drive was pulling more current. Replacing the drive has reset the clock, but ignoring this further will put you in the same position eventually. Get a reactor.


----------



## KennyW (Aug 31, 2013)

Agreed on the reactor. Even if you aren't getting switching transients- especially if there is a bit of phase imbalance you could easily just have enough current distortion that the peak current is high enough to blow the fuses. 

I would also check the voltage phase-to-phase on the line side and make sure it's balanced as well. Even with reactors I had a system once with 2.5% voltage imbalance and the peak currents were crazy - easily 4 times RMS. Like jraef says it's also hard on parts in general to have such high peak currents.


----------



## Electrorecycler (Apr 3, 2013)

Wow! Thanks for the detailed explanation. In most almost all cases here our source transformer is far more than 10x the KVA of the VFD. The reactor has been ordered. If it solves this issue then it gives me the opportunity to install them on all our other small VFD's. The one thing I find funny though is that only one of two drives seems to be responding this way. I never have an issue with the other.


----------



## JRaef (Mar 23, 2009)

Electrorecycler said:


> Wow! Thanks for the detailed explanation. In most almost all cases here our source transformer is far more than 10x the KVA of the VFD. The reactor has been ordered. If it solves this issue then it gives me the opportunity to install them on all our other small VFD's. The one thing I find funny though is that only one of two drives seems to be responding this way. I never have an issue with the other.


Might be other circuit conditions, hard to know. It might also be related to when the line events occurred and whether or not those other drives were running at that moment. 

Serendipity can be awesome sometimes. I had a winery customer once who had been running his well pumps to a sprinkler system for years, no troubles at all. Then he put in drip irrigation with VFDs for the pumps to match demand and save water and energy, but began having issues with voltage imbalance tripping. So he blamed the drives of course and the drive mfr had them hire me to find out why. Long story short, they _already had _a voltage imbalance issue for 20+ years when a residential development went in next door to the vineyard. Every day in the summer around 6:00 PM when the people living there would get home from work and start their AC units and AEKs, the random distribution of single phase loads would unbalance the local 3 phase grid. But for that 20+ years, the vineyard never ran the pumps in the late evening because they used sprinklers and didn't want water on the leaves at night (mildew), so they had no idea. By changing to drip irrigation and adding the VFDs running the pumps at low speeds for longer periods, now the pumps were on when the voltage went unbalanced, the drives saw the effects and tripped.


----------



## Jhellwig (Jun 18, 2014)

Another issue that could cause this is automatic power factor correction. Most inverters spec not being on a system that has automatic caps. If there are any near by or up the line it might cause it.


----------



## KennyW (Aug 31, 2013)

Not sure of the other drives are also PF40's, but as you mentioned an ABB, one possibility is that a 20hp ABB unit is in the size range that commonly comes with an internal DC Link Choke, which would have a similar effect to an external line reactor.


----------



## JRaef (Mar 23, 2009)

Jhellwig said:


> Another issue that could cause this is automatic power factor correction. Most inverters spec not being on a system that has automatic caps. If there are any near by or up the line it might cause it.


Good point, and distance between the PFC caps and the VFDs would make a difference.


----------



## JRaef (Mar 23, 2009)

KennyW said:


> Not sure of the other drives are also PF40's, but as you mentioned an ABB, one possibility is that a 20hp ABB unit is in the size range that commonly comes with an internal DC Link Choke, which would have a similar effect to an external line reactor.


I know the ACS550s come with the "swing choke", but I don't think there is any sort of DC bus inductor on the 355s. I might be wrong though. Still, that could be an explanation, or even maybe that the others already HAVE a reactor somewhere ahead of them, just not in the same box, so he doesn't see it as part of the installation.

As I took it, they were the same size anyway, 3HP. Not likely to have a DC bus choke no matter who makes it.


----------



## KennyW (Aug 31, 2013)

JRaef said:


> I know the ACS550s come with the "swing choke", but I don't think there is any sort of DC bus inductor on the 355s. I might be wrong though. Still, that could be an explanation, or even maybe that the others already HAVE a reactor somewhere ahead of them, just not in the same box, so he doesn't see it as part of the installation.
> 
> As I took it, they were the same size anyway, 3HP. Not likely to have a DC bus choke no matter who makes it.


Ah you're right. I don't know where I was getting 20hp. :no:

At 3hp basically nothing would have one. I agree like you are saying that in essence, the unfortunate fact is that with issues like this, there could be any number of "random" factors involved. The fact that some drives do it and some don't unfortunately probably doesn't mean much. A clamp meter that measures "crest factor" or a current clamp on a scopemeter would give some good insight. 

That said line reactors for drives that small are just about free. might as well just do it as if it were a PM and call it a day.


----------



## JRaef (Mar 23, 2009)

Yep, sometimes "random" makes you the hammer, sometimes it makes you the nail. I did a fish plant in Alaska once and they had some sort of event that took out almost every one of their VFDs, over 70 of them, all at once, different brands, all running for several years. The only ones that survived were the 30 newest ones I had put in, which make me look great! 

But honestly I had no idea why because at that same time, that product (gen 1 ABB "GX" series drives) were becoming grenades on me almost everywhere else I installed them and I was a goat for a dozen other customers.


----------

