Beefing Up DC Power Supply for Single Phase Input to VFD

[JohnMcNutt]

I am making this post because the previous thread has digressed from the original subject, but good ideas were coming.

I am working on a PowerFlex 400 22C-B049A103 that was mis-spec'ed because they forgot to take into account the derating to 35% for single phase operation. I have determined that upgrading the rectifiers and caps might be a viable option for this unit to work to near full power on single phase.

Since it is rated to run at 35% on single phase, it seems reasonable to me that by tripling the rectifier and capacitance, it should be able to run near full power.

I opened it up today and I found a type of rectifier that I don't quite understand. It has three main terminals which I can understand, AC, negative and positive. The diode on the positive one may be an SCR from the looks of the schematic printed on the side. It has a small terminal soldered into the PCB further up as well as another small one coming from the main line, which I may have misrembered. The number printed on it is 11085 R which I did not find on the web.

From there it goes through a large, 2-terminal inductor.

After that, it runs to the caps which are 2200 μF.

My questions at this point for those more familiar with this unit are:

Is that really an SCR on the positive side of the input? Is that how it controls inrush current, so as to charge the caps some other way until it can turn on full conduction in the rectifier? Or is it possible that the inductor is enough to limit the inrush?

I have included some crude drawings of how it works.

 

[Skogsgurra]

Yes, the thyristor is used for inrush.

The inductor is not for inrush. It is there to improve the rectifier's conduction angle, which avoids catastrophic power factor (distortion, not displacement).

You can replace the controlled rectifier and use a simple bridge rectifier. The inrush must then be controlled using a resistor (simplest) and a timer.

The power factor will suffer when you put three times more capacitance there. You can master that by increasing the inductor accordingly. An external inductor will probably be your only option.

In all: is it really worth it? Just go back to the vendor and tell him you made a mistake. He will probably be happy to take the original unit back and sell you a properly sized one.

Also (yes, I am that kind of a boring person) if you do not really understand what you are doing, please don't! Exploding electrolytic capacitors have killed tougher guys than you.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[JohnMcNutt]

Thank you for the reply. The trouble is, I am trying to clean up someone else's mess, and funds are limited.

We are not getting billed on the power factor, so I wonder how bad it would get if we just left the inductor as-is. I am thinking that we could parallel the unused rectifier module with one of the phases, and there is room inside the drive to mount another one along with controlling it to turn on after the inrush is done, for a total of 4. Then add an external capacitor using the terminals provided for that purpose.

 

[ScottyUK]

If you are determined to do this then I suggest you leave the drive internally intact and build a separate rectifier and capacitor bank in an adjacent enclosure. You need to consider protection and heatsinking as well as capacitance value and rectifier capability. You need to be honest with yourself whether you are working beyond the scope of your knowledge, because you are working with energy levels which could quite easily kill you. My opinion - for what it is worth - is that you may be in that position, but it is a question which only you can answer.

 

[Skogsgurra]

" The trouble is, I am trying to clean up someone else's mess, and funds are limited."

If they are so limited that you cannot afford to buy the correct drive, then you shouldn't have started the project at all.

There are drives available on Ebay.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[JohnMcNutt]

" The trouble is, I am trying to clean up someone else's mess, and funds are limited."

If they are so limited that you cannot afford to buy the correct drive, then you shouldn't have started the project at all.

There are drives available on Ebay.

That is what I am leaning towards.

If I was to add another drive (this would be my first choice of what to do) it would be supplying only the second stage of cooling in this large A/C unit. So if it failed, then the room would get a bit warmer but it's been that way for 2 years now already.

That being said, I'd hate to waste scarce money on a piece of junk. What does anybody think of these?

http://www.ebay.com/itm/220V-VARIAB...548?pt=LH_DefaultDomain_0&hash=item2a1eb87154

They are all made in China anyway, as is the A-B drive I am now working on.

 

[Skogsgurra]

If 220/240 V is what you need - then go for it. They also have 380/400 V units.

The looks are like most commodity VFDs and I think you are safe. Those inverters are made by millions and that wouldn't work if there's a problem.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

What was wrong with continuing the other thread?

That drive says it will take 1-phase input but it doesn't say if there is a derating or not.

Does the rectifier looks something like a SKKL?

http://www.semikron.com/products/data/cur/assets/SKKL_92_07911210.pdf

The pre-charge will likely be done using another set of 3 small diodes and a series resistor with the SCR's in those main rectifiers turned-on once the caps are charged. The smaller pre-charge diodes can be circuit board mounted. If the rectifiers are brick devices then you could likely replace using the largest devices with the same physical size.

 

[Skogsgurra]

Correct! There's a diagram a bit down in that ad. It clearly shows a three-phase mains connection.

Oops! Same mistake again.

Forget my 29 Apr 13 0:05 comment.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[JohnMcNutt]

Well, here is an update. The unit came straight from China and it looks a couple generations old. The terminals are very small for what it handles, and the whole unit is pretty small. I like the heat sink on the back, just a bunch of large, open aluminum fins with a tiny DC fan blowing across them but no internal passages for cooling air to get clogged up like the A-B one has.

It works fine on single phase other than a minor problem that it trips out on the hottest days due to IGBT overheat. This is likely due to the fact that it is located near the electronics of the machine where hot condenser air can get to it, and though there is some ducting to prevent this, it is inadequate and I will fix it as soon as I get around to it.

I am satisfied. The model is Hyria SL275-EE.