Inrush current

what are the make and model of drives?

I think the key question here is "are you using high-inrush current motor-rated fuses?". The inrush current for the variable speed drives is going to be very high so it's no wonder you are blowing fuses.

Line reactors are a possibility but check your fuses first. If your main protection powering the drives is still operating after this then sequential starting of the drives is probably your best bet.

It may be a good idea to check your main 350A contactor powering the drives also, it may be damaged due to contact arcing due to the combined inrush of the drives.

Check your variable speed drive manual for more information. It should contain details on recommended drive fuse and/or line reactor protection as well as expected inrush current.

First off, you do NOT have "drives", you have SCR power controllers. Since you say they are feeding transformers, they need to be what are called "Phase Angle Controllers" which can come with a soft start (ramped voltage) option. If, for some reason, someone mistakenly designed the system without them or changed them out with what are called "Zero-Cross Variable Time Base Controllers", that would be the problem. Zero-Cross controllers work fine when feeding heating elements directly, but they are NOT to be used on transformer fed elements such as you are describing. Why? Because, they CANNOT CONTROL INRUSH! Given that this appears to be the problem you are describing, I am conjecturing that that s what happened. The only way for you to control that inrush is to have a soft start option on a Phase Angle Controller. So here is a suggested course of action:

1. Learn a lot more about those controllers. The fact that you are calling them "drives" already indicates that you don't know what you have. You cannot hope to solve a problem when you have no idea what you are working with. Investigate and proceed accordingly.
2. Assuming you DO NOT have Phase Angle Controllers, jettison them and start over with the proper devices. Make sure that whichever one you buy to replace them has the Soft Start feature (or option installed).
3. If you DO have Phase Angle Controllers, find out if thy have Soft Start (ramping) as a feature or option. If it is a feature already, find out why it is not working, because it sounds as though it isn't. It may just be that nobody turned it on or has somehow adjusted it out of effectiveness!
4. If they are Phase Angle but don't have Soft Start, see if it can be added in the field and do so. If it can't, agan, jettison them and start from scratch.

Hope this helps.

JRaef, you wrote:

you do NOT have "drives", you have SCR power controllers

Agree, I was baffled at the terms and thought that someone might have tried using variable-frequency-drives to power transformers and then heaters! I've never heard any drives manufacturer say that their units can be used for that and I'm most certain that VFDs shouldn't be used in this application.

jws29720.
Just to add to what JRaef said, are you starting up the power controllers at the same time? That is, have you tried starting them one-at-a-time? It's possible that one controller will not draw enough current to blow the fuses.

Ouch.... I appreciate your answer but it kinda hurt but yes it helped...I went back and read several of your other post and you seem to be very knowledgeable...May I ask what is your title... I will go back to that plant tomorrow and see if I can find some more info on them and see what I can come up with....Once again thank you and hope you have a good week.

JW

Thank you all for the response.....I am sorry to have said the word (drive)when I sould have said SCR Controller.The controllers have not been changed out that I know of and the fuses or the same as they have been for the last ten years according to the Maintenance Manger because he has been there that long.I pulled the prints to the machine and it does not indicate what they are,but to make a long short.The controllers are a late 80's or early 90's model because the machine was put in 1992. I have one small paper on the drive itself which does not give to much information on the controller which I think the name of it is Opac....I do know that the controllers or proportional to the input reference because I have run each controller from a different location .They controlled good from the honeywell controllers but I did not check to see if they had the ramp option.I asked how they started the unit up from it being cold after sitting a week or so and I was told......That they cut all controllers down to zero and bring each one up at a time...So that should take care of them ramping up all together.I noticed the main contactor was pitted due from arching and I noticed it bounced one time....So I went back Saturday when the plant was down and installed temporary diconnects instead of the contactor to see what would happen........The results were when I threw the disconnect switch in with the honeywell controller down to zero which is .8vdc reference .....it rattled the wires going to the primary side of the transformer and blowed the fuses and also I removed half the secondary load ffrom it....I do know that the entire plant was down and there was no load on the system.....I was trying to eliminate the contactor for that one reason.....Because I have seen contactors burnt up before and had inrush current but this one has got me......Everything is still original.....Now I do not know if the controllers have the ramp option but I will try to find out more info on them.....But I do know the oven has been down for 3 or 4 weeks due to being moved .....but same wire and disconnect but turned 180 degrees... I appreciate your help and your time and your answers more than you will know.

When was the last time it ran normally and with the 10 yr old fuses? What changed?

I would check and double check what wires got moved. Put new contacts in the contactor and leave it wired as it was-It dropped out while you were watching? Check everything that is supposed to seal it in--contacts, aux contacts, stops, e-stops, O/Ls all the way back to neutral or com.

This machine ran for over 10 yrs. as you comment about the fuses and the maintenance guy--something was moved and now you are bypassing and rewiring?

If it was my problem, I would troubleshoot it and find the failed component or field device after I confirmed the wiring was reconnected properly.

Don't use a megohmeter on anything unless you isolate the SCRs(or VFDs) as it can destroy them.

How can I transplant the respone that is in "general section" into this discussion?

Why did you post the same question twice? In preparing my response to the other entry, I've missed all the good stuff here.

Good question, lots of detail.

Copy and paste.....the reason I did that is because no one was responding

OK, since you say it was working fine and now it is not, we can assume it was properly designed and has the necessary features for the application. That leaves only 2 remaining possibilities.

1. Something is no longer functioning properly. The first thing I would elimiate is the SCRs. One or more of them could be shorted and depending on design, wouldleave you ith no restriction on current flow. That seams unlikely from your description of there being 0.8V output when you turn t on, but without knowing exactly how and what you are measuring, it is still a possibility. Do you know how to check SCRs? If not, respond and I'll give you some general instructions.
2. Something was disturbed or mis-connected when moved. A classic example of this is when some conductors were disconnected and pulled back into their conduits, the insulation was nicked and is now allowing a high resistance ground (earth) fault. I say high resistance because you are apparently not getting a big arc-flash bang.

As suggested earlier, disconnect everything in the power circuit and megger each section.