VFD question

Certainly not on this newsgroup, where most posters think 'Ohm's law' is merely a rough guideline and that the laws of physics don't apply to them.

Reply to
Michael A. Terrell
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The OP has a problem with a VFD with an overvoltage condition on it's input. Please explain how your hack job on the output will fix that.

Reply to
Michael A. Terrell

Three transformers on the individual phases isn't 'parallel'.

Reply to
Michael A. Terrell

Oh I see....and the moon is made of cheese, too....

Certainly seems true in the case of YOUR stupidity.

Reply to
PrecisionmachinisT

You may be able to increase the maximum voltage setting up to 255 volts using function code 2 , see page 7-3.

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Reply to
Benny Fishhole

Oops, make that function code 3 :)

Reply to
Benny Fishhole

Yawn...

Reply to
Michael A. Terrell

I don't see, in the original question where it is specified that the OV error is coming from the VFD or from the mill. If the error condition is in the VFD, then of course you have to fix the input voltage. If it's on the mill, and you're only using the VFD to generate 3-phase and not as a speed control, it would seem perfectly reasonable to correct the voltage at the output, assuming you had three smaller transformers as opposed to one large one.

Please explain why that wouldn't work. I'm not trying to play gotcha - I really am interested in whether there is a reason not to do this.

Reply to
rangerssuck

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Both of those support my position. Nice try. Did you even read them? They have nothing to do with either VFD or RPC installations. At the 2 MW rating in the articles, they are small substation grade transformers. Not something you could use for a home shop.

Reply to
Michael A. Terrell

It's not about "protecting" it's about eliminating alarms and making the machine run.

No three phase motor is going to have an issue with 249V, that is within acceptable limits.

Well duh. Do you think that three transformers found in someone's odds and ends pile are going to be identical?

Reply to
Pete C.

We're talking about an RPC, not a VFD. As for overvoltage, 249V on single phase 120/240V service is not an overvoltage condition per utility standards.

Reply to
Pete C.

Which can't be done with transformers on the output. Just like you can't fix a low line voltage with transformers on the output. I recently repaired a pipe bending machine that was reporting low line voltage. It had two problems: The idiot electrician used a single pole breaker and fed 120 VAC to the machine. With the proper breaker and connecting his red wire to the other pole instead of the neutral gave me

208 volts, but the machine was sluggish. A boost transformer took it to 244 volts, and it perfomed like it was supposed to.

Then there is no need for transformers to drive the machine's motor, is there?

Duh, yourself. It depends on who's shop you find them. I had three identical 208 primary 6 v 400A secondary transformers for years. They came from a set of four, mounted on a steel plate and connected to give

1600A at 6 volts. I have had up to 35 identical transformers from mainframe computers. We scrapped tractor trailer loads of mainframes that ran on 208 three phase 60 Hz and had tons of good used transformers until the local surplus places ran out and bought 2000 to 3000 pounds from us. I sold most of them by the pound.

Even if the transformers aren't exactly the same, matching turns ratios can be run in parallel without problems.

Reply to
Michael A. Terrell

A common RPC uses a 208 Volt three phase motor, not a 240 volt single phase.

Reply to
Michael A. Terrell

Big job changing them out?

Whatcha charge to fix it for me?

Gunner

Reply to
Gunner Asch

Do you have a soldering iron? Get some solder wick to remove the solder from the old capacitors. Solder in new capacitors.

Digi-Key, Mouser, etc for new caps, and solder wick, if needed.

Reply to
Cross-Slide

Probably not that hard at all, perhaps 15 minutes work. The biggest problem would be getting exact replacement parts that would fit without any hacking.

Hmm, I don't do this sort of work for hire, I'm busy enough making new gear. There ARE places that do these repairs regularly, and would be likely to have the parts on hand, by make and model. But, I don't know who to direct you to. Watch out strongly for vultures who take your unit apart and then want $1000 to even put it back together without fixing it.

Jon

Reply to
Jon Elson

Depends on the construction of the VFD in question. I'm not familiar with the one which you have, but I suspect it is like my 7-1/2 HP Teco/Westinghouse one, which has big screw terminal electrolytic capacitors secured to a common PC board. For a quick check, verify which terminal of each is positive, and run it with the cover off and clip leads to similar rating capacitors outside the box. If that fixes it, replace the caps in the box. Should just be the terminal screws, and some kind of body clamp for each cap.

Not sure what values to expect, but expect the voltage rating to be about 350 VDC or higher, and probably 5,000 uF or so -- in other words, fairly large caps.

FWIW I picked up at a hamfest a few years ago a pair of caps which should be a bit too big for the VFD in question. The rating is 4700 uF @ 350 VDC, and it happens to have a 12mm stud on the bottom to attach it to the structure of the case. The case, instead of being totally cylindrical, is squished to form about a 12-sided figure, so you can get a grip on it with a (large) wrench while you are tightening the nut on the bottom stud. (The screws are metric too -- but missing from the caps as purchased. IIRC, they are M6x1 screws, but I could be mis-remembering that.

A pity I can't get more of them. These I am keeping for when the caps in my big VFD die. I think that they cost me $5.00 each at the hamfest -- but from a normal vendor -- who knows?

O.K. From Mouser, $141.00 to $188.00 -- and they're all non-stocked. :-(

You should be able to do it yourself, if it passes the clip-lead test above. I'm on the wrong side of the country to be worth shipping it here and back. And I don't know what the caps would cost anyway. Perhaps Jon is closer to you.

If the clip-lead test passes, and the caps don't fit the current mounting point -- mount them externally (wires not too long) but protect them from chips. :-)

Hmm ... come to think of it -- the caps could still be fine, and the terminal screws securing them to the board could be a bit loose.

Good Luck, DoN.

Reply to
DoN. Nichols

I sent you an email with a local phone number of a guy that might be able to help you.

Reply to
Michael A. Terrell

Why change them? If you have an external DC choke terminal, you can add outboard caps without surgery.

Reply to
David Lesher

Because he's a bumbling fool who most likely doesn't even know what they look like.

Reply to
PrecisionmachinisT

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