Buck XMFR questions

Greetings all you AC Electrical Types, My shop is supplied with single phase juice (notice my proper use of technical terms) at 245 volts average. The voltage does vary a little, usually about 2 volts either way, but sometimes as high as 250 volts. I've never seen it drop below 240, but then I don't monitor it 24/7. I have just checked it many times over the years. I'm in a rural area near the beginning of a line that serves many homes so maybe that has something to do with it. Anyway, I use a rotary phase converter to supply 3 phase power to my CNC machines. Setting the xmfr taps in the machines allows me to supply the machines with power that is within specs. Except for the Miyano lathe. It has xmfr taps for the Fanuc control but not for the VFD spindle drive. The Fuji spindle drive has a switch that allows for either 200 VAC or 220/230 VAC operation. So I am supplying the spindle drive with voltage that is too high. And the problem with that is when the spindle decelerates the spindle load meter goes above 120%, the spindle drive shuts down, and sets an "overvoltage" alarm. The drive uses regenerative braking and the power supply must be capable of supplying 22kVa, according to this link:

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Anyway, after such a long winded description I'm wondering if I could use 3 xmfrs buck configured to supply power to just the spindle drive. I don't know what effect, if any, the regenerative braking would have on any xmfrs connected between the rotary phase converter and the spindle drive. If I can use buck xmfrs I need to know what xmfrs I can use and how to wire them. Thanks for reading this far. Eric

Reply to
etpm
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-1985.pdf

A perfect example of where the use of a buck boost transformer is indicated.

Reply to
PrecisionmachinisT

If you ever get to Central Oregon (Redmond), I will give you three buck/boost transformers that will work for you. I will have to check the voltage, but I think it is 20 volt buck/boost. They were part of an old wave solder machine. I can give you the exact spec if interested. I would love to get rid of them.

Paul

Reply to
Paul Drahn

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I have some doubts this is your problem at all. Most VFDs will handle a wide range of input voltages. I'd check you braking resistors to be sure they are still working. There is also the possibility the transistor that turns on the braking resistor has gone bad. With 22 KVa this must be a pretty big motor and VFD, so the braking resistor must also be pretty big.

Jon

Reply to
Jon Elson

Actually it's a very common problem at least with the older fanuc drives.

But, I suggest he should also check the manual over really good; IF there is a jumper setting that specifies an external braking resistor and there is not an external resistor installed then it will also cause overvoltage to occur on the dc buss.

Reply to
PrecisionmachinisT

I echo the above.

Can you disable the regeneration and use a {?different} braking resistor? It's designed to pump power back into the grid vs waste it; and I suspect your rotary converter does not like it.

Besides, wasting is the good old American way!

Or can you feed the VFD with the single-phase line?

Reply to
David Lesher

Oh, it is a regenerating VFD! That is different, if true. Yes, then the input voltage could interfere with the regeneration system. Usually regeneration is only used on really big drives, 100 Hp and up, as it definitely costs a lot more to do. But, maybe they chose this to avoid having to have a large braking resistor with cooling fans.

Not likely a 22 KVa drive would accept single phase, and especially if a regenerative drive it definitely would not.

Also, the drive may be very unhappy with a rotary phase converter. Not exactly sure what an RPC would do when the drive starts sending 3-phase energy back to it, but it is pretty likely it does NOT behave like a true 3-phase supply. My guess is the "wild leg" would surge.

Jon

Reply to
Jon Elson

I agree with Jon. I had a 5hp VFD running the spindle of my Gorton Master Mill (3hp motor) and not long ago..it started doing the same thing yours is doing. Worked fine in cold weather..but started giving me fits in warm. I went so far as putting a full blown buckboost "transformer" on it..and raised the voltages both up and down from my normal 248 volts. Same thing.

Finally the VFD simply went into OV alarm. Some component inside has failed/changed value, and it needs to go to the repair house.

Yours may be headed down the same path.

Gunner

Reply to
Gunner

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Greetings Paul, I would drive down to Redmond to get those. I have a friend who lives in Portland so maybe I could visit with him. Weekends would be best for me but I could drive down on a weekday. Before I plan the trip it would be best to have the specs so that I know they will work. How big and heavy are these things? Thanks, Eric

Reply to
etpm

I think the wild leg is surging. But the drive is operating at about

20 volts above its rating. And it really is a regenerating drive according to the manual. The drive is inside a cabinet with little ventilation so I can't imagine there would be enough cooling to get rid of the heat from braking resistors. All the machines I have seen with braking resistors have them outside of the cabinet that holds all of the electronics. Before I bought this machine it was connected to a Phase Perfect solid state phase converter the drive didn't have overvoltage faults then. Only when the Phase Perfect had some failing filter caps and when the power coming into the building would spike. Eric
Reply to
etpm

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The drive is not a Fanuc drive, it is a Fuji product. But I will check to see if there is a way to add an external braking resistor. Eric

Reply to
etpm

The issue is the regen system must dump the scavaged power

  • somewhere*. With 3ph grid, that's no issue. But with your rotary inverter, it sounds like it is.

The only answer that springs to mind is adding a 2nd load to the rotary converter; a lamp bank or resistor bank. It would have to absorb the shed power. It's not going to be small.

Reply to
David Lesher

Hmm ... this sounds like it is missing a braking resistor. Perhaps it has one and it has fried/opened?

But it is possible that starting from the higher line voltage, it might be more likely to trip the limits. (And, BTW, have you checked how well balanced your rotary converter is? It is common for one leg to be significantly higher than the others, and this can be helped (somewhat) with tuning capacitors -- though since you are driving multiple machines with it, it makes the balance just right for *one* machine, and perhaps worse for others.

It might be easier to use a single buck transformer on the single phase 240 VAC into the VFD. But your 22 KVA says that it will need to handle something like 91 A (so 100 A for a round number), which is a massive transformer. Hmm ... voltage needed to drop your peak 250 V to 240 V for a reasonable maximum would be 10 V (maybe 12 V would be easier to find), which would be about a 1.2 KVA transformer -- not quite as bad as it could be. Your VFD might need to be a bit bigger to provide sufficient spindle output power from single phase (and check that it will *run* from single phase). Anyway, you would need the power to be Wye, not Delta format -- at least on the output side of a big transformer to allow the buck-boost to work cleanly.

BTW It is likely that the rest of the machine (separate from the VFD and spindle motor) really only needs single phase, which it derives from one phase of the three phase input, so you likely don't need the the rotary converter at all for that machine.

Good Luck, DoN.

Reply to
DoN. Nichols

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Greetings DoN, The machine control does indeed need 3 phase power. The Fanuc control uses a 3 phase xmfr to condition the voltage for the control circuits and of course that power is rectified but it looks like the servo amps use 3 phase directly. Of course somewhere in the amp the ac gets rectified. The spindle drive will only accept 3 phase power. There is no braking resistor option either. Looking at the manual it says that the power is dumped back into the 3 phase source. If I do use 3 buck xmfrs to lower the voltage will the xmfrs themselves be able to absorb enough energy to be useful in that regard? Or will their only advantage be the lower voltage? The xmfrs that Paul Drahn offered to me will drop the voltage 20 volts. This is fine according to the manual. When set at the 230 volt setting the drive will operate all the way down to 198 volts. ERic

Reply to
etpm

How would I add a resistor bank that wouldn't be using power all the time but still be able to absorb surges from the spindle drive? I use the phase converter to run several machines at once in my shop. It doesn't seem to matter if only the one lathe is running or if the spindles on other machines are running, the spindle drive still alarms out when decelerating. Not stopping though, just decelerating. Eric

Reply to
etpm

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I will be at the plant Monday evening and will get all the specs then.

Paul

Reply to
Paul Drahn

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Thanks Paul. I'm hoping your xmfrs will solve my problem. Eric

Reply to
etpm

You can't...

Reply to
David Lesher

Maybe I should have said using significant power. Eric

Reply to
etpm

I know that the Bridgeport BOSS-3 (and at least through BOSS-6) used three phase to power the steppers -- but used a single phase for each stepper. A big three phase transformer to provide the voltages for the stepper drivers, with a "mag amp" (saturable reactor) to reduce the voltage when the stepper was stopped or moving slowly, to avoid the motor overheating. (The idea of a constant current supply to the stepper had either not yet occurred (1975), or it was too complex for affordable electronics at the time.) But, if you had enough current available from single a phase, and replaced that three phase transformer with one large single phase one, or three smaller ones (depending on what was available) you could happily run that from single phase. Then you would only need three phase for the spindle motor, and today a VFD is the way of choice for driving a single motor.

BTW Running that from a rotary converter increases the chances of blowing the transistors used to drive the stepper. They were 2N3055 transistors, nominally rated for 60 or 80 VDC. The ones which I pulled from the old electronics on my Bridgeport test out on a curve tracer to something on the order of 120 to 180 VCE max. I suspect that they were selected for the higher breakdown voltage. Anyway -- with the nominal 80V being applied at full speed to the steppers, and with an unbalance from a rotary converter, it would be very easy to over-voltage the transistors at their normal rating. (Of course, I pulled the stepper motors and replaced them with DC servo motors, and was pretty close to being ready to add the computer and LinuxCNC to put it back in service when the lubricator set it all on fire, keeping me out of the shop until it is cleaned up (a liter of Vactra No. 2 makes a *lot* of dark sooty smoke, and a *lot* of heat, too. :-(

Of course -- and once it is DC -- who cares how many phases were used to produce it. :-)

The VFD? Others are available, of course. It sounds like you have a particularly large one, however, so a VFD which will accept single phase would be rather expensive, unless you were really lucky. I have (or perhaps had? I haven't been able to test it, and the case is partially melted) one which was good for 30A input and output at 240 VAC, and it would quite happily run from single phase. This comes up to just short of 10 HP using single phase ratings. A bit over 15 HP with three phase power input, I believe. It was all of $100.00 at a hamfest in the mid 1990s IIRC.

One which is nice in terms of the power bill (how many HP *is* that spindle motor?), but which complicates things with a rotary converter, as others have mentioned. No real bets on what will work or not.

They will try to dump it back into the rotary converter.

That sounds worth trying -- especially at the price. (Depends on how far you have to drive to get them, of course. :-)

Did he have a current rating on the secondary?

You are getting your three phase from a rotary converter. Is it wired to give you a Wye "Y" neutral point, or is it delta? I think that for the buck-boost, you really want Wye to simplify the setup. Delta would need isolation transformer in each phase as well. If you connected the primarys to a delta source, and hooked the secondarys between the corners of the delta and the load you would be applying correction voltages at a weird phase relationship to the voltages at the corners. But if you have your rotary converter using a Wye connection in the idler motor, you should be fine.

Good Luck, DoN.

Reply to
DoN. Nichols

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