Buck XMFR questions

wrote:


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
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    [ ... ]

    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.
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wrote:

The phase converter output is wired Wye. The spindle motor is 15 HP. And some jobs peg the load meter at 120%. Just briefly. The chips coming off sound like hail hitting the inside of the lathe. Eric
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    [ ... ]

    Wye -- good! That makes the buck-boost easier to set up.
    O.K. 15 HP *That* is impressive.

    Make sure that the interlock on the door is good. :-)
    I'm not sure when I will lose connection -- but the way they are talking, we should have a significant period without power, so don't be surprised if I stop posting in the next few days. (Just to the west of the Washington DC Beltway in Virginia, and lots of rain and wind promised. :-(
    Good luck,         DoN.
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I have real 2 phase high voltage (two of 3 legs from the power substation). (The Two phase high voltage was used here when this place had a Sawmill on the site. Three phase is on both sides of the highway with local poles for houses. My trunk off the main line is 1400' long. I had 1600' but lost 200 in a bad storm and a tree fall.)
I have only single phase 240 in the shop. I spin up 3 phase in a rotary and drive six transformers. Six ? - yes three large voltage step up transformers with boost transformers on each output.
It has been working just fine. The core of the transformer absorbs energy from voltages / currents dumped to it in the primary or secondary.
Since it goes into the core, it can come back out. When the tool goes below that voltage / current value the transformer magnetic field will start to collapse and will generate voltage / current as it needs.
Just like the motors do when they come under load and the load eases off. Under load it draws more current to keep on going when the load slacks off the magnetic field in the core begins to collapse and generates the voltage/current.
Martin
On 10/27/2012 1:59 PM, snipped-for-privacy@whidbey.com wrote:

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technically, he only needs 2 buck boost transformers to do the job but 3 smaller ones would do it just as well.
And in your your case, it sounds like (probably) you actually do have three phase power, just that it's in the form that is commonly called "open delta"

http://www.youtube.com/watch?v=V7_ulixULiw

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