Autotransformer question

On 06/28/2010 09:50 AM, Ignoramus28517 wrote:


To make 20-80VDC for your drive? That should be good. What current do the drives pull? You need to size your caps so that the instantaneous voltage to the drive never falls below 20V -- I calculate from the peak voltage to 20V in 1/120th of a second, assuming maximum line sag (usually 100V if it's a 120V line). You really have more than 120th of a second, but that's an easy number to remember and it's conservative.
A surplus place may have something for less money; since it's a one-off you won't care if it's not a production part. I like Surplus Sales of Nebraska (http://www.surplussales.com ) and Herbach & Rademan (http://www.herbach.com ). I've never had a problem ordering from them. I took a quick look expecting to find joy and they may not have exactly what you want -- but it's not a bad idea to look.
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Yes, 20A at 80 VDC. Drives are limited to 20-80 VDC. (they refuse to work if voltage is outside this spec)

15A per drive continuous, 30A per drive "peak".

I will have 20,000 uF.

It does not look so.
A lot of used toroidals (and transformers in general) are sold with bundles of wire stocking out, with no clue as to which are primary, secondary etc.
I will just get one at DigiKey and will get on with my life. Hopefully by the end of this week, I will have a working power supply and will not need to worry about it ever again.
i
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On 06/28/2010 10:45 AM, Ignoramus28517 wrote:

Since this actually touches on things I do for work and it's specific, I feel compelled to add a disclaimer: burn down your shop, destroy your neighborhood, turn you and your family into crispy critters and it's _not my fault!_. (I.e., this is free advice and worth every penny of it).
The voltage will ramp down proportional to current and inversely proportional to capacitance. At 30A, that's (30A)/(20000uF) 1500V/sec. In 1/120th of a second, the voltage will drop by 12.5V.
With 100V into the transformer, you'll get (50V)(100V)/(120V) 41.6VRMS, or 58.3V peak to the rectifier. Assuming a 1.4V drop (full bridge) that's 57V peak (with rounding up) to the cap. Drop that by 12.5 and you're down to 44.5V at the trough of the wave.
Check the current capabilities of the caps -- 30A out means lots more than 30A in when the rectifiers are actually conducting. Ditto the rectifier maximum instantaneous current -- it'll be much higher than 30A.
--

Tim Wescott
Wescott Design Services
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On 06/28/2010 11:55 AM, Tim Wescott wrote:

I forgot to mention -- there really ought to be handbook solutions to all this. I know that the ARRL Handbook used to have them for tube-amplifier power supplies, back when radio amateurs actually built their own equipment.
They may still, but I'd have to take a look.
--

Tim Wescott
Wescott Design Services
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On Mon, 28 Jun 2010 11:50:44 -0500, Ignoramus28517

Thats pretty much what an OmniTurn CNC lathe uses.
Check on the Left side of the control http://picasaweb.google.com/gunnerasch/ForSale2#5487884689516471026
Gunner
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Looks good.
i
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On Mon, 28 Jun 2010 11:50:44 -0500, Ignoramus28517

If you full-wave rectify and filter it with just a capacitor, it will give you more like 12A at 60-70VDC without overheating.
You could also consider something like this (probably overkill) http://cgi.ebay.com/Kikusui-55-20L-DC-Power-Supply-0-55V-0-20A-1100W-/220509258468?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item33576042e4
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Why would it not give me 20A?
i

http://cgi.ebay.com/Kikusui-55-20L-DC-Power-Supply-0-55V-0-20A-1100W-/220509258468?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item33576042e4
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On Mon, 28 Jun 2010 13:37:53 -0500, Ignoramus28517

Because the current it draws from the supply is not sinusoidal- it will be drawn in pulses at the peak of the sine wave, so the I^2*R heating of the windings is higher than for a sinusoid.
Here is a decent application note on the matter-- I use 1.7 as a rule of thumb, the document says 1.6~1.8 so we are in complete agreement- see the bottomline in the table on the last page.
http://www.audiofaidate.org/it/materiale/20_020_020_001_PowerTransformer_FilterRatings.pdf
Note that the output _power_ will be only 20-30% less than 1000W, because the filtered voltage is higher than the unfiltered.

Actually, now that I see your actual requirements of 30A peak per axis, a power supply of this type would definitely NOT be a good idea- it will current-limit at the rated current and your drive will stop working, whereas a xfmr + filter will only be limited by the circuit breaker you put in the primary.
BTW, toroidal transformers draw absolutely wicked primary surges on power up sometimes (depends on their magnetization state when they were shut off and where in the cycle it's turned on), so you may want to put a slow circuit breaker on the primary and a fuse on the secondary after the filter cap.
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I'm curious, I've always used 2:1 transformers (with bridge rec. and cap.) for AMC A8 series servo drives because they are a perfect voltage match and very cheap. They've never been a problem. But I have told a few paying customers to just do this. Bit of a different thing once you cross that line in the sand. Do torroidal have significant safety or other benefit?
Karl
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Karl, what is your line voltage where you are?
My voltage is 124 VAC, usually.
That translates into 62 VAC on the secondary, and 87 volts after rectification, and I believe the drives shut down from that.
i
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I get 124 to 126 all the time. Don Foreman had told me what parts to get for regulators. But i tried without them and never had trouble. Its been eight years. And I know of many others doing the same.
Karl
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On Mon, 28 Jun 2010 14:44:33 -0500, "Karl Townsend"

They don't have as much external field so they're better for audio, instrumentation and CRT applications where that matters. They're supposed to be smaller, maybe more efficient (depending on materials used).
OTOH, they take more time to wind, so they tend to be more expensive (basically all the copper wire in each winding has to be wound onto a kind of bobbin that passes through the center of the core, then it's unwound onto the core. Ordinary E-I transformers can have the bobbins wound before the core is introduced, on a sort of special purpose CNC lathe. It's a lot faster as it's one process, not two, and a number of cores can be wound at once, and the machines also tend to be simpler and cheaper.
Here's an older toroidal winder in action:
http://www.youtube.com/watch?v
q4Ic6UL7c&feature=related
Higher production bobbin winder:
http://www.youtube.com/watch?v=xwVaNlyO_P8&feature=related

Single spindle machine
http://www.youtube.com/watch?v=MU2Wf76leRA&feature=related

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OK, I got it

http://www.audiofaidate.org/it/materiale/20_020_020_001_PowerTransformer_FilterRatings.pdf
Yep
I have 20a motor fuses on the incoming 220v. I was wiring that yesterday.
I think that it will work fine. In my mind, 30A "surge" amounts to fairly huge torques that probably are unnecessary. It would mean accelerations enough to make the mill dance on the floor or at least shake.
My back on the envelope calculations suggest that at 80v, I could get 150-200 IPM move (feed) rates. That's a lot. At 30A the motive force would be appx. 500-600 lbs.
i
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On 06/28/2010 11:37 AM, Ignoramus28517 wrote:

Because 20A out of the DC supply is way more than 20A RMS out of the transformer. You're pulling current out of the transformer for maybe 1/400th of a second at a time (whatever a 37 degree conduction angle is at 60Hz), which means that to hit an average of 20A you'd have to be pulling much more than 20ARMS.
The transformer current rating is there to keep the coils from dissipating too much power, and virtually all of that is I^2*R losses, which is RMS current.
Decrease the cap values and it'll be better (at the cost of more ripple), but you'll never get it down to 1:1.
Cadillac supplies of this type use a choke-input filter; i.e. the rectifiers feed a honking big coil that then feeds a cap. They have far better voltage regulation as long as you're pulling some minimum current at the cost of buying that honking big coil and lugging it around, and at the cost of the output voltage being about 90% of the transformer's output RMS voltage, less the rectifier drop.
Use a choke-input supply, and the RMS transformer current will be almost exactly 100% of the DC output current, instead of way more.
choke ___ DC out o----. ,------o--->|-----o----UUU-----o-------o AC in )|( | | | )|( .--)--->|-----' | o----' '---o | --- | '---|<--. --- cap | | | '------|<--o | | | | | === == GND GND (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
--

Tim Wescott
Wescott Design Services
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Tim, what is the part where I put ***** above?
Do I understand you right, that by adding a choke I would improve regulation?
If so, what size choke and cap would be appropriate here? I have some chokes that seem to be the right size.
i
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On 06/28/2010 12:42 PM, Ignoramus28517 wrote:

They are the four diodes inside your bridge rectifier.

Yes -- but keep in mind that it's probably not necessary, just neato.

Oy. It's handbook time.
For 3A ripple current, you want about 9mH (milli henry). Follow that with a 4000uF cap and you'll get 1V of ripple -- follow it with your 20000uF cap and you'll get -- uh -- lower ripple.
You have a pretty wide choice of choke values -- if you're just plucking them from a bin it's much more important that you get one that'll handle the current, then pair that up with your astonishingly large (for this application) 20000uF caps.
Note that at no-load, without a ballast resistor, your output voltage will still rise to 70V or so. I wouldn't worry about this, as it's still under the 80V limit imposed by your amps.
--

Tim Wescott
Wescott Design Services
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On 06/28/2010 07:28 AM, Ignoramus28517 wrote:

You've already bagged the idea, but to answer your question:
The kVA rating of a piece of AC gear is the maximum design RMS voltage (i.e. 220V) times the maximum design RMS current (i.e. 14A), when it's hooked up normally.
The reason that it's a "kVA" rating and not "watts" is because when you are driving reactive loads (i.e. inductive or capacitive) you can have lots of current without any real power being consumed -- yet in an electrical machine, much of the loss (and hence heat build up) is caused not by the power going through it, but by the applied voltage causing core losses, and the current through the coils causing I^2*R losses.
Had you hooked it up backwards as you had planned, you would have had to assume that the wire up to the 380V terminal was lighter than the wire up to the 220V terminal, and derated that 3kVA rating. The safe rating would have been whatever the current would have been out of that 380V terminal -- a PE with experience with that sort of hacking might be able to figure out a higher current rating than that, but would have to take care that the transformer wouldn't experience spot over heating.
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Ignoramus28517 wrote:

I'm putting my $5 that you will be scrapping the underpowered drives and power supply and replacing them with items that will power the servos at the same levels the Bridgeport engineers spec'd them for immediately after you attempt your first project requiring reasonably heavy cuts.
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I will be the first to let you know that.
The motors are rated at 30 amps peak, 15 amps continuous, 145 VDC. At that current they develop stall torque 3.2 n-m. or 2.4 ft-lbs. Converted to force 2.4*2*12*10*0.8 = 460 lbs of force.
The speed will be about 2x less.
These drives do not lose out on the torque/force front much and the speed would be about 55% of the max speed derived from motor nameplate.
It is also not a given that the original Bosch drives were capable of delivering nameplate torque and speed to the motors.
i
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