Power supply help

[oeffner]

Hi All I'm an old f*rt trying to learn cnc machines by building a cnc router. I have been collecting "stuff" for a while and am about ready to start the putting the "stuff " together.
I have a big square transformer I would like to use specs: 550va power transformer, input 115v 60hz, 59 v ac output with center tap or 2 29 v ac. I think its like 9.32 amps at 59v ac.
I need to drive 4, 440inoz nema 34 steppers 3.3v ,3.4a .98ohm and 4..3mH inductance
I'm using a breakout board from Campbell, and plan on 4 Gecko 201x drives.
QUESTION?? any way I can use this transformer?? also what do I need for a rectifier and capacitors??
Ive read all the related stuff but am not sure I understand it! thanks

[pfred2]

Have you read this?

http://www.geckodrive.com/upload/Gecko%20FAQ.pdf

It has formulas for the capacitor you need and the maximum voltage you can supply your motors with too. I did the calculations but I'm not posting the answers I got because I'm not sure I understand it all either.

Like how your motors could possibly draw 4493 watts at their maximum input voltage just seems wrong to me? But ignore that, I mean it has to be wrong right?

32 * SQRT MH = Drive supply voltage

32 * 2.073644 = 66.35661 V ^2 = 4403 / .98 ohms = 4493 watts! Blow dry my hair.
V^2/R = W

I never was very good at math.

[vger]

That 4493 watt value would be true if.... the motor was a pure resistance, and the drive was not current limiting. As for your transformer, if you use the full winding, 60 V, and use a full wave bridge, then you will end up with about 85 V on the cap. This is because the peak voltage is 1.414 times the RMS value of 60 V. If your transformer has 2 secondary windings with 2 wires connected together as a center tap, then you could wire the two windings in parallel to give you 30 V (I'm rounding off) at 18 Amps. That would still give you a little more than 40 V peak which is what you would have on the capacitor(s).

Post a pic of your transformer secondary wires if you can.

Steve

[pfred2]

I could, but I don't think it'd help oeffner, the original poster out much.

Those drives he makes mention of they don't have to draw 7 amps do they? Because your way he is so close to it working. If he could cut down each driver by an amp he'd have it. It is still an awfully close shave though, even at that. Lets just hope those drivers can go into power saver mode when they're not moving.

Hey oeffner, until you're done with all of your electronics enjoy some pie:
http://fixitnow.com/wp-content/uploa...ie-450x450.gif

The right slice of that pie will always tell you if what you plan on hooking up is half baked, or a cooked goose!

[H.O]

Hi,
At 4.3mH Geckodrive recomends no more than 32*SQRT4.3 or 66V as already been pointed out. Rectifing 59VAC yields around 83VDC which would be too much for both the motor and for the Gecko - which has a max voltage rating of 80V.

As Steve says, if the transformer has two secondaries then you might be able to rewire it to get half the voltage which may not get every bit of performance out of those motors but should work pretty well.

If that's not possible then perhaps winding off a couple of turns from the secondary is an option to reduce the voltage. Winding ON a couple of turns "the other way" is yet another option that might be possible.

Or use a half bridge rectifer set-up, that will also get you around 40VDC.

In any case, 550VA @ 60 V is ~9A. You have three motors @ 3.4A and acording to Geckodrives Whitepaper on stepmotors each drive will never draw more than 3/4 of what the current is set to. This means if you set each drive to 3.4A you'll get a maximum current draw from the powersupply of 3*3.4*0.75=7.65A.

/Henrik.

[oeffner]

the transformer is a big square one so unwinding is kind of out. I have ordered a Toroid (round transformer)for the cnc router and I'll just use this one "center tap" at (measured with load) 34vac at 48v rectified to test everything and maybe drive a plasma cut table to play with. thanks all for your help

[pfred2]

It can be hard to just fall into the exact "right" transformer for some power supply needs. But it does seem that the one you have should work pretty good. Good luck!

[matridium]

Ok so how does one size a power supply for maximum efficiency?

I have been researching this and have only found some information that was useful. Over at http://campbelldesigns.net some of the calculations he has in the document on building a power supply have me confused. I went to CNC zone to look for info and the Gecko site for advice. Some of the formulae at Gecko site are confusing as well and they don't match Campbell. I know there is more than one way to skin a cat, but the numbers I get with either method seem wrong.

I am getting the KL34-180-90 motors that have are 90v 7.5 amps
If I buy the KL- 7220 from Kelinginc.com that they have coupled in a kit for the KL34-180-90 motors will they work fine, will I lose much torque, or rpm?

So what I have thought about is to order 3 power supplies and use one for each driver and motor.

Order 1 of these: PS-8N80R12 from ANTEK
Order 2 of these: PS-8N80 from Antek

this way I can power the motors with no problem I'm at half the rated voltage of the gugong drivers, and the PS-8N80R12 has a 12V output as well and I can use this to run small fans and such.

Does anyone see a problem with the above selections of power supplies? I don't mind the price; because I just fell my time is more important than paying 25.00 to 50.00 less by making my own...

Just looking for a validation that the above PS are ok to use, or if anyone has a different supplier, or better way to do what I want to do..

Regards
Mike