Variable autotransformer

[Pxsi]

I have just got settled in a new house and am in the process of resetting up my shop. While I’m doing that I would like to upgrade my CNC mill.
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I currently have a grizzly 8689 mill 2d CNC’ed using the original lead screws with 60oz steppers geared down 2.5 to 1 with timing belts. This is controlled by a pair of G201s powered by a regulated power supply putting out 48 volts at around 6 amps, and a big cap.
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This setup is great as long as you want to chug along at .125IPM and cut circles that look like lemons. It was good to get my feet wet in CNC and milling, but its time for something more than a toy.
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The G201s are all I want to keep; I want to replace everything else. My plan is to start at the power supply and work my way down.
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Ok now that you have a little background, I’ll get to the point. I have a Variac Variable autotransformer that I got off eBay a long time ago. The seller had no idea what it was, and all I knew is that it was a big industrial transformer, so the price was to good to pass up. It is beat up but looks to be in working condition.
I set it up with a 12A full bride with no load at my old house but got cold feet after it blew several 7 amp fuses between the wall and the transformer. Now that I have a new house, better shop, better equipment and a lot more knowledge I’m willing to give it another try. My question at this point is whether it is even a good idea to use a variable autotransformer for a CNC power supply, or if I should just box it back up and go get a toroid transformer off eBay.

I already have a pair of 12A full-wave bridge rectifiers and caps so I’m good there.

To get a rough outline of what I’m going to be powering; I plan to replace my wimpy 60oz steppers with some 425oz ones. I would like to replace the lead screws with acme with anti-backlash nuts or ball screws. Unless some one can give me a major reason not to I’ll go the acme route. My old system could jog at 10IPM and that is fast enough for me, it’s the cut speed and the 0.017 backlash that bothers me. After I get my X,Y table set up I want to CNC my Z.

I have narrowed the Variac Variable autotransformer down to one of two models, one puts out 20A at 150Volts and the other puts out 20A at 215Volts. I’m still researching exactly what model it is so I will report back any more info I find.

Sorry for being so damn wordy but I find its better to be over specific that vague. If nothing else I’ve confused the hell out of you.

link to Variac site
http://www.variac.com/staco_VD_120_1ph.htm

Link to photo of a transformer that looks like mine, but mine is missing the knob and looks like it servived a war. "the coils seem to be good"
http://upload.wikimedia.org/wikipedi...sformer_01.jpg

I could use advice on any or all aspects of my upgrade!

Pxsi

[Al_The_Man]

Look for previous posts here about the danger of using Variacs for power supplies, they are auto-transformers and therefore not isolated from the incomming supply.
Al.

[Ron111]

PXSI,

Welcome to the forum,

I agree with Al, it would be best to build a simple linear power supply.

If it is in your budget ballscrews with preloaded ballnuts are a good option to eliminate the backlash, but the preloaded ballnuts are a little pricey (somewhere around $125 I think). Another option is where you can buy 2 $22 ballnuts (5/8") and preload them your self for less than $50 an axis.

Just a couple of thoughts, look forward to watching you project develop,
Post some PICs and ask for help when you need it, the guys will jump in.

Ron

[awright]

You stated that the Variac with a full bridge rectifier blew fuses in the line supplying the Variac. The guys here are correct that your problem is the fact that the Variac provides no line isolation.

I presume that you had the negative output from the rectifier bridge tied to ground. If that is the case, one leg of your bridge is short-circuited since both ends of that leg are grounded - one by tying the negative output from the bridge to ground and one because the common terminal on the Variac is grounded. Draw up your circuit with the grounded points shown and you will see how the bridge leg is shorted. This problem is one reason that Variacs are unsuited for use as the main power supply transformer. Safety is the other reason due to the absence of line isolation.

But don't throw away the Variac. They are extremely useful for varying the voltage to your equipment to test the equipment for supply voltage tolerance.


awright