Power Supply Help

[beausdn]

Hi I am a newbie and need lots of help here. I am building a power supply for my cnc router and have some questions (first of many no doubt). I have noticed that most psu's use a toroid transformer, I was wondering if a variac is a suitable substitute, I know they are a little dirty with the brush and all but I have a 120Vac in 0-140Vac out variac allready. Since it is not a toroid transformer there more electrical noise, I have 3 Gecko 320's comming (hope the noise won't be a problem). I also have a couple of 21,000 uf caps and a 600V 35A bridge rectifier. I am using 3 Ametek 40 Vdc 900 Rpm motors that I have adapted using US Digital 500 PPR encoders. For now I am going to use acme thread, and go to ballscrews later if the thing works. At 8 tpi I will direct drive them (if my calculations are correct 197 IPM) I don't intend on overdriving them. I would like to use the variac if possible I'm sure if I keep spending money my wife will freak!

[ViperTX]

What's the current rating of the variac...I suspect that it's only a few amps.

[Al_The_Man]

Variac is not a good idea, they are auto-transformers and as such are not isolated from the main supply. It is possible to have a potential of 120v from the DC power supply to ground.
Although ideal, you do not need a toroidal, a square laminated TXr will do, toroidals are just more effecient.
Al

[beausdn]

The variac is rated 120 Vac 7.5 Amp. However AL The Man came up with a good answer, since this thing is really a variable auto transformer, an open neutral presents a hazzard. I just found this thing in the shop at work and was hoping to save a few bucks but it looks like will have to find a proper tx, or risk sending 168 Vdc through the system YIKES! THANKS AL!

[ViperTX]

Yep, without an isolation transformer you might encounter some nice arcing if you tried to measure anything in the circuit with another AC power piece of test equipment.