Re: [CAD_CAM_EDM_DRO] Where to buy transformers?

In a message dated 05-Aug-02 18:31:02 Central Daylight Time, hutchins@...
writes:

>
> As to size... I just purchased 3 DC motors that run at 40V nominal, 60V
> max, with a peak current draw of 14A/each. The guy I bought them from
> recommended that I run them at 48V for CNC use. I know that I won't ever
> run all three at peak current, but would a 10A (500VA) transformer actually
> be sufficient, or should I shell out the $$$ for a 20A unit?
>
> Also, I've seen several posts that say I should multiply the AC voltage by
> 1.41 to get the DC rectified voltage. In my electronics books, however, it
> says that most AC voltages (such as the 110V household voltage) are already
> specified in rms form. When I'm buying a transformer, do I get a 48V
> secondary, or a 34V secondary?

DeLesley: PEAK current-draw? For what duty-cycle? How long at a time? A
ten-amp transformer might survive a ten-second 40% overload once every five
minutes, but there are SO many "depends..." in there! Best to be SAFE than
try to save $20 and find it "just won't quite do!" I'd go with the 20A size.
That way, you could run TWO at once, I imagine, for a few seconds, every few
minutes, so long as you dropped back to just a few amps "in between" for
those few minutes.

This multiplying the RMS voltage by SQR(2) gives you the PEAK DC voltage, "no
load". If you LOAD the DC-supply to 30% to 100% of the "amp rating" of the
transformer, you should read the D.C. is near the RMS voltage (look at that
on a 'scope, and it looks like a poorly-built roller-coaster ride!). If
you want 48 VDC at some horrific number of amps (anything over 10 A., in MY
scaredy-cat opinion!), then go for a 48 VAC-RMS transformer!

Next topic: Don't know what city you are in. Here in Houston, there is at
least ONE good "surplus house" where one can "nose around, pick-and-choose",
and pick up some formidable good stuff at a fraction of the "new price".
California has more of "those kindsa places" than any other State, I'd say.
Probably NYC has several, too. New stuff? I have no experience buying
transformers over, oh, say 1 amp, from the good mail-order places. I roll my
OWN when larger than what I can find "off the shelf" like I suggest above.
See my article how to DO this:

<A HREF="http://members.tripod.com/~schematics/xform/xformer1.htm">Roll Your Own Power-Transformers!</A>


Lotsa luck! Jan Rowland, Ignernt ol' Troll


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