Re: [CAD_CAM_EDM_DRO] electrical advice needed

In a message dated 04-Jan-02 14:20:14 Central Standard Time,
apollo@... writes:

> My question is, will that be 67VDC @ 4A? Or would that turn into 67VDC @
> 2.8A, so as to conserve the 192 Watt rating?
> Secondly, I'm trying to determine what the actual current draw will be
> from the 110VAC side... if the transformer can supply 192 Watts, then how
> much current would it be safe to assume it will draw from the AC? I'm
> planning on using four of these (one for each axis), so I'm concerned about
> having a cord & switch rated high enough. I'm assuming the transformers
> aren't 100% efficient, so how much higher than the rated power should I go
> to be safe?

Rick: GOOD questions! Lemme see if I can coax my feeble tiny mind into a
comprehensible answer: First question: The filter-capacitor (if of proper
adequate size) will "lift" the RMS rectified voltage to near "peak", which is
that 67 volts. At "NO load". If you load that DC to 4 amps, the DC (with
ripple) will drop to about 48 volts. (s'posed to, anyway!). So, "sorta",
yes, the 67 VDC would be at the lower amperage. HOWEVER, note: These Gecko
drives were something totally NEW to me, when I purchased my first two, which
I am just-beginning to try out, with some small Size 23 steppers.
INTERESTING: The higher DC, being "chopped" at a high-audio-freq., combined
with the motors self-inductance and the filter-capacitors (also in the Gecko
drive) can provide full-torque, while drawing MUCH less current from the DC
supply! For example, if you have a stepper with coils rated at one-amp
(there are two of those), you might think you'd need 48 or 67 VDC at 2 amps
for that motor. NOT! Would you believe maybe 0.4 amp! The VOLTAGE is much
higher than the coil-rating, and this is "chopped" to give an average "max.
rated" V-A, so the supply is called upon to supply considerably LESS current
than the motors would draw, were their "rated coil voltage" applied as steady
DC, directly to the windings. (Don't forget! You can't GET this magic with
old-fashioned "L-R stepper-drives" using those 50 or 100 watt 6 ohm "ballast
resistors" in series with the +common center-taps of the motor-windings (on
"six-wire stepper motors").

Cord rating? Even if you loaded this transformer to its full 192 watts, at
110 volts (might be as high as 125 VAC, if the utility-supply to your lab is
near-perfect) you'd be drawing only 192/110 = 1.75 amps. A lamp's "zip-cord"
is #18, and that can comfortably handle 3 amps! So, no need for #12 "motor
cable" for the power supply! (If it don't smoke, nor burn your hand when you
touch the cord, it is heavy enough!)

OK? Hope this helps! Jan Rowland, old troll


[Non-text portions of this message have been removed]