Re: [CAD_CAM_EDM_DRO] Re: Simple PSU stuff

In a message dated 12/13/2002 5:52:38 AM Central Standard Time,
echnidna@... writes:

> Assuming each motor may operate simultaneously, the transformer for your
> power supply is not large enough to handle 3 - 2 amp bipolar motors.

Bob:

I thought this way, too, when I first bench-tested my first (only, so far)
bipolar-CHOPPER Gecko-drives (G210). Mariss explained to me why the MAGIC I
observed: I had about 30 Volts, unregulated but filtered ("smoothed" in
English-English; We Yanks call that "filtered"), DC feeding two G210's,
those driving two 3+ amp motors, with two 3+ volt windings, each, i.e., about
20 watts per motor. With BOTH motors stepping simultaneously, meaning that
neither was drawing only 1/3 current "at standstill" like the Gecko drives do
after a second or so pause, the DC from the 30 V supply was less than two
amps! Yet, both motors had "full rated torque". Mariss pointed out that the
supply was supplying WATTS, as the chopper-drive still puts-out about 30
volts with this supply to the motor-windings, but "chopped", to average the
current down to the 3+ amps rating.

If I dare attempt "the math": Say the supply IS exactly 30 VDC, and the
current-rating per winding for the motor in question is exactly 3.0 amps, and
the rated winding voltage is exactly 3.333 volts. I.e., exactly 10 watts per
winding, or 20 watts for this motor. Now, to keep only 20 watts "into" the
motor with a 30 VDC supply, we must "chop" the voltage/current to maintain 20
watts total to the motor, so, with 20 W./30 V. = 0.67 amp, less than ONE
winding's rated current! Yet, it "appears" that 3 amps is flowing in BOTH
windings (and it is!). It's the WATTS, not the apparent total current, as
the VOLTAGE is much higher than the 'rated" voltage (which would be 3.333 V.
at rest).

Yep, it took me a few minutes and some pencil-scratching to "get this", but
it is now (nearly) clear in MY feeble mind!

HOPE this clarifies for you! (Mariss: Did I get it right???)
Jan Rowland


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