Re: [CAD_CAM_EDM_DRO] wire gauge and steppers

In a message dated 4/21/2003 10:21:07 AM Central Standard Time,
madyn@... writes:

> #18 ok to 5 amps... again, guessing..... #18 is used for 10 to 15
> amps at 117, so again, no big deal... Household runs are often in the
> 70 foot range, but at more voltage drop permissible....
>
> What the heck, #16 or #18 is fine...
>

Harvey & Dave:

I have done such connections for years, both "unipolar" and "bipolar". The
new "chopper drives" may send an "average" current approaching the motor's
design-current, but since the DC voltage is much higher (didn't you say 35
VDC supply?), the actual current from that power-supply's transformer and
rectifier is considerably less. However, once the filter-capacitor
(thousands of µF) is connected across that unregulated DC, the current from
there to the drives becomes effectively more.

ANYway, for currents up to 3 amps, #18 is VERY adequate for the kind of short
runs you normally have between the supply and the drives or switches or
connectors. I would have no fear of using #18 for Size 34 and smaller motors
up to six feet, and probably all the way on up to 12' would be OK.

The only time I think you would need #16 would be for Size 42 motors, cables
over, say, five feet.

Now the old fashioned UNIpolar drives generally sent (usually-nearly-) FULL
winding-current to the motors, "wasting" the excess available voltage over
those big power-resistors in series with the +common leads to the motors as
heat. Modern BIpolar drives such as the Gecko G201 and G210 (and other
brands I know nothing about) "chop" the DC so those resistors are not needed.
Those better chopper-drives then DROP the current to about 1/3 the "max.
rated" current when the motors are still for, oh, two seconds. Thus, MUCH
current is NOT flowing, anyway, so even if the wires were a bit small (say,
one gage-size smaller than "mathematically correct"), it'd be fine.

Here's a tip for EVERYONE who never realized it: If some particular AWG size
(say #18) is "right" for a given current (say 3 amps), then simply ADD three
to the gage-number to indicate the size adequate for HALF that current (that
is, #21 would be right for 1.5 amps); and SUBTRACT three gage-sizes for TWICE
the current (that is, #15 would be right for 6 amps.) Of course, these
examples begin with the assumption that #18 is right for 3.0 amps. (I think
like a transformer designer).

Conductors in a cable such as a motor's four our six-wire cable might be
rated for more current that the same gage-size used as magnet-wire in a coil,
but this is simply due to the fact that wasted heat is more easily dissipated
from a loose cable than from a tightly-wound transformer-coil! So, in this
instance, the thing to keep in mind is the voltage-DROP due to the wire's DC
resistance at whatever current.

What did I miss? HTH! Jan Rowland


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