RE: [CAD_CAM_EDM_DRO] wire for motor ?

Hi Steve,

Thanks. Forgot to include the return line. And of course stranded wire is
a different again from solid. (Might even be Litz wire inside the computer
cable)

I used the ARRL handbook for the numbers. The voltage drop across the wire
isn't nearly as critical as one might think. It just means that the wire
acts like the R part of an RL stepper driver while the current is building
up in the winding. Since the chopper drive will be at least 24V and the
motor at full current is only 3V the real problem as you stated so well is
the heat build up in a bundle of wires.

I also like the thicker wire because at least part of a CNC system has a
moving motor and the thicker gauge provides a better mechanical connection.

John Dammeyer



Wireless CAN with the CANRF module now available.
http://www.autoartisans.com/products
Automation Artisans Inc.
Ph. 1 250 544 4950

> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Stephen
> Wille Padnos
> Sent: Sunday, December 10, 2006 10:51 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] wire for motor ?
>
>
> John Dammeyer wrote:
>
> >Hi Dave,
> >
> >If it were 28g solid wire then it has about 68 Ohms
> resistance per thousand
> >feet. If you have a six foot cable then the resistance is
> 0.4 ohms. Your
> >3V 3A motor has a DC resistance of 1 Ohm. Total resistance
> is therefore
> >about 1.4 ohms.
> >
> >
> Unfortunately, there's a common mistake in there. Wire
> resistance specs
> are per conductor, and current flows through two conductors in a
> properly functioning system. :)
> The total resistance from the wire would actually be about
> 0.8 ohms per
> motor phase, split into two "resistors" per phase. Assuming
> that the
> driver can supply 3A, the power dissipated in the cable will
> be as high
> as 2 (phases) * 3A^2 * 0.8 ohms (per phase) = 14.4W. The
> motor will be
> using 18W, since it's 1 ohm per phase, and the driver will have to
> supply nearly 6 V to get the motor to full current.
>
> >Since your driver measures the current through the wire and
> the motor and
> >will be set up to provide 3A it will result in 3.6W of power
> dissipated
> >across the wire.
> >
> >If you made up a cable using 18g wire it's 6.3 ohms per 1000
> foot. Or 0.0063
> >per foot. Power dissipated across the wire is now 0.06W.
> >
> >John Dammeyer
> >
> >
> I have to agree that heavier gauge wire is better. Additionally, the
> AWG specs you see are usually for "plain" wire, not something that's
> molded in several layers of rubber along with several other
> current-carrying conductors. You need to significantly
> derate wire when
> you expect it to get hot (which computer cables certainly are
> not meant
> for), and when you expect it to be bundled together.
>
> I almost never remember wire gauges or resistances, so I
> google for "AWG
> ampacity" (I added the word "bundle" this time), and look at
> the charts
> when I need to :)
>
> - Steve
>
>
>
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