Re: twisting motor cables

--- In CAD_CAM_EDM_DRO@y..., "ahlee1010" <ahlee1010@h...> wrote:

> --- In CAD_CAM_EDM_DRO@y..., "caudlet" <tom@t...> wrote:
>
> Would it be correct to say that signal line are more suseptible to
> noise because it is only 5V while the motor cable may be way above,
> say, 30V? What about motor cable as a source of noise?
>

When you twist the motor cables you are not trying to keep noise out
of the motor but simply reduce the effects of the noise on the other
circuit elements and wiring.

The signal lines are twisted to reduce the effects of induced noise
from inductive sources such as the motor drive.

The voltage in a system only matters as a ratio of signal to noise.
Input channels where the sensitivity is high are the primary problem
sources for cable induced noise. With a sensitive input and a long
cable you can easily induce transients large enough to become false
input readings.

The motor is a power device with a very low input impedance it is
very hard to induce enough noise into the motor cables to actually do
anything. The primary source here is the motor drive so you try and
reduce the interaction between the two phases. Mostly you are trying
to keep the motor drive switching out of everything else.

> > a "pair" of wires. That means wires that make up a complete
> circuit
>
> One of the pair goes to ground. Does the return current always

return

> via th other pair, or is it possible that the return current may go
> via any of the numerous other ground wires?
>
> Thanks
>
> Lee

In an ideal circuit the current returns down the complementary wire
in the signal pair. This however is not always the case as current
will always seek the easiest path to ground. This is one reason the
shield is connected at one end only in many cases, this ensures that
it acts as a shield and not a ground current path defeating it's
purpose.

Brian Punkar
BSP