Re: twisting motor cables

--- In CAD_CAM_EDM_DRO@y..., "caudlet" <tom@t...> wrote:

> the motor end). If you want to twist wires, then twist each motor
> PAIR together; it does not matter that more than one of the wires

may

> be connected to the same ground at the power source.

I believe that there had been a recommendation to use twisted pairs
of signal wires. How about signal pairs? Is this the reason why
standard parallel port have 8 separate pins allocted as ground pins.
So for 3 axes we must have 6 separate ground wires (3 steps and 3
direction) going from the parallel port to the 3 separate drives? And
how would this come in when we have a drive like gecko which uses a
common +5V?

> > 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?
>
> Yes. The factor is the S/N (signal to noise) ratio. Also the

input

> circuitry is much more sensitive in most systems than outputs.

Much

> like an audio amp (poor example but best one I can think of) you

have

> to shield inputs but the speaker outputs are unshielded cable. In

a

> real rough sense we have "chopper" systems that operate in the mid

to

> upper audio range. Granted, the waves are square rather than sine
> and contain a rich set of harmonics, but motors are real forgiving
> about the shape of the wave and act as big inductors to help negate
> the higher harmonics. Any time you have high current and voltage
> moving a heavy load there is the possibility of noise generation.
> Running wires (mini inductors) next to each other and pumping large
> amounts of signal through one, will induct a signal in the other.
> Wires have to be close to have their magnetic fields overlap and

the

> longer they run in parallel the better the cross coupling (and

signal

> transfer). Analog circuits that have amplification built in have
> much lower S/N requirements. Digital circuits have much higher

noise

> immunity. Of note here: Wires in the same shielding (metal

conduit)

> have better coupling than in open air. There are other factors as
> well but do not normally come into play at non RF frequencies.
> Things get really weird up in the areas above 500MHZ where even

short

> pieces of wire become an antenna (like PC board runs)
>
>
>
>
> > 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?
>
> Current flows in both conductors in equal and opposite directions.

If

> the circuits share a common ground than the common ground carries

the

> sum of the currents. The most current will flow in the "closest
> ground" wire meaning the one with the most direct path back to the
> common ground. It can also flow in parallel grounds much like
> parallel resistors. Using twisted wire around a common ground is
> pointless (and may do more harm than good.) If you use a
> conventional bipolar bridge motor controller (Gecko's and most
> others), the motor wires do not share a common ground (at least on
> the motor end). If you want to twist wires, then twist each motor
> PAIR together; it does not matter that more than one of the wires

may

> be connected to the same ground at the power source. In critical
> circuits the designer will often make it a point to have a central
> ground at the point closest to where ground is established
> (Transformer centertap, bridge negative, Large filter cap negative,
> etc) and have seperate ground runs from different parts of the
> circuit. Ground is relative. The further you are away from it the
> more signal that can be induced onto it. If lightning strikes

close

> to a ground running into a building you can get hundreds of volts
> inducted in the short run. Electrical equipment can be presented
> with over voltages and/or reverse current flow...a BAD thing.
>
> Confused? Ground loops, noise, and EMI are the bane of designers.