More Re: Drive mfrs recommendations for MOTOR cables ?

Mariss, and others who have replied,

Thank you.
Your answers so far point out WHY I have asked the question(s). Also
why I'm asking more in this post<G>

On one hand, we have our resident maker of drives which are nearly
universally praised saying that #22 at 7A, under 10 feet is okay for
steppers (with some data and calcs to "support" his answers; i.e.,
higher voltage is better if using #22.) But still only a 1% loss
(drop?) at the higher volts using #22... Hmmmm<g>

AND we have ANOTHER EE who says "Use as big as will fit", with a
recommendation of 16 or 18 gauge generally... And his "general"
advice is consistent with this "bigger is better" approach. Some
others have echoed this "general recommendation" of #16 or
#18 "oughta work". Including a wizened old troll... Hmmmm<g>

In a world in which nearly any electrical device can be "virtually
modelled", why don't we have some definitive answers? Or at least a
methodology to arrive AT the truly CORRECT choice? (In fairness,
Mariss HAS provided some clues as to how this might be determined.)

But back to my questions. I'm looking/hoping for exact answers to
specific scenarios. There can be a LARGE difference in cost when a
CNC machine is being constructed "on the cheap". For many of the
reasons pointed out by the responses. As the wire size goes up so
does the connector size, etc... Olflex, Belden, and Kabelschlepp
wires are not inexpensive, so if using the "correct" size means
smaller, the savings overall can be significant!

So where(at what points) ARE we making the choice to reduce our
success by trying to use smaller,cheaper, easier to fit and bend
choices? (unknowingly)

I talked to a Belden App engineer many years back at a time when the
robotic cables line (infinity?) was having problems and its release
date kept being pushed further into the future. She said that #22
would be okay for 20 foot cables at 2 amps... (The voltage being
discussed was 36VDC.) But she didn't want #22 used for 3.5-4Amps at
this length...

And it is interesting that the responses seem? to reflect that a 20
foot cable for a cnc machine is long? A 4x8 foot cnc gantry router
will need a MINIMUM of 16' if cable trays are used (from drive
enclosure to z axis). 20-24 feet is not uncommon for this type of
machine. And 35' is not unheard of...

Many commercial desktop CNC machines use DIN connectors (like a pc
keybd). Or molex with crimp and insert pins. Or DE-9 connectors.
The "D-sub" connectors, as was pointed out are rated for 5 amps, but
#20 is a tight fit, at best. AMP's "molex" line of .063 and .093 pin
size connectors will handle larger wire and still be quite
inexpensive. (Works well as long as "oiltight" is not needed.) The
DIN connectors need an experienced hand when soldering as they are
prone to melt.

So, a few more questions:

First, where IS the "use next gauge" point in the previously given
setups?

Next, I have heard that foil shielding breaks down quickly when used
in a reciprocating motion. What will be the effect of this?

Moving on, I too, despise/dispute the "monster cable and oxygen-free
copper" claims as it always seemed kinda weird that this HUGE cable
was terminated in a thin pin, which then was slipped into a
copper "spring" contact at the back of the speaker. And this typical
speaker connector "would have" made BETTER contact without the neat
little machined gold plated pin on the end of the "monster" cable.

I bring this up not to discuss stereo merits, but we have a similar
thing happening with our connectors. Especially when we "cram" a
larger gauge into the D-subs or DINs... But also we have a "bigger is
better" mindset in our field TOO. Is IT based on FACT, or tradition?
Have there been changes in drive types, or materials which make
the "old" recommendations just that; OLD recommendations?

Is there any difference in cabling a unipolar drive versus a chopper?
What about a bilevel?

So finally, I'll ask our list EE's another set of questions (AND
hopefully we'll still hear from othe "others" selling drives on both
these and the previous questions):

What are we losing when we put #18 into a D-sub or DIN? How can the
ratings of the D-subs be so high with the pins so much smaller than
wire diameter of a similar rating? Should we crimp AND solder, or
just crimp? Or just solder? What are the tradeoffs? What is the
effect of skin-effect on this wire-connector-connector-wire set of
transitions?

Thank you all in advance,

Ballendo

P.S. ONE more... At what point does the DIFFERENCE in cable length
between axes become significant? In the 4x8 gantry example above, the
x axis cable might be only 3-6 feet long... And the y might be 14'.

--- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:
> Ballendo,
>
> Excellent question! I just did some measurements so here's my
> recommedations:
>
> Ours, like most switching type drives, generates substantial
> switching noise on the motor cables. Good practice is to have some
> sheilding so as not to interfere with radios, TVs and other
equipment
> sensitive to RF interference.
>
> STEP MOTORS:
>
> Use a 5 conductor sheilded cable. Connect 4 of the leads as
required
> to the motor. Use the 5th lead to connect to the motor case at one
> end and to your "single point" ground at the other end. Connect one
> end of the sheild to this same "single point" ground. Do not
connect
> the other end of the sheild. See (1) for reasons.
>
> Braid or foil, it makes no difference. If twisted pairs are
> available, use one pair per motor winding.
>
> Wire guage (within reason) makes very little difference. Using the
> wire guage as the motor leads is usually sufficient. See (2) for
> reasons.
>
> Very long motor cables (>20ft to 200ft) present a special problem.
> Series chokes of about 100uH should be used on each motor lead at
the
> drive end. See (3) for reasons.
>
> SERVO MOTORS:
>
> Use the same techniques for the motor wiring as for step motors.
The
> encoder wiring should be a seperate sheilded cable. Use a twisted
> pair cable like CAT-5. For single-ended encoders, have only one
> signal per pair, the other wire should carry ground or +5VDC.
Again,
> ground the sheild at one end to your "single point" ground. See (4)
> for reasons.
>
> (1)The windings on a step motor are on the stator. They form 1
plate
> of a capacitor. The wire insulation and the bobbin they are wound
on
> forms the di-electric (insulator) of a cap and the motor case forms
> the other plate of a capacitor. This capacitor can be as large as
> 3,000pF (measured) on a size 42 DC servomotor and stepper.
>
> MOSFETs switch very rapidly and have a dv/dt of 2V/ns on the motor
> leads. The capacitive current then is 6A (I=C*dv/dt). This pulse
> current is substantial and must be returned to ground to insure it
> does not interfere with encoders in particular.
>
> (2) Wire gauge contributes to the total motor resistance and
> inductance. Inductance can be figured at about 2nH per inch. On a
> 20ft cable this would amount to only 0.5uH per wire and 1uH for the
> round trip. This is negligable compared to the 1mH to 50mH
inductance
> the motor has.
>
> Resistance produces a voltage drop when current passes thru it. The
> effect of resistance is voltage drop subtracts from the voltage
> available to the motor.
>
> I measured the resistance of some 22 guage, 5 conductor sheilded
> cable (from Home Depot) I have around. A 10ft length measured out
at
> 0.35 ohms for a round trip (10' out, 10' back). At 7A this
resistance
> would drop 2.45V and dissipate 17W.
>
> At high speeds step motor phase currents for a 7A motor drops to
> 2.33A maximum. The voltage drop then becomes only 0.82V. This
amounts
> to 3.5% performance hit whe using a 24VDC power supply and about a
1%
> performance drop at 80VDC. It can be neglected.
>
> Heating in the cable is 1.7W per foot at 7A. This would make the
> cable slightly warm to the touch but should not be a problem
> otherwise.
>
> (3) The same test cable as in (2) measures 600pF wire to wire
> capacitance for a 10ft length, or 60pF per foot. The wire to sheild
> capacitance was 100pF per foot.
>
> This works out to 3,200pF of capacitance load on the MOSFETs for a
> 20ft cable. The resulting charge and discharge currents are large
> (see 1) and can begin to fool some switching drive's current sense
> circuitry. The result would be squealing and hissing noises from
the
> motor and possibly reduced phase current due to the drive
prematurely
> terminating the switching cycle.
>
> Series chokes in the motor leads (100uH) at the drive serve to de-
> couple this capacitance, allowing the drive to function normally.
>
> (4) A length of CAT-5 cable I have measures out at 30pF per foot.
> This would form an RC time constant of 3uS for a 10' length and 10K
> pull-up resistors in a typical encoder circuit. This 3uS would be
the
> width of the cross-talk pulses if both conductors in the pair were
to
> carry both encoder channels. This may degrade the quadrature
decoder
> and cause false feedback signals. Use ground or +5VDC (encoder
supply
> power) on the other wire in the pair to prevent this cross-talk.
>
> Mariss
>
>
>
> --- In CAD_CAM_EDM_DRO@y..., "ballendo" <ballendo@y...> wrote:
> > Hello Dan, Dean, Mariss, Hans, Tim,?,?,
> >
> > I'm wondering what our list member step drive mfrs recommend
using
> > for cabling FROM their drives TO the motors?
> >
> > Shielded?
> > Individually? Overall? Braided? Foil?
> >
> > Twisted pairs?
> >
> > RE: Gauge... Any modifications to the "standard" amps ratings as
> > supplied by the wire mfrs when the wire is used for PWM drives?
> >
> > Effect of wire that is "too big", if there is any?
> >
> > Now how about a few specific apps (I've tried to pick
> some "typical"
> > values here):
> >
> > 12VDC PS, 2A drive, 1A motor rating, 3 foot cable length?
> > Same with a 12 foot cable?
> >
> > 36VDC PS, 2A drive, 2A motor rating, 10 foot cable length?
> > Same with 5 foot cable? 20 foot cable? 4A motor?
> >
> > 60VDC PS, 5A drive, 4A motor rating, 18 foot cable length
> > Same with 5 foot cable? 6A motor?
> >
> > Does drive type (unipolar, bipolar, bilevel) affect these answers?
> >
> > Any other thoughts or considerations? (besides the need for oil
> proof
> > outer covering if used in a machining environment)
> >
> > Thank you in advance,
> >
> > Ballendo
> >
> > P.S. Any special consids for servos, versus steppers? (Just the
> > motor. We'll assume everyone knows to keep the encoder signals
away
> > from the drive current) Can I just use SJO to the servos?