More Re: Drive mfrs recommendations for MOTOR cables ?

Ballendo,

I can only speak to the electrical effects of wiring. I treat wire as
any other passive electrical component that has a combination of R, L
and C.

My answer was based on considering the consequences of two extremes.
One is where R, L and C can be neglected, the other is what are the
effects of using the smallest practical wire guage (22) where the
effects of R, L and C are significant.

The former would require superconducting, infinitely thin wire. We
can all agree this would inccur zero losses and the length of
cableing would not matter.

The latter has significant R and C, those effects were taken into
account and found to be tolerable.

At one extreme there is a zero performace hit, at the other, it is
3%. Using larger guage wire will whittle away at that 3% if you think
it is worth the effort.

The problem is there is no "correct" solution because of two
conflicting goals; zero parasitics (no R, L and C) as one goal,
inexpensive, light and flexible wiring as the other goal. Both cannot
be satisfied simultaneously.

As a side bar, take look at a step motor winding wire guage. I'll bet
it will be less than 22 guage and if you unwind it, it will be much
longer than 20 ft.

Finally, the type of drive does not matter, be it bipolar, bi-level
or other. Current is curent. The exception is switching type drives
that may be fooled by capacitice currents and prematurely terminate
switching cycles if they are not properly blanked to ignore them.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "ballendo" <ballendo@y...> wrote:
> 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?