More Re: Drive mfrs recommendations for MOTOR cables ?

--- 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?