Re: [CAD_CAM_EDM_DRO] More Re: Drive mfrs recommendations for MOTOR cables ?

Hi Ballendo, et all,

Some additional thoughts - I think it was my comment "bigger is better"
regarding wire gauges but this does need qualification here goes:
"within reason". I think most folk have gleaned the key issues and
answers from the various posts. Anywhere from 16G - 22G multistrand
cables will work. Use judgement as to what you select within this
range, based on price, availability, the connectors you will use, your
motor amperage, the max cable lengths etc. Most complex theoretical
design reduces to simple "rules of thumb" when it is applied to the real
world - the trick is to have the judgement to sensibly apply these rules
of thumb. It helps to understand the theory behind those rules and most
of the theory has been discussed (issues of inductance, capacitance,
resistance etc.) for its relevance.

I run 4 conductor foil shield 18G cables for my BP clone mill which uses
6A per phase steppers on all axes. I chose this gauge partly because it
is a convenient size to work with, it fits nicely into my AMP/Tyco CPC
connectors and it is relatively rugged and durable in a workshop
environment. It also was available in 100' rolls for C$34 (~US$20). I
made sure that all cables were given large loops (6-8" rad) to
distribute the flex over a greater length of cable. In 14 months and
~1300 hours of use, I have not observed any problems with this cable.
On your concern about cable flex and aluminum foil shielding - based on
my many years of experience with foil shielded cables in a mobile
environment (connecting electronics together on buses) we never
experienced problems with the foil shield breaking down - even where it
received a lot of flexing. (We used AMP CPC connectors) Where there is
cable movement near the table, I think if we design for reasonable
radius cable curves e.g. greater than a 2-3" radius (the bigger the
better) you need not worry. If we were designing 500 - 1000 IPM rapid
traverse machines running on a 24/7 basis then super flexible cables
with a braided shield would be mandatory and the extra cost justified.
FYI, I tried to find a reasonably priced shielded super flexible cable
but could not (at the time) find someone to sell it by the foot.

For those designing large area gantry router tables or plasma cutters -
I think the cable issue is more complex and probably needs greater
consideration for superflexible cables (Olflex etc.). This is because
the cables (particularly the long axis) usually run in cable trays and
there is a lot more movement of the cables. Here, a key issue is
mechanical fatigue in the cables. Often speeds are much higher than on
a mill table. With so much potential movement I personally, would
recommend a super flexible cable with a conductor gauge suitable for the
amperage of that application.

I recall that DIN connectors were originally designed for audio
applications (connecting microphones etc.) But they seem to be much
higher rated than I expected. Looking at the tech data at this link:
http://catalog.tycoelectronics.com/TE/docs/pdf/5/39/157935.pdf
I see the contacts were tested at 250 Volts and 7.5 amps - wow! They
will work just fine in a home CNC application! One benefit of them over
a D connector is that they will be easier to solder a larger wire and
they are designed to terminate a cable shield. They also have a good
cable strain relief. Note, they're still only spec'd to 20G.

Skin effect is only a design issue/concern when you are working in the
High Frequency range (megahertz and up).

For significantly different cable lengths, use your judgement. Maybe a
slightly heavier gauge cable is justified (or should be considered) for
a 14' run as compared to a 3' run.

Running an 18G conductor to a single D-Connector pin is going to be
difficult to terminate (solder or crimp) and while it can be made to
work, it is kind of like connecting two 3/4" ID water hoses together
with a 1/4" ID coupling. You will still get some water flow, but the
coupling "resistance" will be significant. Ditto in a D-connector pin.
Also, the space betwen contacts is pretty tight - increasing the
risk of shorts. (i.e. "dead geckos"...) The pin contact resistance
will, at high continuous currents, become significant and start to warm
up and degrade its useful life. Eventually if you try and draw more
current (unlikely in our applications) you will start a fire... In
other words, use a conductor wire gauge that the connector is designed for.

In terms of how to terminate a connector - follow the manufacturer's
instructions as closely as possible. Take your time and take care to do
it right the first time.

The AMP/Tyco website has TONS of data at http://www.tycoelectronics.com/
including detailed instructions on how much wire to strip, how to
terminate etc. This data is backed up by decades of manufacturing
experience.

That's it from me on this topic...

Cheers, Peter



ballendo 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?
>>>
>
>
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