Re:305oz Steppers! More torque, Scotty!!!! (Shes' breaking up, I cannah hold'er)

I see the site has been adjusted to say 6.8mH now. But
anyway...What problems could be expected running a 6V
motor at 20x it's voltage rating?

Thanks!
John

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "John Hansford" <kz1927@...>
wrote:

>
> The Hobbycnc site says the inductance is 2.3mH...does
> that make them sound any better?
>
> $45.00 *seems* like a good price...
>
> Thanks!
> John
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "ballendo" <ballendo@> wrote:
> >
> > >In CCED, "Dave Rigotti" <drigotti@> wrote:
> > >HobbyCNC is pleased to announce that our CNC "Packages" are now
> > >available with 305oz steppers! We also have them "ala carte"
> >
> > Hello,
> >
> > Another increase in stepper motor torque! Sounds great!
> >
> > (But there's a fly in the ointment, and nobody seems to be telling
> > you about it. IMO it's time somebody did...)
> >
> > Let's review a couple things about stepper motors, and let's look at
> > some published facts about motors likely to be VERY similar to the
> > ones just announced, and the earlier/formerly "greatly increased
> > performance" motors. (Quotes just above are for MY emphasis, NOT a
> > direct quote from someone else.)
> >
> > I don't know for sure where Dave R gets his motors; but I do know
> > that most of us selling motors with these specs are using the same
> > motors... Made by the MS motor company in China, and many in the USA
> > will source them from John at KelingINC.net
> >
> > So, let's review:
> >
> > Most of the time the Stepper torque published and mentioned is
> > HOLDING torque. It's the biggest number, so it sounds the best for
> > marketing. It's also the way steppers have been identified for
> > decades. (Probably due to the previous sentence!<G>) BUT...
> >
> > It's important to realise... The motor will ONLY have this much
> > torque when it is STOPPED. As soon as it starts to turn, the torque
> > goes DOWN. FOR ALL stepper MOTORS. Regardless of drive type (bipolar
> > vs. unipolar)
> >
> > How FAR down, and how FAST the torque drops, and along what "shape"
> > or "Curve" depends upon some things about the motor, AND some things
> > about the drive and power supply.
> >
> > Every motor is a generator when it turns. This is what causes the
> > motor to lose torque as it goes faster. The important criteria for
> > this are the motor power supply voltage, and the motor coil
> > inductance. An additional factor is the drive type, unipolar vs.
> > Bipolar.
> >
> > So let's look at the numbers for the Motors Keling sells as having
> > the specs listed on Daves HCNC website. (Where the new 305 oz.in.
> > motors are listed as 6v,2.0A) Here are links to both 305's and the
> > 200's as formerly sold by hobbyCNC (and still available from
> > http://www.cncresource.com ):
> >
> > http://www.kelinginc.net/KL23H286-20-08B.pdf (2.15Nm, 305 oz.in.)
> >
> > http://www.kelinginc.net/KL23H276-30-8B.pdf (1.4 Nm, 200 oz.in.)
> >
> > Note: A Nm is a Newton-meter, which equates to about 141 oz.in.
> >
> > (Again I'll mention that I don't know exactly where Dave gets his
> > motors; so these may not be the IDENTICAL motors he's selling. But
> > I'd bet that if not, they're VERY close!)
> >
> > The first thing to notice is that the coil inductance for the 305 is
> > over THREE TIMES as high (6.8mH) as for the 200 (2.2mH)...
> >
> > Note: mH is milliHenry's, a measure of inductance. Which is the
> > motor coil's resistance to changing current direction.
> >
> > What this means (that matters to us in the DIY-CNC arena) is that
> > the 305's are going to be a better generator of electricity than the
> > 200's. Which is NOT good when you're looking for torque at high
> > speed. Especially when you're limited in your ability to increase
> > power supply voltage to compensate. More on that in a minute.
> >
> > So the motor turns and as it turns it generates a voltage. This
> > voltage is called back EMF (ElectroMotive Force; which is the old
> > name for voltage. And it's why Ohm's law is E=IR instead of V=IR).
> > Back EMF basically subtracts from the power supply voltage.
> > Which means that the motor will behave as if it is being powered by
> > a supply voltage equal to the original power supply MINUS the back
> > EMF.
> >
> > Keling doesn't show torque speed curves for all its motors and the
> > ones it DOES show have the bipolar torque curve (because bipolar
> > drive are more efficient, so that curve makes the motors LOOK
> > better). Point is, we can't "see" the torque curve. But we CAN see
> > that the back EMF will be higher than the 200's (due to the 6.8mH
> > vs. 2.2mH that we CAN see on the spec sheets.)
> >
> > Translated into CNC-speak, the 305's will lose torque very quickly
> > compared to the 200's as speed goes up. In fact, I wouldn't be
> > surprised to find that the 200's outperform the 305's at "typical"
> > speeds used in most DIY-CNC projects... (just a guess right now;
> > I'll know for sure very soon!<G>)
> >
> > Let's get to the other factors in play here:
> >
> > Power supply voltage. If we can raise the power supply voltage, we
> > can push the 305's to higher performance. And this is true for ANY
> > stepper, up to a point of diminishing returns--and motor heating--
> > that Mariss of Gecko has empirically determined to be about 20-25x
> > motor nameplate voltage.
> >
> > Side Note: It is STILL BEST to always use the LOWEST power supply
> > voltage which will give you the results you NEED. Sizing your PS to
> > the max that the drive can handle "just cuz" is bad engineering. And
> > that can lead to other unseen and unconsdered problems that you'll
> > bat your head against later...
> >
> > The 305's have a 6 volt nameplate rating. The 200's are rated for
> > 2.76V. (using ohms law to solve since this spec isn't on the linked
> > sheet. Note that many of the "200's" imported into the USA were
> > marked as having 1 ohm coils; in that case the motor volts will be
> > 3V.)
> >
> > So as long as we use drives which have about double the voltage
> > capability we may actually see the performance increase with the
> > 305's. BUT...
> >
> > HobbyCNC drives are based on the Sanken SLA7062 (old revs) and
> > SLA7068 (pro) driver chips. These chips have a MAX voltage rating of
> > 44VDC. (We use these in our CNCResource drives as well; they're good
> > chips.) Mariss has pointed out that a unipolar drive semiconductor
> > has to be able to withstand DOUBLE the supply voltage. So these
> > chips are probably using a 100v architecture, with the de-rating to
> > 44V acounting for die variation and reliability improvement.
> >
> > HobbyCNC also recommends a 24VAC transformer secondary for the power
> > supply. This will provide approximately 34VDC when rectified and
> > filtered as suggested at the HCNC site. If you're going to buy these
> > 305's, it would be a GOOD idea to increase your power supply voltage
> > as much as you dare towards that 44VDC limit. If you do that the
> > back EMF will be subtracted from this larger "number", and the
> > torque will be carried out further along the torque-speed curve.
> >
> > Wait there's more...
> >
> > The 305's have a case thats almost 3-1/2 inches (88mm) long. The
> > 200's are 3 inches (76mm). That in itself "may" not be too bad; see
> > my P.S. following this post!<G> BUT...
> >
> > The ROTOR INSIDE the motor is ALSO longer. More mass. Put simply,
> > slower. (all else being equal. Which I've pointed out IS the likely
> > case where the power supply voltage is concerned due to limits of
> > the driver chip used.) Another thing which doesn't get much "press"
> > her in the build-it-yourself CNC groips is soemthing called
> > mechanical impedance matching. Which can be thought of simply as:
> > putting a v8 on a bicycle "might" give you problems! You've got to
> > match the driven load to the driving motor (AND its defining
> > parameters like drive type, power supply voltage and such!) to get
> > anywhere near the deisred and expected performance.
> >
> > And there's another thing called detent torque that we shold
> > mention. This is the effect of the reidual magnetic force acting on
> > the mechanical shape of the rotor and stator stampings inside the
> > motor. It is higher for the longer motors, and its effect comes
> > right off the top of your torque expectation. It could be likened to
> > a magnetic "friction" that must alwyas be overcome. Shorter motors
> > usually have smaller detent torque.
> >
> > These tow factors are part of why you've read of drives being
> > adjusted for larger motor types. Their mass and mechanics are
> > different and this directly affect the results.
> > You won't see this mentioned when only holding oz-in is being held
> > up as THE decision maker!
> >
> > The point of all this is to say: Don't just look at the biggest
> > number oz.in. quoted to decide your purchase!!! There's a LOT more
> > to good CNC machine design than simply "bigger is better".
> >
> > And I'm taking the time to type this because when someone comes out
> > and says only, "Bigger motors, get 'em here!" MY job gets harder.
> >
> > Because then I! have to explain all the things above to OUR
> > CNCresource customer who's hearing "bigger, better, bigger, better"
> > in his or her mind due to the one-sided push of "Performance" sales
> > being done by others in the DIY-CNC arena...
> >
> > Now let's be fair and explain exactly WHERE the 305's are gonna be a
> > good choice. Where you need LOW speed torque. Please keep in mind
> > that this is low speed AT THE MOTOR. You can have a very high speed
> > machine that has low motor speed. Belt and rack/pinion driven axes
> > are good examples. The tradeoff there is step size/ machine
> > resolution.
> >
> > Folks who've seen my writings over the years know that I often talk
> > about the "balance" of DIY-CNC design. It's fun for me to see the
> > design evolution of many "home builds" over on "the zone"; where
> > after MANY iterations they finally empirically arrive at what a
> > balanced design approach will give in the first or second
> > pass...
> >
> > Finally, let's mention that the bipolar drives offered by Xylotex
> > trade increased performance due to drive type for lower power supply
> > capability and lower amps/coil specs, compared to HCNC and
> > CNCresource.com driver boxes. FWIW, Xylotex are great drives, and
> > Jeff supports them well.
> > The net effect is that all three perform similarly with similar
> > sized motors. (I've run all three in our lab and the empirical
> > results support this statement. There ARE ways to skew the results
> > in favor of the unipolar drives; which is why we're currently
> > offering the SLA based unipolar drives at CNCresource. We'll be
> > adding some bipolar drive options this summer.
> >
> > Anyways, hope this helps,
> >
> > Ballendo
> >
> > P.S. About those increasing motor lengths... thise of us in steppers
> > used to expect that the motor body length would fall into roughly
> > three lengths. We called these single, double and triple stack
> > motors. This due to the fact that inside the rotor WAS divided into
> > 1,2,or 3 distinct areas, or "stacks" of laminations. When the new
> > range of Chinese motors came out; the old rules were blurred.
> >
> > Chinese mfrs. were "stretching" the "old" definition of what single
> > stack and double stack meant... And pushing the holding torque
> > number above all else...
> > The first wave was sort of a stack and a half; then double stacks
> > like the 200's. Now we're seeing triple stack motors being used in
> > what were formerly "single stack" places. Because everybody's going
> > after the "torque" by chasing the single HOLDING torque number, as
> > explained above.
> >
> > Anyways, when I designed the Wood Duck CNC routers, we were using a
> > 2-1/4" body length motor. Allowing for the "growth" I'd been seeing;
> > I designed the machine to be capable of handling 2-3/4" bodied
> > motors. Then the 3" motors hit the market. And everybody felt
> > they "had to" have those (I've since re-designed to accomodate
> > these, but I'm NOT going to keep adding just to "keep up with the
> > Jones's"!<G> Especially when the "Jones's" is based on a partially
> > explained truth... (We mfrs. are not operating in a vacuum, and we
> > have to either educate or explain our competitors moves to our
> > potential customers. Sometimes it's easier to just go with the flow.
> > But there comes a time when the truth needs to be spoken.
> >
> > There's more to good CNC machine design than maximising oz-in in the
> > motor spec! (FAR Better to maximise oz-in in the axis travels!) The
> > two are NOT the same thing.
> >
>