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

John,

Nothing wrong with doing it. Just don't try to do it with the HobbyCNC
driver board. It's only rated to 44Vdc. (That's just over 7x the rating.
Not anywhere near 20x)

I don't know of any Unipolar drive rated to 120Vdc. :-)

Cheers,

Peter.

John Hansford wrote:

> 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.
>> >
>>
>
>
>
>
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