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

>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.