Re: NEMA17 current recommendations

[AnWe79]

Just a word of warning, don't get motors specced higher than 2A.
I got 2.5A motors before I understood the implications.
It means the pololus (stepsticks in my case) can't drive the motors at full current, and you'll never reach the specced torque.
The motors run very cool though, barely lukewarm with the stepsticks running full blast.

I suppose the downsides with low current motors is that microstepping won't be as exact as you'd think.
Nophead explains it better than i can ever hope to.

I think the reasoning is equally valid for authentic pololus, but the component values are different.
I also guess that the motors may run hot (PLA melting hot), but I'm not sure about that one.
I've done the mod mentioned in the article and I'd reccomend it to anyone with a steady hand, good eyesight and decent solder skills (those resistors are tiny).

If you want to really know how a specific motor/driver combination will perform (and if there are any gotchas),
Nophead has an excellent writeup on Motor maths. It may seem daunting, but is well worth the read.

NB: The motors will probably still work fine even if all this is not optimal for your driver/motor combintaion, but microstepping (and audible noise) will be worse.

/Andreas

[Larry Knopp]

Awesome reply, Andreas.
Thanks.

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[John Driggers]

Couple of things - so Andres' is right, BUT with a caveat - the operating temp of most stepper is in the 80-100c range, so running them "under rated" is a good thing - they stay cool.  The rule of thumb is to run them at 70% of their rated current.

If your motors are using printed mounts, you can get away with running them at spec'd current - and UNLESS you go for the new, not quite compatible purple Pololu's, you will NOT get the full two amps out of the drivers before they go into thermal overload and barf on themselves....  With heatsinks and some ok cooling, I can get about 1.3 amps out of the "standard" ones.

 

On Saturday, January 19, 2013 1:09:41 PM UTC, Rsqdivr Mark wrote:

Hi all.  Absolute newbie here.  I've pretty much decided that for my first printer, I want to go with the MM design (1.5, RAMPS, etc).  My head hurts from pouring over all the specs and variations.  I've been carefully considering the motors.  I have found NEMA17 steppers ranging from 0.4A to 1.8A all providing plenty of torque.  Since the pololu a4988 has a rated current of 2A, but anything over 1A gets into cooling issues, wouldn't I want to opt for the lower current motors if the torque was roughly equivalent?  But so many of the motors I see people quoting are of the ~1.7A variety.  Something tells me there's a catch here that I'm not considering.  Thanks for your help.

Mark

[Lee]

Also, here is a good list of models of steppers that people are using for reprap and where you can buy them.

http://reprap.org/wiki/Stepper_motor#NEMA_17_suppliers

I've been using the Kysan motors on that page without problems. They're rated at 1.5A, but I'm sure that I drive them at less.

[Rsqdivr Mark]

Thanks for your reply.  I have ordered the Pololu "black" versions.  Are these the ones you refer to as purple?  I understood they had better cooling because they were 4 layer, etc.

I was considering one of these two sets:

http://www.aliexpress.com/store/product/5pcs-NEMA17-78-Oz-in-CNC-stepper-motor-stepping-motor-1-8A/704350_554160664.html

http://www.aliexpress.com/item/3D-Printer-Stepper-Motor-Nema-17-Stepper-Motor-Double-Shaft-42x48mm-2-8v-1-68A-44Ncm/660739056.html

Do you see any issues with these paired with the Pololu black driver (RAMPS v1.4, 12v/30A power supply)

Thanks again.

[AnWe79]

Those look good.
I've never heard of the brands, but I hadn't heard of Wantai before I got mine either so take that for what it's worth :)

Unless you need the double shafts I'd go for the first ones, more bang for the buck.
Just watch the shipping costs, they seem high to me, but it may be totally different for your location.
Self sourcing my kit I found that it's often wiser to get as much as you can from one place at slightly higher item cost, multiple shipping costs add up fast.

/Andreas

[Rsqdivr Mark]

Thanks all for the advice.  So is there any problem with getting those rated at lower voltage?  For instance, I can find some at 1.8A but only 3.4Vdc that provide 4N.m of torque.  Doesn't the Pololu adjust the voltage to maintain the set current (limited by the potentiometer)?  I have the 30A/12Vdc power supply.  So, is it ok that I get motors that say they're rated/operating voltage is less than 12v?

Mark

[AnWe79]

Voltage "ratings" for stepper motors are confusing. They usually specify the steady state voltage that is needed to reach the rated current (and torque). That value is just a consequence of the winding resistance.. Most repraps use chopper drivers where the voltage is not constant, so that value is largely irrelevant. 12 V should be safe, what matters is that the voltage doesn't approach the insulation breakdown voltage of the windings. 12 V shouldn't be anywhere near it. Many reprappers drive this type of motor at 24V with no problems.

The short answer is that you can just ignore the voltage rating, (unless you are going to use some unusual driver circuit).

/Andreas

[Triffid Hunter]

On 20 January 2013 00:09, Rsqdivr Mark <jblea...@gmail.com> wrote:

Hi all.  Absolute newbie here.  I've pretty much decided that for my first printer, I want to go with the MM design (1.5, RAMPS, etc).  My head hurts from pouring over all the specs and variations.  I've been carefully considering the motors.  I have found NEMA17 steppers ranging from 0.4A to 1.8A all providing plenty of torque.  Since the pololu a4988 has a rated current of 2A, but anything over 1A gets into cooling issues, wouldn't I want to opt for the lower current motors if the torque was roughly equivalent?  But so many of the motors I see people quoting are of the ~1.7A variety.  Something tells me there's a catch here that I'm not considering.  Thanks for your help.

The ideal stepper for reprap is NEMA17 size, rated 1.5A to 1.8A or less, 1-4 volts, 3 to 8 mH, 62oz.in (0.44Nm, 4.5kg.cm) or more of torque, 1.8 or 0.9 degrees per step (200/400 steps/rev respectively), for example the kysan 1124090/42BYGH4803 or the 17HS8401

Stepper motor torque is proportional to current, however at rated current the motor will be sitting at around 80c which we consider too hot, you can burn yourself by touching them, and they will melt PLA motor brackets. However we don't want to lose too much torque just to cool the motors. Our pololus push about 1.25A on a good day, so motors in 1.5-1.8A range are ideal. Dissipated heat is proportional to current squared so it drops rapidly with only a small increase in motor's rated current.

Stepper motor "voltage" is simply the voltage required to push the rated current into the winding resistance. It does not take into account winding inductance, or the fact that when the motor spins, it generates an opposing voltage. If the driver runs out of headroom with its supply voltage, then current falls and so does torque. You want the winding resistance (hence "rated voltage") to be as close to zero as possible. The 1-4v range works well.

When microstepping, this is even more pronounced as the limited supply voltage causes the tops of the current waveform to be chopped off, causing the motors to not track their position properly until the curve drops again, which causes discontinuities at speed and hence lost steps.

And of course the more torque your motors have, the faster your printer will run, and it will also be more reliable at lower speeds so you want as much torque as possible given the above constraints.

http://ultimachine.com/content/kysan-1124090-nema-17-stepper-motor

http://www.aliexpress.com/store/product/5pcs-NEMA17-78-Oz-in-CNC-stepper-motor-stepping-motor-1-8A/907217_523595194.html

are a couple of links to motors that satisfy the requirements of a reprap

For those curious folk among you, a zero ohm stepper would work fine. It would run stone cold at any current although the torque would peak at some point and go no higher due to finite magnetic permeability. Our drivers would not have difficulty driving it- when operating in proper current mode chopper drive, the winding inductance dominates, and the resistance is simply a parasitic loss of power. The drivers would run no cooler nor hotter unless the winding inductance was also extremely low. However, it would ONLY be able to be driven by a current-mode chopper drive- PWM drive or linstepper or plain h-bridge would pop fuses or catch fire trying to handle it.