Ebay stepper motors ?

I found a set of 5 stepper motors here, I am a complete noob and I am not to sure if I should buy - is 0.4NM-1 a lot of torque . I also noticed they are only 1.2A max - that is quite low and would mean they would generate a lot of heat ?

but what concerned me most of all was that the motor cords are dis-connectable (all of the other reprap stepper motors that I have seen have the cords attached - see the link) but more importantly the motor was only 40mm deep compared to the 47mm depth listed in the BOM in the wiki (I am planning on building a Mendel90).

Any advice on stepper motors would be great (should I buy a more powerful motor for the extruder ?)

thanks so much , in advance

Jeremy

The motors you're looking at are mid-range. The shorter motors (34mm) are typically have ~30 oz-in holding torque, the 40mm ones (like yours) are ~50 oz-in and the longer 47mm ones are ~70 oz-in. The separable cable is a nice feature which makes assembly and disassembly of your printer easier. The current of 1.2A doesn't mean anything by itself - other than that's the max. current your drivers will have to put out. In this regard, lower current means that your driver chips won't generate as much heat. Unless you are building a really large printer or are really wanting to push your accelerations, these motors should be OK. As for the extruder, if you are using a Wade's or other type of geared extruder, they should be fine. They may be a little small, though, for a direct drive extruder unless you are using a pretty small diameter filament drive wheel. I've used this size motor successfully with a 7mm dia drive wheel, but I wouldn't want to use it with a 10mm dia drive wheel.

The holding torque specification for steppers, however, is a largely useless number. Most motors of a given size have more or less the same power output (power = speed x torque), but with different windings and internal geometries you can get high torques at low speeds or lower torques at higher speeds. The voltage rating of the motor v. the supply voltage you are using also dictates how much power you can get out of the motor. (You want your supply voltage much higher than the rated voltage.) To sort all this out what you really need is the speed-torque curve for the motor, which many sellers do not supply. If you can find out who the manufacturer is, they will usually have speed-torque graphs for every motor they make. When you look at the speed torque curve for different motors, you'll see that the torque drops off as the speed increases. But for some motors, the torque drops off really fast, and for others, the torque delivery is much flatter across a wider range of speeds. With most printer designs, you'll want your motors running at 5 - 10 rev/sec. You want to make sure that the motor still has a reasonable amount of torque in the 5-10 rev/sec range.

Note that sometimes you'll run across NEMA 17 motors with really high holding torques - 80+ oz in. This is usually achieved by creating really tight gaps between the rotor and stator, and using really sharp tooth geometries. This usually results in a more rapid drop-off in the running torque, and it also decreases the accuracy of the microstepping. That is, making a 1/16th microstep will not move the motor exactly 1/16th of a full step.

well that's an in-depth answer, thanks for all the information. I never really thought about this, but i always thought higher holding torque equals higher power output.

Well thanks from my side too. I'd the same "problem" but the explanation was awesome. Thanks a lot Lobo. It would be grate if I could rate your post.

Thanks so much Lobo !!

- ill try and serch out the speed vs torque curve

do you know if there would be a problem using the 40mm size instead of the 47mm - ie in terms of mounting?

if i used the motors at max current would my motor mounts be in danger of melting ? would it be good to get the motor mounts printed in ABS ?

thanks heaps

Jeremy

I don't think they'd be in danger of melting. Stepper motors can get pretty hot in some cases, but I think if you're running them above ~80C, then you're probably using more current than spec. PLA should be fine at those temperatures.

Lower current steppers can still achieve high torque if the inductance is high. The tradeoff is that you are energizing a coil with many more windings to generate a higher magnetic flux.. but just like an RL circuit, if you increase the inductance, then the frequency at which you can operate the motor is reduced. Thus your tradeoff is manifested in torque vs speed. For two motors of the same torque rating, the one that has a lower inductance (and therefore higher current rating) will be operable at a higher speed. Because of this, it is often an advantage to use motors which have current ratings that are comparable to the max current of the driver.

Steppers on 3d printers can usually be underdriven because the torque requirements aren't all that large. This keeps the motors from heating up substantially and reduces the noise that they operate at.

Thanks Iquizzle.

Would these be alright ?

once postage is taken into accout they cost the same as the ebay listing. 1.8A would be better ?


thanks