Overdriving water cooled stepper motor

[Noobster]

Let´s say i have all the cooling (and voltage) for the stepper motor is there still a limit to the torque-current relation ? I am building a 3d printer with a water cooled hotend and was wondering if it makes sense to drive the stepper above the rated current (like way above, since the watercooling is already in place) therefore dodging the normally required gearing?

[Benta]

Let´s say i have all the cooling (and voltage) for the stepper motor is there still a limit to the torque-current relation ?

Of course there is. It's called core saturation, and don't expect a lot of leeway here. Motor manufacturers only use just as much iron as necessary (cost).

[thm_w]

You are better off adding voltage instead of current: https://www.designworldonline.com/improving-high-speed-performance-of-hybrid-stepper-motors/
and tweaking the motor type to what would best suite your use case.

Having high idle current doesn't really help you, since a 3D printer is experiencing very little external force on the motors, where holding torque is needed. Unless you have a belt driven bed or something non-typical. But then that motor is usually stationary and can be made larger.

More gearing and smaller motor seems to be trendy currently if we are talking extruders.

[Noobster]

Saturation was the term i was missing. I found this "Because of magnetic saturation, there is no advantage to increasing the current to more than 2 times rated current. At that point the increase in current won't increase the torque. At about 10 times rated current you run the risk of demagnetizing the rotor." (https://www.orientalmotor.com/stepper-motors/technology/stepper-motor-basics.html). 2 times torque sounds quite good though. Thanks for the replies

[Siwastaja]

Yeah, magnetic saturation basically means the core material loses is ability to "amplify" the magnetic field, relative permeability, which would be originally several thousands. It does not happen suddenly but gradually, and even when all is lost, you still have permeability of 1.0 (that of air), so there is no limit to the torque, but you get less and less for given amps * turns. This means now you need to put thousands of amperes more to get what you would normally get by just adding 1A. Quite obviously, the resistivity of the winding copper wire is the problem here, and the windings just vaporize in seconds.

This "diminishing returns" game might already start at the rated current, or slightly above it. In absence of thorough specifications, the only way to know is to test and measure.

If you have a better-than-normal cooling available and efficiency is of no concern to you, then you surely can drive the motor into the "diminishing returns" region; the saturation is not abrupt but gradual. On the other hand, if you just mount some water cooling blocks outside of the motor, you may be overestimating your cooling efforts. The windings inside do not couple to the motor iron that well; there is a lot of air there, and some varying amount of varnish which might be better than air, but still crappy as thermal conductor. As most of the heat is produced in the copper windings, and their resistivity goes up with increasing temperature, forming destructive positive feedback, by exceeding rated current, hotspots form somewhere in the middle of the windings, until the varnish burns, causing short circuits, or the copper simply melts.

[Noobster]

Thanks, that was my thinking as well and the reason i asked. If there would have been a lot to gain i would have looked into optimizing cooling, like potting the windings etc. But good to know there is some performance to be gained. Strange that no motor manufacturers have come up with a super weight optimized stepper for use in extruders.

[thm_w]

There are many low weight stepper motor options.

https://www.bondtech.se/product/ldo-nema14-round-20mm-pancake-motor/
https://www.pololu.com/product/2298

[langwadt]

There are many low weight stepper motor options.

https://www.bondtech.se/product/ldo-nema14-round-20mm-pancake-motor/
https://www.pololu.com/product/2298

very low torque, is that enough for an extruder?

[thm_w]

There are many low weight stepper motor options.

https://www.bondtech.se/product/ldo-nema14-round-20mm-pancake-motor/
https://www.pololu.com/product/2298

very low torque, is that enough for an extruder?

People are using 3:1 to 6:1 gear ratios with these motors. Gear is fine as a small amount of backlash doesn't affect extruding when appropriate retract distance is used.
That puts it in the same range as a standard Nema17 (40-60 Ncm holding).

[Noobster]

Small light motor yes, but also weak. I thought of a motor designed for minimum weight. Have not seen something like that. Gearing is fine but introduces more opportunities for errors. There was a video on YouTube showing gear runout producing artifects in the print (i think it was mirageC)

[thm_w]

70g is very light already, don't chase whatever the current 3D printing trend is without knowing it has value or not.
The artifacting you are talking about was due to a low tolerance two piece extruder gear with a large amount of runout (not perfectly round). The problem was not that reduction gearing was used.

https://www.bondtech.se/2023/03/09/turning-lemons-into-lemonade-chapter-two/

[Noobster]

I think the runout of the pressed on plastic gear was also quite high. You are right though that you get deminishing returns at some point. I remember my first printer where i made a hobbed bolt (M8 bolt cut with a tab lol, this was the standard back then), even that worked.