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BenjaminPoilve / Liplo

A simple desktop plotter
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Hot stepper motors? Try this #1

Open Ianmcmill opened 5 years ago

Ianmcmill commented 5 years ago

You use 12v stepper motors. Try motors which are spec'd at 3 to 4v. Those 12v stepper motors are inappropriate for 3d printers/plotters etc. I have a bunch of 12v steppers and they collect dust in my shelf because they get too hot. Adjusting current on the driver board does not help here. Their resistance is way too high (30ohms) 4v steppers have around 7ohms which is why they don't get so hot.

BenjaminPoilve commented 5 years ago

Thanks for the input!

My physics must be a little rusty, but if the torque is proportionnal to the input wattage, to get an equivalent wattage at lower resistance I would need to pump more Intensity (W=UxI) which would create much more heat as intensity is squared in Joules' Law (P ~RI^2). So for equivalent torque, wouldn't there be more heat?

I think the crux of the problem is that stepper are always overpowered to avoid any potentiel step losses, which leads to overheating. Most of the 3D printer I've seen use 12 or 24v steppers, which run pretty hot (~80°c)..

Ianmcmill commented 5 years ago

All physics aside. My 3d printer steppers are rated 2.0A 3.6V. Okay not quiet 0.4A. Nevertheless. I have never had any steppers rated above 4V in the datasheets. They are driven by a A4988 with the voltage set to 0.6V using the potentiometer of the driver. They barely get 35°C. The torque is good. Not loosing steps. Same on my polargraph. I have a hard time pulling the belt to get it into home position when the motors are powered. If they would reach 80°C my PLA parts would warp and sag. I always shake my head when I see printer with heat sinks or active cooling. Sidenote: Same goes for A4988 or DRV8825 drivers. Heat sinks are all they need. My ramps board is runs without active cooling but heatsinks for years.

https://www.omc-stepperonline.com/de/nema-17-bipolar-59ncm-84oz-in-2a-42x48mm-4drahte-w-1m-kabel-and-verbinder.html The less resistance the coils have, the cooler.

Ianmcmill commented 5 years ago

And I forgot one thing. Your Liplo plotter is a really nice designed machine!

BenjaminPoilve commented 5 years ago

Well, my (possibly flawed) understanding of physica always prevented me from trying such low voltage steppers! I'll sure give it a try in my next project. Thanks for the insight, any idea why such a faulty design is so common? I have had both makerbot and printrbot printer and I am pretty sure both use 12v motors and are prone to overheating! To avoid the issue I tried using sensored DC motors, but at the pricepoint I wanted to attain, the motors were much to fragile. Thanks for the compliment, it means a lot!

Ianmcmill commented 5 years ago

I jst realized a thing about my steppers and my driver. I think this is important to add to this topic. I measured the Vref on my drivers and I did set them so that the stepper motors run at around 40% of their rated current. I have had no problems with losing steps. Even on my 3D printer. I found this out when I used this calculator https://v6zmvq5nk5.codesandbox.io/ and adjusted my steppers accordingly. They became very hot. Then I decreased the Vref again down to a point where the motors had enough torque to move the X and Y gantry and didn't lose steps.

On a plotter like yours, where even less mass has to be moved (no extruder/hotend) you could also go down to maybe 30% of the rated current. As I said, my 3D printer runs fine with 40%. The motors are even more quiet.