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Selecting the right motor for a Prusa Mendel

Posted by indieflow 
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indieflow
Selecting the right motor for a Prusa Mendel
September 10, 2011 05:50AM
Hi all,

I'm just planning my first Prusa Mendel build with a few friends and just after a bit of advice on NEMA 17 stepper motors! Now we've seen that thingfarm.org sell these [thingfarm.org] for £69.60 and was wondering if these [www.ebay.co.uk] would work instead, as obviously a hell of a lot cheaper and covers the holding torque approx. 49Ncm?

Any advice/help or links to other sellers would be greatly appreciated!

Simon

Edited 1 time(s). Last edit at 09/10/2011 05:55AM by indieflow.
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nophead
Re: Selecting the right motor for a Prusa Mendel
September 10, 2011 06:30AM
The current is too high for the reprap electronics. You can run them at lower current but the torque will be less. Probably OK for the axes but not the extruder. These hit the sweet spot of the Pololu drivers: [www.zappautomation.co.uk].

[www.hydraraptor.blogspot.com]
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indieflow
Re: Selecting the right motor for a Prusa Mendel
September 10, 2011 06:52AM
Dam thought they looked to good to be true!
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indieflow
Re: Selecting the right motor for a Prusa Mendel
September 10, 2011 08:04AM
Does anyone know of any cheaper sources of NEMA 17 stepper motors?
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misan
Re: Selecting the right motor for a Prusa Mendel
September 10, 2011 06:36PM
@nophead: Do you mean both choices, right? I've checked and both motors are 2.5A or 2.3A while stepstick's driver is 2A max. (of course you can always power them with lower current and you'll obtain lower torque but they still may work ok). I'm guessing the guys at Thingfarm's Prusa motor kit is expected to work ok for Prusa.
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nophead
Re: Selecting the right motor for a Prusa Mendel
September 10, 2011 07:21PM
I was referring to the eBay ones but the Thingfarm ones do seem to be similar. It is almost impossible to get 2A from a Pololu / Stepstick, even with a heatsink. Since the current available is limited, the torque from these high current motors will be be limited to about 1.65/2.5 * 48 / 43 i.e. 74% of the torque you can get from the Zapp ones. I think that would limit the extruder max flow rate and for a given torque the drivers will run a lot hotter.

When you use the Zapp ones the thermal limit of the driver is around the same current that the motor gets too hot for plastic brackets, so they are a good match.

Edited 1 time(s). Last edit at 09/11/2011 02:48AM by nophead.

[www.hydraraptor.blogspot.com]
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indieflow
Re: Selecting the right motor for a Prusa Mendel
September 11, 2011 02:21AM
I'm confused now! So are you now saying the ones for auction on eBay would be ok, just that they will not run at 100% due to the current limitation of the stepper motor driver?
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nophead
Re: Selecting the right motor for a Prusa Mendel
September 11, 2011 02:59AM
Yes the machine will function but you won't have as much torque available, so your maximum acceleration will be less, extruder max flow rate may be lower and the drivers will get about twice as hot, so you will need good heat sinking.

[www.hydraraptor.blogspot.com]
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indieflow
Re: Selecting the right motor for a Prusa Mendel
September 11, 2011 06:34AM
Ah right, thanks for that info then! So basically avoid lol

Am I right thinking then my best bet for my Prusa build would be to go for these? [www.zappautomation.co.uk]

Thanks once again for your advice/info, I think I'm slowly getting there! lol
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nophead
Re: Selecting the right motor for a Prusa Mendel
September 11, 2011 07:11AM
Yes they are what I use but you probably want the A version with a single shaft rather than the B version with 2 shafts. Either are fine for the Prusa but the A versions seem to come with longer wires, YMMV. B is has a short shaft on the back for attaching a shaft encoder.

[www.hydraraptor.blogspot.com]
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misan
Re: Selecting the right motor for a Prusa Mendel
September 11, 2011 10:46AM
Just one detail: "B" version is dual shaft, and shaft is cylindrical, but "A" version shaft has a filed flat, which may be a problem (or not).

[www.zappautomation.co.uk]

Edited 1 time(s). Last edit at 09/18/2011 02:49PM by misan.
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indieflow
Re: Selecting the right motor for a Prusa Mendel
September 11, 2011 01:05PM
I hope the flat isn't a problem as I just ordered the A versions!!!
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misan
Re: Selecting the right motor for a Prusa Mendel
September 11, 2011 01:16PM
I can only think z-axis might be happier with no flat (x,y and extruder might prefer to have a flat so as to prevent gear slipping on the shaft). I'd say it all depends on what you are attaching each motor's shaft.
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devijvers
Re: Selecting the right motor for a Prusa Mendel
September 12, 2011 03:02AM
> These hit the sweet spot of the Pololu drivers: [www.zappautomation.co.uk]

@nophead do these motors also hit the StepStick sweetspot?
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nophead
Re: Selecting the right motor for a Prusa Mendel
September 12, 2011 04:02AM
As far as I know the stepstick is the same as Pololu, so yes.

[www.hydraraptor.blogspot.com]
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Lanthan
Re: Selecting the right motor for a Prusa Mendel
September 13, 2011 06:05PM
Yep. But the Stepstick has thinner tracks in the motor outputs.
Probably doesn't matter, anyway. (?)
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John Meacham
Re: Selecting the right motor for a Prusa Mendel
September 13, 2011 10:22PM
Those will be fine.

The polou/stepstick is a true current limiting stepper driver so the motor specs when it comes to current/voltage are not relevant. The polou drives the motor at a much higher voltage, 12V or 24V probably depending on your design and since the motor is rated for 3.1V this means it only needs a fraction of the amperage for the same holding torque. 3.1V and 2.3A is what you can safely just connect to the leads and let sit there safely, not how the motor is actually used in practice.

John
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Lenbok
Re: Selecting the right motor for a Prusa Mendel
September 14, 2011 03:52AM
The guys who make the ones in that ebay ad look to have something fairly similar (to my unexpert eyes) to the one recommended at zappautomation - what do you think of the 42BYGHM809 on [www.wantmotor.com] - maybe it would be a good alternative for those not in the UK?
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nophead
Re: Selecting the right motor for a Prusa Mendel
September 14, 2011 07:54AM
Looks very similar but with half the step angle.

[www.hydraraptor.blogspot.com]
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richardw347
Re: Selecting the right motor for a Prusa Mendel
September 18, 2011 02:46PM
Thanks everyone for the info, I was hunting for a good place to get steppers for my second Prusa. I picked up some
SY42STH471684A's from Zapp and they're really good. I especially like the extra long wires, made wiring everything up so much easier!
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aranciam
Re: Selecting the right motor for a Prusa Mendel
June 10, 2013 07:09AM
I need to the motors for my I3 single frame, so i am on [www.zappautomation.co.uk]

there are far to many to chose from..nophead..which one shall i got for?

cheers
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misan
Re: Selecting the right motor for a Prusa Mendel
June 10, 2013 07:26AM
This one [www.zappautomation.co.uk] would be my first choice.
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