6Ohm Steppers vs 1.25Ohm 2.5A

  • Hi all. Slightly cheeky as this is asking about a kit that doesn't have a Duet. But hopefully you'll afford me a little pardon as I have used many Duets and plan to use more soon! I know there is quite an expertise in stepper choice on this forum.

    The kit in question is a p3steel based system (first of two) that is running a ramps system. Bought because I require a familiarity with the RAMPs/Marlin system and also had an interest in converting my Duet 0.6 mendel onto a p3steel chassis.

    Yes a creality/anet would have been cheaper but wanted something that could be a solid work horse with future expansion capacity.

    The advert stated: "Wantai Nema motors with 4.8 kg / cm torque and 2.5A maximum current" Which stated a resistance of about 1.25Ohm on a few sites although I do need to double check that. The motors that have been supplied seem much higher resistance ~6.5Ohm and are own branded (ie some supplier offers a laser marking/branding service). I've a feeling that these are going to be slower due to the high inductance (normally hand in hand with higher resistance?) and 12V VIN limit? Getting 2.5A through those motors at 12V would be impossible. Lower inertia of these smaller motors will probably be a disadvantage on the heavy Y too. What are peoples thoughts on the change in motor spec from advertised?

    I've emailed the supplier and will give them a chance to resolve before naming and shaming. This isn't the only issue. Advert stated LCD and that's missing, print quality is poor, one part missing, and no english assembly guide.

    The drivers are DRV8825 onto a RAMPs board. I believe these can cope with 2.2A, probably with fans.

  • @doctrucker Some of the information I have:

    Stepper voltage is not so important, but there is a guide maxvolts = sqrt(inductance)*32, so e.g. 1.8 mH => max. 42 V. The official voltage of the datasheet is not important.
    Fast steppers are those with low inductance and small mass (short ones).

    According to https://hackaday.com/2016/08/29/how-accurate-is-microstepping-really/ DRV8825 has problems with microstepping, I would exchange them with TMC2130 or similar.

  • If you're using DRV8825 because they are on the board, one fix is to use TL smoothers they really do work with 8825's ( but still a bit of a kludge)

  • @stewwy nice, didn't know them!

  • Thanks for the comments. I was concerned that what appeared to be a low resistance high current motor had been swapped out for what appeared to be a high resistance potentially high inductance motor with less inertia. Are there any potential benefits of higher resistance ( and probably inductance) motors with less inertia?

    If anything i was thinking I needed a larger motor on the moving bed to avoid a large inertia mismatch.

    I'll read the hackaday more thoroughly later but I'd be interested to see how the behaviour changes with appropriate ineria matching.

  • administrators

    Motor inertia depends on motor size, not directly on resistance/inductance.

    High inductance/low current motors need less current than low inductance/high current, so they are better with low-current drivers such as TMC2100, TMC2130 and TMC2208. Also if you use stall detection, you should get finer control over the stall sensitivity and perhaps more accurate stall detection. The disadvantage is that for a given motor supply voltage, the speed at which the motors can no longer maintain full torque is lower. See the motor EMF calculator at reprapfirmware.org.

  • Yes the received motor was shorter, that's where I was getting the reduced inertia from, not the change in electrical properties.

    I'll have a closer look at what the maximum speeds are likely to be with the new motors.

  • After a bit of hunting around these were the stepper motors that were supplied with the kit:


    ...and these look like they might be the ones that I received, although the screws on the rear of the motor don't look quite right:


    Lower torque, higher inductance but also a lower current. I will run figures later but it may well balance out to a similar performance with the replacements being slightly more efficient in terms of current draw compared to max holding torque.

    Still not sure why you would change motors and not be up front about it on the advert.

    Edit: The supplier has just responded to a web chat to say that isn't the correct data sheet but is taking time to respond with the correct link.

  • All the supplier would say was that:

    The new ones are better

    same specs of Pruda


    I can't see an easy listing of what the Prusa motors are, they also appear to be laser marked to the Prusa branding.

  • @doctrucker said in 6Ohm Steppers vs 1.25Ohm 2.5A:

    I can't see an easy listing of what the Prusa motors are, they also appear to be laser marked to the Prusa branding.

    I have not found anything about them either when I was searching a while ago.

    But then again: just using the same motors as Prusa without matching the mainboard does not make much sense. The MK3 does not run on 24V because they just found a cradle of 24V PSUs somewhere in the back of the warehouse... 🤦

  • administrators

    @doctrucker said in 6Ohm Steppers vs 1.25Ohm 2.5A:

    All the supplier would say was that:

    The new ones are better

    same specs of Pruda


    I can't see an easy listing of what the Prusa motors are, they also appear to be laser marked to the Prusa branding.

    Does the machine use 24V power? If so then lower current/higher inductance motors should be OK, and they may also make stall detection easier.


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