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:

    https://cdn.solarbotics.com/products/datasheets/3303_0_datasheet.pdf

    ...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:

    https://www.omc-stepperonline.com/download/17HS15-0854S.pdf

    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

    Prusa

    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

    Prusa

    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.



  • Just had another 'conversation' with them and they may now be releasing the specs for the motor, but not refunding me for the unbranded Arduino I just cooked due to the limit switch wiring not being clear enough in their Spanish documentation that covered an alternative board rather than mine!

    In addition the firmware had thermal runaway protection was disabled, and used 1.0.2, rather than the latest 1.1.9-bugfix.

    The look into the RAMPs system has certainly been useful (now running again on my spare proper Arduino!) and it'll be interesting to have the two archetectures running in parrallel to really see some of the benefits the more recent boards have realised.

    @dc42 Just 12V on this system. I think to go higher with the Arduino/RAMPs you need to supply the 5V separately to avoid cooking the 5V regulator.

    Very tempted to make this my first YouTube product review!


  • administrators

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

    @dc42 Just 12V on this system. I think to go higher with the Arduino/RAMPs you need to supply the 5V separately to avoid cooking the 5V regulator.

    That's correct. Even with a 12V supply, if a graphical LCD is included in the system then the 5V regulator on the Arduino is likely to overheat anyway. RAMPS was an OK solution when it came out, but now that most users have LCDs attached and 24V power is popular, it's well past its use-by date IMO.



  • @dc42 I agree with you there. The performance of the current systems are far superior.

    My rational here was I wanted to learn a little about the Marlin firmware as I had a potential contract with a local education centre who were planning to use the Ender-3 systems as a teaching aid which the students then get to keep. After reading into the systems a little more I realised that there was a whole heap of minor tweaks that may need to be done to make them build as originally intended (e.g. better quality bowden couplers etc) but also some critical safety mods to the firmware that were needed to enable Thermal Runaway protection.

    So I could have bought the ender 3 which was likely to need a lot of fettling/fiddling (like my Ormerods & Mendel) or try out a P3Steel based system which I had hoped would give me a reliable (if not a little slow) work horse. I started to look into ordering the parts myself but saw this kit with all the bits to get going.

    As it happens the contract approval looks like it is going to fail in the finance department, despite the project funding being on a use-it-or-lose-it basis and the project owner being very keen to proceed. The up side is I have a nice solid printer available, but equally the downside is of course I'll have to fund it.

    A definite positive is that I have much better idea for what I want to replace the Mendel frame with, and this years xmas present consists of various bits that are required to do that.

    I'm seriously contemplating pricing up what I could pull together a Maestro based bundle of parts for based on the P3Steel, but I have heard so many people say the margins are tight and I'm not sure what I can bring to the table that they didn't.



  • A snippet from this morning's chat to enjoy:

    09:19 [supplier name clipped]

    Stepper motors don't depend voltage

    09:21 Me

    Wrong.

    Higher voltage allows the peak current to be reached quicker (as it is fighting against the inductance - which you won't state) which allows the peak torque to be sustained at a higher RPM.

    09:22 [supplier name clipped]

    I will send you the specs of our motors/Prusa motors


  • administrators

    So they've changed the motor specs to be similar to the ones Prusa uses without changing to 24V power at the same time? Crazy.



  • Yeah, I still can't find anything definitive on the Prusa motors. Following the other comments here I mentioned the 24V for prusa thing and they said:

    09:18 Me

    You changed motors from what was advertised and said it's what prusa use. Prusa use 24V and a different control card so how is that releavent to my machine?
    09:18 [supplier name clipped]

    Thank you for the feedback
    09:18 [supplier name clipped]

    The last firmware is available for [product name clipped]
    09:19 [supplier name clipped]

    Yes Prusa is the same motors for 12V board ( MK2S)
    09:19 [supplier name clipped]

    and 24v board (MK3)

    Call me pessimistic but I panicked a bit when I saw 2.5A rated motors swapped out for motors that are set with a 450mAh current limit. I get that the 2.5A motors may never have been run at 2.5A and de-rated and the higher resistance motors may be able to get to full current quicker as it was so much lower. If this was a great performance swap you'd think they'd just be up front about it rather than take so much verbal beating to release the spec. I've a gut feeling cost is the key driver here.

    All that said I'm not willing to name and shame just yet as I've not all the information and I also have a gut feeling this is a one-man-band with a very fancy website who has clearly over stretched himself. I've sympathy for that and if I do review it I will do my best to find positives to balance the negatives a little!



  • Looking forward to doing a review on this printer. Latest issue (after shelving it over yuletide) came to light this morning after finally soldering the 14AWG wire to the MK3 Alu heat bed. Measured resistance and I see 0.7 ohm. That's neatly about 200W and I see no shorts to the aluminium. I make that 17A through a 11A poly fuse. That along with disabled firmware thermal protection is boarding on a real liability for them. Checked against a spare MK3 board and it has a much higher resistance and thinner tracks.


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