Stepper motor Inductance: 5.4mH ?



  • Hey,

    I've got my eyes on a nema 23 stepper motor with a Rated Current/phase: 2.8A , Inductance: 5.4mH and Voltage: 3.2V
    .
    Using 24v psu and Duet 2 WIFI.

    Can I run that nema with regarding the recommended Inductance - <=4mh ?

    Cheers.


  • administrators

    Yes you can. You can use the motor EMF calculator at reprapfirmware.org to calculate an approximate maximum speed before torque drops off.



  • If putting aside the speed and torque, I thought the issue was toasting the board when using a motor with higher Inductance than recommended. Or did I get that wrong ?


  • administrators

    @CJ-BRAVO said in Stepper motor Inductance: 5.4mH ?:

    If putting aside the speed and torque, I thought the issue was toasting the board when using a motor with higher Inductance than recommended. Or did I get that wrong ?

    It's not about toasting the board, it's all about being able to maintain torque at high speeds.



  • @dc42

    oh...

    So switching back to toasting - what are my limitations regarding stepper motors and fried boards? just the rated current?



  • @CJ-BRAVO You could fry your board by running the steppers at the Duet 2 Wifi's maximum output of 2.4A and explicitly omit active cooling (of the backside) - you could even "improve" this by putting into an insulated enclosure. 😉



  • @wilriker

    okey dokey.

    So 2.4A is the only obstacle ? just to make things clear and sorry for the silly questions :
    If I take the specs of the motor as I mentioned before -
    Rated Current/phase: 2.8A , Inductance: 5.4mH and Voltage: 3.2V

    Math goes like : ( 3.2/24 ) * 2.8A = 0.37A ?
    So I can actually use a much higher current/voltage I want to ?



  • @CJ-BRAVO

    no you run the motors at 70-80% of their rated current. you ignore the rated voltage (if its below 12V)

    so 0.8* 2.8 = 2.24A



  • @wilriker said in Stepper motor Inductance: 5.4mH ?:

    @CJ-BRAVO You could fry your board by running the steppers at the Duet 2 Wifi's maximum output of 2.4A and explicitly omit active cooling (of the backside) - you could even "improve" this by putting into an insulated enclosure. 😉

    ...but shouldn't the thermal protections should kick in to stop you killing the drivers?

    If I push current too high on the v0.6 boards I get a layer shift where the drives have cut out and come back in again, or was that something else?



  • @Veti

    ok.

    So to make things clearer for me, I should separate between the rated current of the drivers (2.4A) and the total amp that the duet can take?

    I understood that I do the whole calculation thing ( ( 3.2/24 ) * 2.8A = 0.37A ) to figure out the total current I can put on the board - in this case I can use a few 2.8A motors on the duet with no worry but I should take notice if exceed the rater current for driver. right?



  • @CJ-BRAVO said in Stepper motor Inductance: 5.4mH ?:

    but I should take notice if exceed the rater current for driver. right?

    the drivers will shut down if they get to hot and you can not set the current higher than allowed.

    do take notice of the fuse
    7.5A for the VIN going to the heaters and stepper motors



  • @DocTrucker said in Stepper motor Inductance: 5.4mH ?:

    ...but shouldn't the thermal protections should kick in to stop you killing the drivers?

    Yes, they will. But depending on how determined you are to fry your board you could probably still accomplish it - but that requires pure will to do so.

    @CJ-BRAVO When it comes to stepper drivers (the ones used on Duet boards that are current chopping) you can ignore any voltage in the system. They will only care for current. On Duet 2 you have a firmware limit of 2.4A - if you set it to 154A it will still not exceed 2.4A.

    @Veti The type of this fuse will only blow if it is a multiple of the rated current (2-10x depending on the actual fuse used) and only is there for a short-circuit protection. You can run all steppers at 2.4A without blowing the fuse.



  • @wilriker he he, fair enough! 😄

    ...in part checking for my own 'procedures'!



  • @wilriker said in Stepper motor Inductance: 5.4mH ?:

    You can run all steppers at 2.4A without blowing the fuse.

    hey, thanks for the answers.. still what to verify -
    "You can run all steppers at 2.4A without blowing the fuse." : because the whole "motor voltage" (3.2v) divided by the power supply (24v) times the rated current (2.8A) comes down to "not really" 2.8A ?



  • @CJ-BRAVO again, forget about the voltage when it comes to stepper drivers. It's only the current that matters - the same for the fuse.

    In theory 5*2.4A (=12A) is larger than the 7.5A fuse but since this fuse will not blow below 15-20A continuous current draw it won't be a problem.

    Voltage only matters for top speed without losing torque.

    EDIT: sorry for all this nonsense. I was replying to about 3-5 totally different questions in my head. 🤦


  • administrators

    @CJ-BRAVO said in Stepper motor Inductance: 5.4mH ?:

    "You can run all steppers at 2.4A without blowing the fuse." : because the whole "motor voltage" (3.2v) divided by the power supply (24v) times the rated current (2.8A) comes down to "not really" 2.8A ?

    Correct. At low motor speeds, using 2.8A peak current, the power required per motor will be 2.8 * 3.2 watts, times 5 motors makes 49W. If the stepper drivers were 100% efficient, the current drawn from the 24V supply would be 49/24 amps which is just over 2A.

    In practice the drivers are not 100% efficient, so the current draw will be a little higher. The current draw will also be higher when accelerating, but acceleration doesn't last long.



  • @wilriker
    🙂 so which part of what you said should I ignore ?

    @dc42
    thanks!



  • @CJ-BRAVO said in Stepper motor Inductance: 5.4mH ?:

    @wilriker
    🙂 so which part of what you said should I ignore ?

    Everything I said regarding input to the drivers.



  • @wilriker

    so just make sure not to exceed 2.4a per driver and regarding duet board is fine cause its actually not drawing that much for the drivers. or something like that.



  • @CJ-BRAVO Unless you recompile the firmware with the limit increased you cannot exceed 2.4A/driver output current anyway. All you have to do is to match your steppers to perform well up until this limit (or even below). Also you need to actively cool your board when going beyond 2A/driver.

    To get to problems on the input side you will probably have a hard time finding steppers that would get you there (i.e. steppers that have a rated voltage at or above your PSU voltage). And even if you manage to find those they would have to be moved at a crawl-speed or lose torque basically immediately. Short: there is nothing to worry on input side.


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