I saw on a popular site simple indicators for attaching to the axle of an extruder motor (Prusa printers). Those gadgets are spinning due to movement of motor, both clockwise (pulling filament) or counterclockwise (retracting). But all of those things are attached to motor axis by small neodymium magnet (round, 8x3 mm). I searched informations how such magnets affects for stepper motors and I read that magnet field can significantly change magnetic field of the motor. On presented movies for gadgets which I saw, the motor seems to have no trouble with rotating but AFAIK neodymium magnets have really strong magnetic field and I am curious how its centric orientation due to motor axis:

  • inhibits rotation
  • if yes, how much it raises the temperature of motor?
  • how it affects for electronic of filament sensor?

Currently I use Prusa i3 MK3 printer and during long (~10h) printings an extruder motor is enough hot and I don't want make it hotter. I want to print and attach such "rotation indicator" but the fact of used magnets made me started to thinking about magnetic field of motor. Or maybe those changes are so marginal to think about them?

  • $\begingroup$ How strong of a magnet are you using? Only a small magnet would be necessary to attach it on I guess. $\endgroup$
    – 0scar
    Mar 10, 2019 at 7:55
  • 2
    $\begingroup$ Also, how sciency you want your answer to be? $\endgroup$
    – Trish
    Mar 10, 2019 at 9:18
  • $\begingroup$ I've updated my question for sizes of magnet which is suggested to use (round, 8x3 mm). I want simple answer, like: "You can use it safely, in this case it's not to be worry about" or "it's not a good idea, because influence of the field will be too strong and will cause...". Thank you. $\endgroup$ Mar 11, 2019 at 18:41

1 Answer 1


The best way to know is placing the magnet on the shaft; if you see that motor starts shaking or stops this mean that you affects the motor operation, but I think if any magnet interference can be deprecated due internal coils of the motor during operation.

The motor works with some coils in the rotor and some magnets in the stator (motor frame). But the shaft is to far from the coils to be affected for the magnetism of one small neodimiun magnet which needs at least 7mm to trap another small metal objet or for induction sensing and 2mm as maximum for creating electrical flows (generation); this values are afected if the magnet is placed over other metal part reducing his atraction field, and the armor is too big for an small magnet 8x3mm.

You can attach some indicator with a double sided adhesive tape (3M) if still are worried about affecting your extruder.

If your motor extruder is heating during extended usage periods you should adjust the motor current, for example: you motors is rated 0.5A you should calibarte the current for 0.48A or 0.45; this could reduce the torque force of the motor by a little. Or maybe the current current :D is calibrated above 0.5A thats way you are getting over heating.

  • $\begingroup$ I don't see where your heating is referencing to $\endgroup$
    – Trish
    Mar 18, 2019 at 12:48
  • $\begingroup$ Thank you. The answer is enough for me. I'll check the gadget with magnet. I'll hope it won't raise the temp of motor significantly but when it will I'll search how to calibrate the current. $\endgroup$ Mar 18, 2019 at 21:39
  • $\begingroup$ No, this feature won't raise the temperature. $\endgroup$ Mar 19, 2019 at 7:01

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