I am going to build a very simple brushed DC motor. The rotor will not be enclosed in a stator (the rotor will be built on a bench with free space around it), and I will using permanent magnets to make the external magnetic field.

I plan to compare the torque generated on the rotor with a non-iron core, with the torque generated on the rotor with an iron core (the two cores will have the same dimensions). I will do this at stall torque, and I think this will help me find the permeability of the iron core (assuming the non-iron core has a known permeability). The permeability isn’t constant in a ferromagnetic material such as a material made up of iron but knowing the permeability at stall torque should be enough for this assignment.

The best way to compare the two different torque values would probably be to just build a single rotor with a single coil and have the ability to just interchange the cores since this hopefully will eliminate some error when comparing the torque. This would mean that the coil can't be permanently attached to the first core, although the coil also has to be attached strong enough so that it doesn't fall off when the rotor spins. I got this suggestion from an electrical engineer, but I can't get in touch with him anymore. I could also build two different motors and try to make them as similar as possible to eliminate error, but the first option I think is better if it's possible to accomplish.

The dimensions of the cores will be 30x28x90mm (it's a rectangular shape, I couldn't find a cylinder), and I have the ability to cut and drill the iron core that I’ve bought, although I can only cut and drill across the short end. I plan to put the axis of rotation across the short end, but I don’t know if I should make the axis by drilling a hole (and inserting a small steel rod), or by cutting the core into two pieces and attaching each piece to a bigger prebuilt axis. This decision could influence my main issue, namely how I can switch the cores.

Any help is greatly appreciated, and please let me know if the question is unclear or badly formulated, and I will explain more about my assignment.

  • \$\begingroup\$ The rotor WILL be surrounded by a stator ... that's what those permanent magnets are. \$\endgroup\$ May 4, 2021 at 16:40
  • \$\begingroup\$ Ok, what's the "outer shell" called that generally enclose the rotor then? \$\endgroup\$
    – Max
    May 4, 2021 at 20:43
  • \$\begingroup\$ Sometimes it's the stator, sometimes it's a case, sometimes it's even the rotor. Yours sounds like what might be called an open frame motor. \$\endgroup\$ May 4, 2021 at 22:13
  • \$\begingroup\$ Ok good to know \$\endgroup\$
    – Max
    May 5, 2021 at 15:58

1 Answer 1


this is not your plan but perhaps build wood or plastic core with holes drilled in and insert or remove metal rods or build a bottle-like core and fill it with iron powder or ball bearings.

  • \$\begingroup\$ Haven’t thought of that! You mean make it hollow inside? I have already bought a core, so do you think 3d-printing a hollow shape, winding a coil around it and putting it over the core is good idea? Because then I would have the opportunity to remove the iron core but use the same coil. It shouldn’t matter for the magnetic field if the second coil is hollow or has a plastic core since air and plastic has about the same permeability. \$\endgroup\$
    – Max
    May 4, 2021 at 20:41
  • \$\begingroup\$ if you can 3d print, tyhen yes that's a good approach, ensure it doen't get too hot if you're using a fusion prnter. \$\endgroup\$
    – Jasen
    May 5, 2021 at 11:28

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