I am trying to do a simple project, a brushed DC motor, in my free time to check what I learn in lessons in real life and I've encountered something I didn't think of before while studying the EMF concept. That is the magnetic force on the metal rotor.

Since the only thing I can use for my rotor is some steel screws on which I am winding my coils in a 4 pole configuration, the magnets in my motor create lots of inertia on the steel screws and motor cannot overcome it so it's not spinning but there is stress on the motor which I can feel from the vibrations. I don't have a large DC power supply and best I can do is 12V. Next, I will try to use thicker wire because I am currently using 0.4mm copper wire and will upgrade it to 0.8mm which in theory will increase my current up to 4 times hopefully and therefore emf will increase 4 times as much but still I am concerned if there is another way to overcome this magnetic pull on the rotor.

  • 1
    \$\begingroup\$ That's called cogging torque and if you turn the shaft on any PM magnet motor you can feel it. You can minimise it by precision machining the steel parts in the rotor to a suitable design, or by eliminating steel (all iron) from the rotor which is probably easier for you. \$\endgroup\$
    – user16324
    Dec 5, 2020 at 14:28

2 Answers 2


There is a reason why DC motors have a convoluted shape to their rotor magnetics: -

enter image description here

Picture from here.

In the picture above, the light grey rotor magnetic material presents to the stator an almost perfect "circle" of magnetic material and thus the "static" effects of the permanent magnet stator are equalized in virtually any rotor angular position. This means it can freely rotate with very little cogging torque. Maybe you need to read up on cogging torque?

the only thing I can use for my rotor is some steel screws

If you are using screw heads, the static magnetic forces are concentrated where the screw heads congregate. To overcome this either make your rotor without magnetic material (a commonly done thing) or try and increase the surface area of each rotor pole a lot more.

Here is another way that cogging torque is reduced. It uses a skewed or twisted rotor: -

enter image description here

The twisted rotor will inevitably have less of a detent/cogging problem.


The magnetic field of the rotor and the stator push on each other. That's what makes the rotor turn. That magnetic fields cause mechanical effects is a given, else the rotor wouldn't move.

The mountings for the rotor and the stator have to be able to stand up to the physical forces acting between the stator and rotor, else the motor just blows apart instead of the stator rotating.

It is no accident that the housings (which hold the stator) are mostly cylindrical and made of metal. The inside has to be cylindrical so the rotator can rotate. A cylindrical outside is also the strongest form that can be made with the least materials. The housing is usually metal because it contains the magnetic fields better - but also because the metal housing is stronger than plastic or cardboard.

If there were no "stresses" between the stator and the rotor, then you could build a motor with a cardboard housing and expect it to run. In reality, the forces would pop your cardboard apart in short order.

Even model DC motors sold in kits have to make sure they can handle the forces between the stator and the rotor.


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