DC Motor Working Principles: Are Both Explanations of the Rotor Valid?

[UMath1]

TL;DR Summary

Should the rotor be considered as a magnet that experiences a torque based on the magnetic field it is surrounded by or as a circular wire which experience force = I x B which then produces a torque?

I've seen two different explanations on the working principle of DC motors:

One considers the rotor as a loop of wire that experiences a torque because current flow is perpendicular to an external magnetic field. The other describes the loop as an electromagnet which then experiences a torque due to the external magnet field.

Are both valid? And if so is the total force experienced by the wire the sum of the two or just one? It seems like double counting if both are counted.

 

[essenmein]

Both are valid and are ways of solving for the same thing, net magnetic torque generated. You don't add them. Current in a wire within a uniform magnetic field experiences a force because the current also generates a magnetic field which then interacts with the existing field to produce a force.

 

[sophiecentaur]

One considers the rotor as a loop of wire that experiences a torque because current flow is perpendicular to an external magnetic field. The other describes the loop as an electromagnet which then experiences a torque due to the external magnet field.

You can go back further than that and consider two parallel current carrying wires. Either wire can be considered as producing a field and there will then be an I X B force on the other.

You could also consider a brushless motor which uses a permanent magnet rotor and a stator coil through which the current direction is switched at an appropriate time to keep the torque in a constant direction - thus doing without a messy commutator and brushes.

Afaik, there are fewer and fewer electric motors of the kind that you have been using as a model, although it has to be said that a motor with electromagnets in both stator and rotor has advantages in that it will work with AC too without needing rectification.