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I have watched several videos about this topic. According to these videos, the torque of DC motors relies on the Lorentz force (a force that acts on moving charged particles in a magnetic field). However, the working principle of BLDC (or stepper) motors is based on the force between magnetic poles.

My question is: are these two type of forces different, or is it the same force which can be calculated with two methods? I guess, there are magnetic-electric and magnetic-magnetic forces in each motor, but why do they mention only one force per motor? Maybe they mention the dominant force?

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  • \$\begingroup\$ By 'magnetic-magnetic forces' do you mean cogging torque? \$\endgroup\$ – Bruce Abbott Sep 27 '19 at 22:15
  • \$\begingroup\$ The torque resulting from the magnetic poles is usually called "switched reluctance torque". In the design of motors for electric vehicles it often contributes about 30% of the torque. As @BruceAbbott mentions it causes cogging so it is undesirable if it provides too much torque as the 'cogging' causes variations in the torque and can make the motor noisy. It has the advantage however that exploiting it can reduce the amount of permanent magnet material required. \$\endgroup\$ – Kevin White Sep 28 '19 at 1:08
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To answer exactly half of your question: if the motor had no iron core (i.e., if you made a brushless motor with magnets on the rotor and with only coils on the stator), then the Lorentz force interpretation and the "coil as bar magnet" interpretation would just be two different ways of viewing the same problem.

But as soon as you wind those coils on an armature, my understanding breaks down -- I don't know if all of the motor torque comes from the coils pressing on the armature, or just some (I suspect that it is not none at all).

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In all electromagnetic energy conversion devices the forces are magnetic. The primary magnetic field is produced by the flow of current in the part of the device called the armature. In most motors, there is a second magnet that can be either a permanent magnet or a second structure with a magnetic field that is produced by the flow of current. The forces involved are derived from the Lorentz force law. That derivation can be found in a physics text somewhere, but most electrical engineering discussions of electromagnetic energy conversion start with the relationship between current and magnetic flux.

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    \$\begingroup\$ "...flow of current in the part of the device called the armature" or stator in the case of a BLDC motor (and the magnets are are on the rotor). \$\endgroup\$ – Bruce Abbott Sep 28 '19 at 1:33

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