# Electromagnetic force in electric motor

## Main Question or Discussion Point

Suppose we have a DC motor with permanent magnet. Moreover, the armature of motor is groove rotor. The elementary electromagnetic force is given by: dF=jxBdv, where j is electric current density , x is vector multiplication , B - magnetic flux density , dv - elementary volume. If we integrate by all the volume, we will calculate only zero, because there is current density j in the groove , but there is no magnetic field B. Otherwise ,there is B in the steel , but no j.
How you can explain that?

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Meir Achuz
Homework Helper
Gold Member
There is B in the groove.

Are you sure?
The magnetic resistance of the groove materials, such as copper, insulation ,air etc is many times more then steel. The magnetic flux pass around the groove, but not inside it.

Meir Achuz
Homework Helper
Gold Member
Are you sure?
The magnetic resistance of the groove materials, such as copper, insulation ,air etc is many times more then steel. The magnetic flux pass around the groove, but not inside it.
The hig magnetic reluctance (not resistance) of those materials is what permits the B field to enter the groove. That is why steel is not used for the armature.

Ok. Is this correct picture or not?
http://www.mfly.de/animation_b.gif [Broken]

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Meir Achuz
Homework Helper
Gold Member
The picture is colorful, but I don't think it is relevant to your question or my answer.

Yes,this is AC motor, but formula for its force must be the same. As we see color of groove is blue and it corresponds to B=0.

Meir Achuz