# Electromagnetic force in electric motor

1. Jan 21, 2007

### batsan

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?

Last edited: Jan 22, 2007
2. Jan 21, 2007

### Meir Achuz

There is B in the groove.

3. Jan 22, 2007

### batsan

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.

4. Jan 22, 2007

### Meir Achuz

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.

5. Jan 23, 2007

### batsan

6. Jan 24, 2007

### Meir Achuz

The picture is colorful, but I don't think it is relevant to your question or my answer.

7. Jan 25, 2007

### batsan

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.

8. Jan 27, 2007

### Meir Achuz

The picture is colorful, but I don't think it is relevant to your question or my answer, since B is not zero in the groove however they color it.