Re: The recoil of a dc motor at start-up

On Aug 25, 4:30 pm, nos...@xxxxxxxxxxxxxxxx (J. J. Lodder) wrote:
Tareq <tareq....@xxxxxxxxx> wrote:
On Aug 22, 1:52 pm, nos...@xxxxxxxxxxxxxxxx (J. J. Lodder) wrote:
Tareq <tareq....@xxxxxxxxx> wrote:
On Aug 19, 10:38 pm, nos...@xxxxxxxxxxxxxxxx (J. J. Lodder) wrote:
Tareq <tareq....@xxxxxxxxx> wrote:
When current is switched on in a DC motor ( consider a simple one of
the type existing in children toys) the rotor starts to rotate and we
can notice that the stator sarts to rotate momentarily in the reverse
direction. How can we interpret this recoil in terms of torques and
forces acting on the magnetic dipoles inside the magnetic material of
the stator ?

The currents in the rotor produce an inhomogeneous magnetic field.
A magnetic dipole (such as an iron atom in the stator)
will experience a net force in an inhomogeneous magnetic field.

It is possible (but quite hard by comparison)
to show that the net result of all those forces
is what you knew already,
namely angular momentum conservation.

Best,

Jan

Thanks Jan. But as in the Stern-Gerlach experiment, atoms with spin up
will feel a force in one direction and atoms with spin down will feel
force in the other direction. The total force hence felt by the atoms
is zero !

Only when there are equal numbers of up and down spins.
In a ferromagnet this will not be the case,
else it wouldn't be a magnet,

Jan- Hide quoted text -

- Show quoted text -

The numbers of up and down spins here differ in the direction of the
magnetic field of the stator. The magnetic field of the rotor which
acts by the force on the stator is in a direction orthogonal to
magnetic field of the stator. Are the numbers of up and down spins
different along any arbitrary direction ??

Yes, excepting only the plane perpendicular to the magnetisation,

Jan

But that's not the reason why the DC motor works. In a DC motor the
stator and rotor fields are parallel each other, at least at one point
of the turn (if the stator is a dipole). Actually it would work even
if all the moments in the stator pointed in the same direction, the
component of the rotor field in that direction and its inertia make
the rest.

guillermo

.

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