Motor theory

Electromagnetic field around conductors

When current flows through a conductor a magnetic field develops around it. The direction of the magnetic field depends on the direction of the current flowing through the conductor. The blue and green dotted lines in the diagram below show the direction of magnetic flux around a conductor

The diagram below show the magnetic fields developed around the conductors as though you were looking into the conductor as the current flows away from you:

The diagram below show the magnetic fields developed around the conductors as though you were looking into the conductor as the current flows towards you:

Force on a conductor in a magnetic field

If a conductor carrying a current is placed within a magnetic field then a force will act upon it, this is the principle of a DC motor. The diagram below shows this:

When no current flows through the conductor the magnetic field of the magnets has no effect on it.

When current flow through the conductor in this direction (away from you) the conductor is forced downwards.

When current flow through the conductor in this direction (towards you) the conductor is forced upwards.

Calculating the force

The force acting on the conductor (if at right angles) can be calculated with the formula below. Where

F = Force (in Newtons)        B = Flux density (in Tesla) - how strong the magnet is        l = Length of the conductor (in Metres)        I = Current flowing through the conductor (in Amps).


F = B I l

Permanent magnet DC motor

If the conductor is held on an axle to that the the conductor is allowed to rotate rather than be forced up and down, then rotational movement can be achieved. The diagram below shows the force acting on the conductor and how it is caused to rotate. The commutator supplies current to the conductor via carbon brushes.

Stator

showing the permanent magnets.

Rotor

The conductor is a length of wire wound around a laminated iron core. The wire is called windings.

Brushes

The brushes transfer current to the windings of the motor via the commutator.