Electric Starter Motor

The Electric Starter Motor converts electrical current into rotary motion. In doing so it converts electrical energy into mechanical energy. The interaction of two magnetic fields produce this rotational force.

The field coils (either electromagnetic or permanent) located in the housing produce magnetic flux lines. Within the stationary field coils is the armature, a loop of wire (a conductor) with one end connected to B+, the other to B-. When current is applied to the armature flux lines circle the loop in one direction on one side and in the opposite direction on the other side. The interaction of the flux lines on the armature and the flux lines from the field coil cause the armature to rotate.

The armature will only rotate to the point where the magnetic force is equal on both sides. (Armature 90° to magnetic flux lines of field) For the armature to continue to rotate, the polarity or direction of current flow must be reversed.

Through the brushes and the commutator, the current flow is reversed as the magnetic forces become equal, causing the armature to continue to rotate.

This constant reversal of current flow in the armature provides continual rotation.

Direction of rotation is determined using Flemings Left Hand Rule. 

• Point your First finger in the direction of the magnetic Field (from N to S).
• Rotate your hand about that finger until your second finger points in the direction of the Current (conventional current, from + to -).
  Fig 2: Identifying Magnetic Field

  Courtesy of BMW OF NORTH AMERICA, INC.

  Loop symbolizes force out of magnetic field. 

• Then your thumb points in the direction of the Movement of the wire.

To increase the force on the wire (armature) do one of the following:

• Use a larger current.
• Use a stronger magnetic field.
• Use a greater length of wire in the field.

To increase torque and speed in the starter motor, more windings in the armature are added, and the field has more pairs of magnets (either permanent or electromagnetic).

Flemings left hand rule: 

The basic law of motors, the direction of force on a wire that is carrying current when it is in a magnetic field.