Physics - The DC motor and the split-ring commutator
The DC motor and the split-ring commutator
For simplicity, consider a single loop of wire placed in a magnetic field
as shown. In this diagram the magnetic field is provided by permanent
magnets, but this field could also be provided by electromagnets. Refer to
diagram (a). When current passes through the wire, the side of the loop AB
will be forced to move upward. The side of the loop CD will be forced to
move downward. Verify this using the right-hand slap rule. A torque  or
turning force has been applied to the loop and the loop begins to spin.
View a video of loop AB forced upward.
When the loop is in the position shown in diagram (b), the force on each
side has the same direction and magnitude as previously and it is of the
same magnitude, yet due to the physical position of the loop no torque is
produced. The torque, or turning effect, is determined by both the size of
the applied force and its orientation relative to the axis of rotation. The
vertical position of the loop means that the magnetic forces are of no
assistance in producing a torque at this moment. However, unless friction
was very great, the loop would continue to spin past position b.
As the side AB moves to the right-hand side of the diagram, a downward
force is now required to keep the loop spinning in the original clockwise
direction. The split ring commutator  allows the direction of the
current in the loop to be reversed and hence the direction of the force on
AB becomes downward. Although point A was originally connected to the
negative terminal of the supply, once the loop has turned sufficiently,
point A is now connected to the positive terminal of the supply. This
allows the current in the loop to change direction.
Note that the force on the side length of the loop is determined by the
magnetic field strength, the wire length and the current flowing. Since
none of these factors alter during the motor's operation, the force on any
side length is constant. However, the orientation of the loop determines
the amount of torque or turning effect this force is able to produce. You
should know the positions of optimal turning effect (positions a and c),
and which loop positions result in zero turning effect (zero torque).
Our model of the DC motor is over-simplified. Real motors have many sets
of loops placed at intervals around a cylinder so that there is always one
set of loops experiencing optimal torque. Alternatively, motors may have
stationary loops of wire and permanent magnets or electromagnets that spin
relative to the loops.
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