A commutator is a common feature of direct current rotating
machines. By reversing the current direction in the moving coil of a motor's
armature, a steady rotating force (torque) is produced. The current in the winding causes the
fixed magnetic field to exert a rotational force (a torque) on the winding,
making it turn. As the rotor's field comes close to aligning itself with that
of the stator, the commutator switches the rotor's polarity, so the motor is
perpetually trying to settle.
The armature is connected to the commutator which rides along
the brushes which are connected to a DC power source. The current from the DC
power source flows from the positive lead, through the brush labelled A1
through one commutator section, through the armature coil, through the other
commutator section, through the brush labelled A2 and back to the negative lead. This current
will generate lines of flux around the armature and affect the lines of flux in
the air gap. On the side of the coil where the lines of flux oppose each other,
the magnetic field will be made weaker. On the side of the coil where the lines
of flux are not opposing each other, the magnetic field is made stronger.
Because of the strong field on one side of the coil and the weak field on the
other side, the coil will be pushed into the weaker field and, because the
armature coil is free to rotate, it will rotate.
In
practice, DC motors will
always have more than two poles. In particular, this avoids "dead
spots" in the commutator. In case of two pole rotor, when the rotor will
exactly at the middle of its rotation (perfectly aligned with the field
magnets), it will get "stuck" there. Meanwhile, with a two-pole
motor, there is a moment where the commutator shorts out the power supply
(i.e., both brushes touch both commutator contacts simultaneously). This would
be bad for the power supply, waste energy, and damage motor components as well.
Another disadvantage of such a motor is that it would exhibit a high amount of torque "ripple" (the amount of torque it could produce is cyclic with the position of the rotor).
In this type of motor ,one pole is fully energized at a time (but two others
are "partially" energized). As each brush transitions from one
commutator contact to the next, one coil's field will rapidly collapse, as the
next coil's field will rapidly charge up.