5
FIGURE 7. COMMUTATION PRINCIPLE
ARMATURE
CURRENT IN
OPPOSITE
DIRECTION
NEUTRAL POINT
BRUSHES
BRIDGE
GAP
INITIAL
ARMATURE
CURRENT
6926
COMMUTATION PRINCIPLE
Commutation is the reversal of current in the armature
conductor at the proper time. In the left–hand side of
FIGURE 7., the armature current flows first through the
darkened portion of the conductor and then returns to
the negative brush through the lighter portion. The
brushes will then “short circuit” the conductor when it
has rotated to the neutral point half way between the
poles. Current flow stops at the neutral point because
each brush bridges the adjacent commutator bars for an
instant. Then when the commutator bars rotate enough
to reverse connections with respect to the brushes the
current will flow in the opposite direction in the
conductor. The direction of current flow in the
conductor changes direction every 180
°
at the neutral
point, that is commutation.
NOTE: Some 78–80V motors require interpole
windings which reduce arcing at the neutral point. The
interpole windings provide this feature by reducing the
magnetic field across the armature conductor at the
neutral point.
DIRECTION OF MOTOR ROTATION
The direction of armature rotation depends upon the
direction of the field and the direction of current flow in
the armature. If either the direction of the field or the
direction of current flow through the armature is
reversed, the rotation of the motor will reverse.
However
if both of the above two factors are reversed at
the same time, the motor will continue rotating in the
same direction. The traction motor direction of rotation
is changed by reversing the field with the F/R
contactors. (See FIGURE 7.). The hoist and steering
motors have a fixed direction of rotation.
MOTOR SPEED
The speed of the motor depends on the voltage applied
and the load. In a lift truck the voltage to the traction
motor is controlled by either a resistor or SCR
controlled circuit. The pump and power steering motors
have full battery voltage applied and change speed with
a change in the load.
