DC MOTOR THEORY
DC Motors
The right-hand rule for motors shows the direction in which a current-carrying conductor moves
in a magnetic field. When the forefinger is pointed in the direction of the magnetic field lines,
and the center finger is pointed in the direction of current flow, the thumb will point in the
direction of force (motion).
Torque
Torque is defined as that force which tends to produce and maintain rotation. The function of
torque in a DC motor is to provide the mechanical output or drive the piece of equipment that
the DC motor is attached to.
When a voltage is applied to a
Figure 5 Armature Current in a Basic DC Motor
motor, current will flow through
the field winding, establishing a
magnetic field. Current will also
flow
through
the
armature
winding, from the negative brush
to the positive brush as shown in
Figure 5.
Since the armature is a current-
carrying conductor in a magnetic
field, the conductor has a force
exerted on it, tending to move it at
right angles to that field. Using
the left-hand rule for current-
carrying conductors, you will see
that the magnetic field on one side
is strengthened at the bottom,
while it is weakened on the other
side. Using the right-hand rule for
motors, we can see that there is a force exerted on the armature which tends to turn the armature
in the counter-clockwise direction. The sum of the forces, in pounds, multiplied by the radius
of the armature, in feet, is equal to the torque developed by the motor in pound-feet (1b - ft).
It is evident from Figure 5 that if the armature current were reversed, but the field were the same,
torque would be developed in the opposite direction. Likewise, if the field polarity were reversed
and the armature remained the same, torque would also be developed in the opposite direction.
The force that is developed on a conductor of a motor armature is due to the combined action
of the magnetic fields. The force developed is directly proportional to the strength of the main
field flux and the strength of the field around the armature conductor. As we know, the field
strength around each armature conductor depends on the amount of current flowing through the
armature conductor. Therefore, the torque which is developed by the motor can be determined
using Equation (6-1).
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