DC MOTOR THEORY
DC Motors
Theory of Operation
Figure 1 Left-Hand Rule for
Current-Carrying Conductors
Every current-carrying conductor has a magnetic
field around it. The direction of this magnetic
field may be found by using the left-hand rule for
current-carrying conductors. When the thumb
points in the direction of current flow, the fingers
will point in the direction of the magnetic field
produced, as shown in Figure 1.
If a current-carrying conductor is placed in a
magnetic field, the combined fields will be
similar to those shown in Figure 2. The direction
of current flow through the conductor is indicated
with an "x" or a "·".
The "x" indicates the
current flow is away from the reader, or into the
page.
The "·" indicates the current flow is
towards the reader, or out of the page.
Above the conductor on the left, the field caused by the conductor is in the opposite direction
Figure 2 Current-Carrying Conductor in a Magnetic Field
of the main field, and therefore, opposes the main field. Below the conductor on the left, the
field caused by the conductor is in the same direction as the main field, and therefore, aids the
main field. The net result is that above the conductor the main field is weakened, or flux density
is decreased; below the conductor the field is strengthened, or flux density is increased. A force
is developed on the conductor that moves the conductor in the direction of the weakened field
(upward).
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