Page 262 - Robot Builder's Bonanza

THE FUNDAMENTALS OF DC MOTORS 231

These wires are wrapped around pieces of
metal. At the end of the rotor is a commuta-
tor, which is used to alternately apply current
to the windings.
Figure 22- 1 shows a simplified diagram of
current passing from a battery, through the
commutator (shown here simplified), and ener-
gizing the rotor, which is in the middle. As the
rotor spins, current is applied to the windings
of the motor in such a way that the rotor is kept
in motion. Only when current ceases to flow
through the windings (or the motor shaft is
physically blocked from turning) does the
motor stop.
Note the two dark gray bars in the motor
diagram shown in Figure 22- 1. These are
brushes, and they serve as terminals to apply
the current from the battery to the commuta-
tor. On very inexpensive hobby motors, the
brushes are often just pieces of copper wire,
bent to a handy shape. On more expensive
Figure 22- 1 Simplified block diagram of a DC motor. motors, brushes are made of conductive car-
Current flows from the battery to brushes, which are bon. Both of these motors are known as
electrically connected to the commutator. Windings brushed motors.
around the rotor are energized, causing the rotor to spin.
Both types of brushes can wear down over
time, which can break the electrical connection
between the battery and the commutator. This is why, when used long enough, a DC motor
will just go kaput.

Brushless motors use electronics, not brushes, to alternate the current between windings.
Brushless motors are used extensively in computer disk drives, “noiseless” fans, CD and DVD
players, and precision electronics. You should know about them, as motors pulled from these
G components and sold as surplus may require additional electronics in order to operate.
You may also encounter brushless motors used in R/C servo motors. These motors tend to be
quite expensive and are reserved for applications where motor failure could be catastrophic,
like suddenly losing control of a $1000 R/C helicopter.

REVERSIBLE DIRECTION
One of the prime benefits of DC motors is that most (but not all) are inherently reversible.
Apply current in one direction— the + and on the battery terminals— and the motor spins
clockwise. Apply current in the other direction, and the motor spins counterclockwise. This
capability makes DC motors well suited for robotics, where it is often desirable to have the
motors reverse direction. Use it to back a robot away from an obstacle or to raise or lower a
mechanical arm.
If you’re buying your DC motors surplus, you may encounter some that are not reversible.
This could be due to the way the motor windings are constructed inside the motor or it could
be due to an intentional mechanical design. Read the description for the motor carefully. It will

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