A
DC motor is an electric motor that runs on direct current (DC) electricity
Theory of
Dc Motor
The speed of a DC motor is
directly proportional to the supply voltage, so if we reduce the supply voltage
from 12 Volts to 6 Volts, the motor will run at half the speed. How can this be
achieved when the battery is fixed at 12 Volts? The speed controller works by
varying the average voltage sent to the motor. It could do this by simply
adjusting the voltage sent to the motor, but this is quite inefficient to do. A
better way is to switch the motor's supply on and off very quickly. If the
switching is fast enough, the motor doesn't notice it, it only notices the
average effect.
Principles of operation
In any electric motor, operation is based on simple
electromagnetism. A current-carrying conductor generates a magnetic field; when
this is then placed in an external magnetic field, it will experience a force
proportional to the current in the conductor, and to the strength of the
external magnetic field. As you are well aware of from playing with magnets as
a kid, opposite (North and South) polarities attract, while like polarities
(North and North, South and South) repel. The internal configuration of a DC
motor is designed to harness the magnetic interaction between a
current-carrying conductor and an external magnetic field to generate
rotational motion.
Let's start by looking at a simple 2-pole DC
electric motor (here red represents a magnet or winding with a
"North" polarization, while green represents a magnet or winding with
a "South" polarization).
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Fig Electric Motor
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Every DC motor has six basic parts -- axle, rotor
(a.k.a., armature), stator, commutated, field magnet(s), and brushes. In most
common DC motors (and all that Beamers will see), the external magnetic field
is produced by high-strength permanent magnets. The stator is the stationary
part of the motor -- this includes the motor casing, as well as two or more
permanent magnet pole pieces. The rotor (together with the axle and attached commutate)
rotates with respect to the stator. The rotor consists of windings (generally
on a core), the windings being electrically connected to the commutated. The
above diagram shows a common motor layout -- with the rotor inside the stator
(field) magnets.
The geometry of the brushes, commutator contacts,
and rotor windings are such that when power is applied, the polarities of the
energized winding and the stator magnet(s) are misaligned, and the rotor will
rotate until it is almost aligned with the stator's field magnets. As the rotor
reaches alignment, the brushes move to the next commutator contacts, and
energize the next winding. Given our example two-pole motor, the rotation
reverses the direction of current through the rotor winding, leading to a
"flip" of the rotor's magnetic field, driving it to continue rotating.
Since most
small DC motors are of a three-pole design, let's tinker with the workings of
one via an interactive animation.
In small motors, an alternative design is often used
which features a 'coreless' armature winding. This design depends upon the coil
wire itself for structural integrity. As a result, the armature is hollow, and
the permanent magnet can be mounted inside the rotor coil. Coreless DC motors
have much lower armature inductance than iron-core motors of comparable size,
extending brush and commutator life.
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Fig DC Motor
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