Brushed DC motors are widely used in applications
ranging from toys to push-button adjustable car seats.
Brushed DC (BDC) motors are inexpensive, easy to
drive, and are readily available in all sizes and shapes.
This application note will discuss how a BDC motor
works, how to drive a BDC motor, and how a drive
circuit can be interfaced to a PIC microcontroller.
PRINCIPLES OF OPERATION
The construction of a simple BDC motor is shown in
Figure 1. All BDC motors are made of the same basic
components: a stator, rotor, brushes and a commutator.
The following paragraphs will explain each component
in greater detail.
The stator generates a stationary magnetic field that
surrounds the rotor. This field is generated by either
permanent magnets or electromagnetic windings. The
different types of BDC motors are distinguished by the
construction of the stator or the way the electromagnetic
windings are connected to the power source.
The rotor, also called the armature, is made up of one
or more windings. When these windings are energized
they produce a magnetic field. The magnetic poles of
this rotor field will be attracted to the opposite poles
generated by the stator, causing the rotor to turn. As the
motor turns, the windings are constantly being
energized in a different sequence so that the magnetic
poles generated by the rotor do not overrun the poles
generated in the stator. This switching of the field in the
rotor windings is called commutation.
Brushes and Commutator
Unlike other electric motor types (i.e., brushless DC,
AC induction), BDC motors do not require a controller
to switch current in the motor windings. Instead, the
commutation of the windings of a BDC motor is done
mechanically. A segmented copper sleeve, called a
commutator, resides on the axle of a BDC motor. As the
motor turns, carbon brushes slide over the commutator,
coming in contact with different segments of the
commutator. The segments are attached to different
rotor windings, therefore, a dynamic magnetic field is
generated inside the motor when a voltage is applied
across the brushes of the motor. It is important to note
that the brushes and commutator are the parts of a
BDC motor that are most prone to wear because they
are sliding past each other.
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