Construction DC motors are made up of several major components which
include the following:
• Frame
• Shaft
• Bearings
• Main Field Windings (Stator)
• Armature (Rotor)
• Commutator
• Brush Assembly
Of these components, it is important to understand the
electrical characteristics of the main field windings, known as
the stator, and the rotating windings, known as the armature.
An understanding of these two components will help with the
understanding of various functions of a DC Drive.
Basic Construction The relationship of the electrical components of a DC motor is
shown in the following illustration. Field windings are mounted
on pole pieces to form electromagnets. In smaller DC motors
the field may be a permanent magnet. However, in larger DC
fields the field is typically an electromagnet. Field windings and
pole pieces are bolted to the frame. The armature is inserted
between the field windings. The armature is supported by
bearings and end brackets (not shown). Carbon brushes are
held against the commutator.
Armature The armature rotates between the poles of the field windings.
The armature is made up of a shaft, core, armature windings,
and a commutator. The armature windings are usually form
wound and then placed in slots in the core.
Brushes Brushes ride on the side of the commutator to provide supply
voltage to the motor. The DC motor is mechanically complex
which can cause problems for them in certain adverse
environments. Dirt on the commutator, for example, can inhibit
supply voltage from reaching the armature. A certain amount of
care is required when using DC motors in certain industrial
applications. Corrosives can damage the commutator. In
addition, the action of the carbon brush against the
commutator causes sparks which may be problematic in
hazardous environments.
Basic DC Motor Operation
Magnetic Fields You will recall from the previous section that there are two
electrical elements of a DC motor, the field windings and the
armature. The armature windings are made up of current
carrying conductors that terminate at a commutator. DC
voltage is applied to the armature windings through carbon
brushes which ride on the commutator.
In small DC motors, permanent magnets can be used for the
stator. However, in large motors used in industrial applications
the stator is an electromagnet. When voltage is applied to
stator windings an electromagnet with north and south poles is
established. The resultant magnetic field is static (nonrotational).
For simplicity of explanation, the stator will be
represented by permanent magnets in the following
illustrations.
Magnetic Fields A DC motor rotates as a result of two magnetic fields
interacting with each other. The first field is the main field that
exists in the stator windings. The second field exists in the
armature. Whenever current flows through a conductor a
magnetic field is generated around the conductor.
Right-Hand Rule for Motors A relationship, known as the right-hand rule for motors, exists
between the main field, the field around a conductor, and the
direction the conductor tends to move.
If the thumb, index finger, and third finger are held at right
angles to each other and placed as shown in the following
illustration so that the index finger points in the direction of the
main field flux and the third finger points in the direction of
electron flow in the conductor, the thumb will indicate direction
of conductor motion. As can be seen from the following
illustration, conductors on the left side tend to be pushed up.
Conductors on the right side tend to be pushed down. This
results in a motor that is rotating in a clockwise direction. You
will see later that the amount of force acting on the conductor
to produce rotation is directly proportional to the field strength
and the amount of current flowing in the conductor.
Whenever a conductor cuts through lines of flux a voltage is
induced in the conductor. In a DC motor the armature
conductors cut through the lines of flux of the main field. The
voltage induced into the armature conductors is always in
opposition to the applied DC voltage. Since the voltage induced
into the conductor is in opposition to the applied voltage it is
known as CEMF (counter electromotive force). CEMF reduces
the applied armature voltage.
The amount of induced CEMF depends on many factors such
as the number of turns in the coils, flux density, and the speed
which the flux lines are cut.
Armature Field An armature, as we have learned, is made up of many coils and
conductors. The magnetic fields of these conductors combine
to form a resultant armature field with a north and south pole.
The north pole of the armature is attracted to the south pole of
the main field. The south pole of the armature is attracted to
the north pole of the main field. This attraction exerts a
continuous torque on the armature. Even though the armature
is continuously moving, the resultant field appears to be fixed.
This is due to commutation, which will be discussed next.
Commutation In the following illustration of a DC motor only one armature
conductor is shown. Half of the conductor has been shaded
black, the other half white. The conductor is connected to two
segments of the commutator.
In position 1 the black half of the conductor is in contact with
the negative side of the DC applied voltage. Current flows away
from the commutator on the black half of the conductor and
returns to the positive side, flowing towards the commutator
on the white half.
In position 2 the conductor has rotated 90°. At this position the
conductor is lined up with the main field. This conductor is no
longer cutting main field magnetic lines of flux; therefore, no
voltage is being induced into the conductor. Only applied
voltage is present. The conductor coil is short-circuited by the
brush spanning the two adjacent commutator segments. This
allows current to reverse as the black commutator segment
makes contact with the positive side of the applied DC voltage
and the white commutator segment makes contact with the
negative side of the applied DC voltage.
