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INTRODUCTION
Electric lift trucks use Direct Current (DC) motors to
convert electrical energy into mechanical energy. The
DC motor usually will provide long periods of service
with a minimum of maintenance. The motor’s
commutator and brush assembly are the first two areas
that generally require maintenance. This section
provides extra emphasis on these two areas along with
the other normal maintenance items. A brief review of
the basic principles of magnetism is included to provide
an understanding of DC motor operation.
MAGNETISM
MAGNETISM AND PERMANENT
MAGNETS
The space affected by a magnet is called a “field of
force.” The extent of this field is determined by the
strength of the magnet and can be detected by the use of
a compass. It is common practice to show the field by
lines which are called “lines of force.” When a compass
is used as a pointer in exploring a field, it indicates that
the lines have direction (leaving the north pole and
returning to the south pole). The magnetic field of a
permanent magnet can be concentrated by shortening
the air gap between the north and south poles (“U”
shaped or horsehoe magnet). The smaller the air gap
between the poles, the greater the concentration of the
lines.
When two magnets are placed so that the north pole of
one and the south pole of the other are close together,
they attract each other. If the magnets are placed with
their like poles adjacent they will repel each other. These
facts identify a fundamental law of magnetism – Unlike
poles attract each other and like poles repel each
other.
Materials differ in their behavior when placed in a
magnetic field. Most materials have no effect on the
magnitude or direction of the field. These are called
“non–magnetic” materials. Other materials (such as
iron) having the property of magnifying or
concentrating a field are called “magnetic” materials.
Magnetic lines seem to penetrate all substances and are
deflected only by magnetic materials or by another
magnetic field.
ELECTROMAGNETIC FIELDS
A magnetic field exists around a conductor carrying an
electric current. The greater the current flow, the
stronger the magnetic field. This field is at right angles
to the conductor. These lines are concentric circles
around the length of a straight conductor. The field dif-
fers from a permanent magnet because there are no mag-
netic poles at which the lines can enter or leave.
UNLIKE
POLES
ATTRACT
LIKE
POLES
REPEL
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FIGURE 1. MAGNETISM AND PERMANENT
MAGNETS
The direction of the lines around a conductor may be de-
termined by the use of the compass. Also if the direction
