due largely to its reliability and simplicity of
construction. The basic induction motor has two main
assemblies or components-a ROTOR and a
STATOR, as shown in figure 6-1. The mechanical
rotation of the rotor is produced through the principle
of electromagnetic induction.
flows through the stator (a circular assembly of
stationary coils or windings) which surrounds the
The alternating current flow in the stator
produces a constantly rotating magnetic field. This
magnetic field induces a current flow in the conductors
of the rotor (a cylindrical or drumlike assembly of
copper bars mounted on a shaft). The induced current
in the rotor then produces a magnetic field of its own.
The magnetic field of the rotor is produced so it
opposes the magnetic field of the stator; that is, the two
fields repel each other. This continuous repulsion of
the rotor field by the stator field results in a continuous
rotation of the rotor assembly around its axis or shaft.
Thus electrical rotation (in the stator) is transformed
into mechanical rotation (in the rotor).
The rotational speed of the stator field remains
constant unless the frequency of the electrical power
source varies. The rotational speed of the rotor is also
constant and is more or less independent of the
workload imposed on it. This is not to say, however,
that an induction motor cannot be overloaded. Under
heavy or excessive loads, the motor tends to draw more
current to maintain speed; this can result in over-
heating and burned-out windings.
The totally enclosed, fan-cooled motor has totally
enclosed windings and rotor. Cooling air is circulated
over the enclosure to remove heat. This motor is used
in locations where the surrounding air may contain a
high proportion of dust, as in a carpenter shop.
Induction motors are usually named by the method
used for starting the motor. Two fairly common types
The explosionproof motor is similar to the totally
of induction motors, classified in this manner, are the
enclosed fan-cooled motor; but, it is constructed to
SPLIT-PHASE MOTOR and the CAPACITOR-
prevent any explosion within the motor from igniting
START MOTOR. Split-phase induction motors are
combustible gases or dust in the surrounding air. This
designed to operate on single-phase current. Induction
motors require two or more out-of-time-phase currents
to produce the continuously rotating magnetic field in
the stator. For this reason, induction motors that must
run on a single-phase power supply are provided with
split-phase windings that make two phases of the
single-phase current. Split-phase motors can be used
to drive a variety of equipment, such as washing
machines, oil burners, small pumps, and blowers. The
capacitor-start induction motor is a variation of the
split-phase motor, but it has a high capacity,
electrolytic capacitor. The primary function of this
device is the storage of electricity to provide
power during the start.
There are a number of mechanical modifications to
induction motors. The most important are as follows:
(1) the splashproof motor; (2) the totally enclosed, fan-
cooled motor; and (3) the explosionproof motor.
The splashproof motor is constructed so dripping
or splashing liquids cannot enter the motor. The motor
is self-ventilated; but, since moisture-saturated air
may be circulated through the motor, the windings are
made moisture-resistant. Motors of this type are most
often used to drive pumps and other machinery where
the moisture content of the air is high.
Figure 6-1.Rotor and stator assemblies of an induction motor.