Three-phase
AC induction motors are commonly used in industrial applications. This type of
motor has three main parts, rotor, stator, and enclosure. The stator and rotor
do the work, and the enclosure protects the stator and rotor.
Stator
Core
The
stator is the stationary part of the motor’s electromagnetic circuit. The
stator core is made up of many thin metal sheets, called laminations.
Laminations are used to reduce energy loses that would result if a solid core
were used.
Stator
Windings Stator laminations are stacked together forming a hollow cylinder.
Coils of insulated wire are inserted into slots of the stator core.
When
the assembled motor is in operation, the stator windings are connected directly
to the power source. Each grouping of coils, together with the steel core it
surrounds, becomes an electromagnet when current is applied. Electromagnetism
is the basic principle behind motor operation.
Rotor
Construction
The
rotor is the rotating part of the motor’s electromagnetic circuit. The most
common type of rotor used in a three-phase induction motor is a squirrel cage
rotor. Other types of rotor construction is discussed later in the course. The
squirrel cage rotor is so called because its construction is reminiscent of the
rotating exercise wheels found in some pet cages.
A
squirrel cage rotor core is made by stacking thin steel laminations to form a
cylinder.
Rather
than using coils of wire as conductors, conductor bars are die cast into the
slots evenly spaced around the cylinder. Most squirrel cage rotors are made by
die casting aluminum to form the conductor bars. Siemens also makes motors with
die cast copper rotor conductors. These motor exceed NEMA Premium efficiency
standards.
After
die casting, rotor conductor bars are mechanically and electrically connected
with end rings. The rotor is then pressed onto a steel shaft to form a rotor
assembly.
Enclosure
The
enclosure consists of a frame (or yoke) and two end brackets (or bearing
housings). The stator is mounted inside the frame. The rotor fits inside the
stator with a slight air gap separating it from the stator. There is no direct
physical connection between the rotor and the stator.
The
enclosure protects the internal parts of the motor from water and other
environmental elements. The degree of protection depends upon the type of
enclosure. Enclosure types are discussed later in this course.
Bearings,
mounted on the shaft, support the rotor and allow it to turn. Some motors, like
the one shown in the following illustration, use a fan, also mounted on the
rotor shaft, to cool the motor when the shaft is rotating.
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