AC Motor Construction
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.
Enclosure
Stator
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 Lamination
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Stator Windings Stator laminations are stacked together forming a hollow
cylinder. Coils of insulated wire are inserted into slots of the
stator core.
Stator Windings Partially Completed
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.
Stator Windings Completed
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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.
Rotor
A squirrel cage rotor core is made by stacking thin steel
laminations to form a cylinder.
Rotor Lamination
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.
Cutaway View of Rotor
Conductor Bar End Ring
Shaft
Steel Laminations
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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.
Frame
Rotor
Stator
Air Gap
Partially Assembled Motor
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.
