AC Motor Construction

AC induction motors are commonly used in industrial
applications. The following motor discussion will center around
three-phase, 460 VAC, asynchronous, induction motors. An
asynchronous motor is a type of motor where the speed of the
rotor is less than the speed of the rotating magnetic field. This
type of motor is illustrated below. The three basic parts of an AC
motor are the rotor, stator, and enclosure.

 

Stator Construction

The stator and the rotor are electrical circuits that perform as
electromagnets. The stator is the stationary electrical part of the
motor. The stator core of a NEMA motor is made up of several
hundred thin laminations

Stator Windings

Stator laminations are stacked together forming a hollow
cylinder. Coils of insulated wire are inserted into slots of the
stator core

Each grouping of coils, together with the steel core it surrounds,
form an electromagnet. Electromagnetism is the principle
behind motor operation. The stator windings are connected
directly to the power source

Rotor Construction

The rotor is the rotating part of the electromagnetic circuit.
The most common type of rotor is the “squirrel cage” rotor.
Other types of rotor construction will be mentioned later in the
course. The construction of the squirrel cage rotor is reminiscent
of rotating exercise wheels found in cages of pet rodents.

The rotor consists of a stack of steel laminations with evenly
spaced conductor bars around the circumference.

 

The laminations are stacked together to form a rotor core.
Aluminium is die cast in the slots of the rotor core to form a
series of conductors around the perimeter of the rotor. The
conductor bars are mechanically and electrically connected with
end rings. The rotor core mounts on a steel shaft to form a rotor
assembly

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 also protects the electrical and operating parts of
the motor from harmful effects of the environment in which the
motor operates. Bearings, mounted on the shaft, support the
rotor and allow it to turn. A fan, also mounted on the shaft, is
used on the motor shown below for cooling

 

AC Generators

AC Generators

  

A basic generator consists of a magnetic field, an armature, slip
rings, brushes and a resistive load. The magnetic field is usually
an electromagnet. An armature is any number of conductive
wires wound in loops which rotates through the magnetic field.
For simplicity, one loop is shown. When a conductor is moved
through a magnetic field, a voltage is induced in the conductor.
As the armature rotates through the magnetic field, a voltage
is generated in the armature which causes current to flow. Slip
rings are attached to the armature and rotate with it. Carbon
brushes ride against the slip rings to conduct current from the
armature to a resistive load

An armature rotates through the magnetic field. At an initial
position of zero degrees, the armature conductors are moving
parallel to the magnetic field and not cutting through any
magnetic lines of flux. No voltage is induced.

The armature rotates from zero to 90 degrees. The conductors
cut through more and more lines of flux, building up to a
maximum induced voltage in the positive direction.

The armature continues to rotate from 90 to 180 degrees,
90 cutting less lines of flux. The induced voltage decreases from a
maximum positive value to zero.

The armature continues to rotate from 180 degrees to 270
degrees. The conductors cut more and more lines of flux, but
in the opposite direction. voltage is induced in the negative
direction building up to a maximum at 270 degrees.

The armature continues to rotate from 270 to 360 degrees.
   Induced voltage decreases from a maximum negative value to
zero. This completes one cycle. The armature will continue to
rotate at a constant speed. The cycle will continuously repeat as
long as the armature rotates