Chemical Engineering July 2022 - 20

Facts At Your Fingertips
Alternating-Current Induction Motors
Department Editor: Scott Jenkins
lternating-current (a.c.) induction
motors are widely used
throughout the chemical process
industries (CPI) and other manufacturing
sectors to convert electrical
energy into mechanical energy to
power rotating equipment, such as
centrifugal pumps, compressors and
fans, as well as other industrial machinery.
This one-page reference provides
information on the construction
and operation of three-phase induction
motors (Figure 1).
A
Motor components
The basic construction of a typical industrial
a.c. induction motor includes
the following elements: rotor, stator,
stator windings and enclosure.
Rotor. The rotor has an iron core
made from a cylinder-shaped stack of
laminated rings around a motor shaft
to which it is attached. The rotor has
conducting end caps on either end
and conducting bars that run through
slots in the laminated metal stack between
the end caps. The assembly
rotates inside the stator on bearings,
which ensure that the rotor remains
centrally positioned with the stator.
The appearance of the rotor and conducting
bars gives rise to the name
" squirrel-cage motor. "
Stator. The stator is made from a
series of stacked steel sheets that
are ring-shaped and encircle the rotor,
while allowing the rotor to move
freely. The stacked metal slices are
laminated with insulating material
and have spaces cut out around
their diameter to accommodate the
copper windings.
Windings. Windings made from copper
wire are distributed in the slots
on the interior of the stator to carry
the supply electrical current that will
induce a magnetic field that will penetrate
the rotor inside.
Enclosure. The enclosure, consisting
of a frame and end bells, protects
the motor. There are several
types of enclosures for different applications,
as designated by the National
Electrical Manufacturers Association
(NEMA; Rosslyn, Va.; www.
nema.org), a trade association and
20
standards-making body. The
four main NEMA enclosures are
the following: open, drip-proof
(ODP) enclosures; totally enclosed,
non-ventilated (TENV);
totally enclosed, fan-cooled
(TEFC); and totally enclosed,
blower-cooled (TEBC) motors.
Induction motor operation
With correct design of the stator
windings and stator slots, applying
alternating current to the stator
will generate a rotating magnetic
field. When electrical current is
applied, the stator produces a rotating
magnetic field needed to rotate
the motor shaft. Typically, three-phase
a.c. electric power is supplied to the
stator so that the three phases are
electrically separated from each other
by 120 deg.
FIGURE 1. In this cutaway image of the interior of an
electrical induction motor, the rotor can be seen inside the
stator, which surrounds it
Meanwhile, the rotor sits inside the
stator core and its design gives it the
ability to conduct electromagnetic
current. As the rotating magnetic
field moves about the rotor, it induces
voltage in the conducting bars of the
rotor. Because of the conductive end
caps on each end of the rotor, current
can flow though the rotor bars. The
rotor then produces a magnetic field
opposed to that of the stator. The opposing
magnetic fields set up a situation
where the opposite poles of the
rotor and stator attract each other
while the like poles repel. As the stator's
magnetic field rotates, the rotor
chases it, driven by the attractive and
repellent magnetic forces.
Motor terminology
The following terms are useful for understanding
the real-world operation
of an induction motor.
Magnetic poles. The stator can be
designed to have a varying number
of magnetic poles around the stator
body. The number of poles has an
impact on the speed and torque of
the motor. For a smaller number of
poles, the speed of the rotating magnetic
field is faster, but the torque is
lower. Adding magnetic poles lowers
the speed of the magnetic field, but
increases the level of torque that is
possible for the motor to generate.
The manufacturing cost for the motor
is higher for larger numbers of
poles, so most motors are two- or
four-pole motors.
Synchronous speed. This is the
speed of the rotating magnetic field in
the stator. Synchronous speed is calculated
by the equation Na = 120f/P,
where f is the frequency of the a.c.
supplied to the stator and P is the
number of motor poles. For example,
the synchronous speed of a four-pole
motor powered by 60-Hz a.c. current
would be 1,800 rpm.
Rated speed. The rated speed is the
rotational velocity of the rotor inside
the motor housing. The rated speed
is always less than the synchronous
speed because the rotor always rotates
slower than magnetic field of the
stator. In fact, the rotation speed of
the rotor must be lower than the synchronous
speed or else there would
be no induction and the rotor would
not be able to create a magnetic field.
Slip. The slip is the difference between
the speed of the rotating magnetic
field of the stator and the mechanical
speed of the rotating rotor assembly.
The size of the slip depends on the
load that is on the motor. For a larger
load on the motor, the slip will be
greater than for smaller loads.
Torque. The term torque refers to
the rotational force generated by
the rotating motor. Induction motors
draw more current as the load is increased,
and consequently produce
more torque.
n
Editor's note: Some material for this column is from Yaskawa America
Inc., Induction Motor Basics, video e-learning module, accessed at
www.yaskawa.com/support-training/training/elearning-curriculum
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2022
http://www.nema.org http://www.yaskawa.com/support-training/training/elearning-curriculum http://WWW.CHEMENGONLINE.COM
Chemical Engineering July 2022

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