INTRODUCTION
Induction motor is a generalized transformer. Difference is that transformer is an
alternating flux machine while induction motor is rotating flux machine. Rotating flux is
only possible when 3 phase voltage (or poly phase) which is 120 degree apart in time is
applied to a three phase winding (or poly phase winding) 120 degree apart in space
then a three phase rotating magnetic flux is produced whose magnitude is constant but
direction keeps changing. In transformer the flux produced is time alternating and not
rotating.
The induction motor is a three phase AC motor and is the most widely used
machine. It has simple and rugged construction, low cost and minimum maintenance,
high reliability and sufficiently high efficiency and needs no extra starting motor and
need not be synchronized.
PRINCIPLE OF OPERATION
The motor which works on the principle of electromagnetic induction is known as
the induction motor. The electromagnetic induction is the phenomenon in which the
electromotive force induces across the electrical conductor when it is placed in a
rotating magnetic field.
The stator and rotor are two essential parts of the motor. The stator is the
stationary part, and it carries the overlapping windings while the rotor carries the main
or field winding. The windings of the stator are equally displaced from each other by an
angle of 120°.
The induction motor is the single excited motor, i.e., the supply is applied only to
the one part, i.e., stator. The term excitation means the process of inducing the
magnetic field on the parts of the motor.
When the three phase supply is given to the stator, the rotating magnetic field
produced on it. The figure below shows the rotating magnetic field set up in the stator.
CONSTRUCTION AND PARTS
An induction motor has 2 main parts; the Stator and the Rotor. The Stator is the
stationary part and the rotor is the rotating part. The Rotor sits inside the Stator. There
will be a small gap between rotor and stator, known as air gap. The value of the radial
air-gap may vary from 0.5 to 2 mm.
STATOR
A row of small stationary airfoils
attached to the casing of an axial-flow
turbine, positioned between the rotors.
ROTOR
RotorV as the name suggests it is a
rotating part of an electrical machine, in
which current is induced by transformer
action from rotating magnetic field.
POWER FACTOR
The power factor of induction motors varies with load, typically from around 0.85
or 0.90 at full load to as low as about 0.20 at no-load due to stator and rotor leakage
and magnetizing reactances. Power factor can be improved by connecting capacitors
either on an individual motor basis or, by preference, on a common bus covering
several motors. For economic and other considerations, power systems are rarely
power factor corrected to unity power factor. Power capacitor application with harmonic
currents requires power system analysis to avoid harmonic resonance between
capacitors and transformer and circuit reactances. Common bus power factor correction
is recommended to minimize resonant risk and to simplify power system analysis.
EFFICIENCY
Full load motor efficiency varies from about 85% to 97%, related motor losses
being broken down roughly as follows:
Friction and windage, 5–15%
Iron or core losses, 15–25%
Stator losses, 25–40%
Rotor losses, 15–25%
Stray load losses, 10–20%.
Various regulatory authorities in many countries have introduced and implemented
legislation to encourage the manufacture and use of higher efficiency electric motors.
There is existing and forthcoming legislation regarding the future mandatory use of
premium-efficiency induction-type motors in defined equipment.
APPLICATION
For industrial and mining applications, 3-phase AC induction motors are the
prime movers for the vast majority of machines. These motors can be operated either
directly from the mains or from adjustable variable frequency drives. In modern
industrialized countries, more than half the total electrical energy used in those
countries is converted to mechanical energy through AC induction motors. The
applications for these motors cover almost every stage of manufacturing and
processing.
Applications also extend to commercial buildings and the domestic environment.
They are used to drive pumps, fans, compressors, mixers, agitators, mills, conveyors,
crushes, machine tools, cranes, etc.
REFERENCES
Electric Motors, 2nd Edition by 3G E-Learning
https://en.wikipedia.org/
https://circuitglobe.com
