INDUCTION MOTORS
Group No: 18
Group Members:
Muhammad Arsalan Khan
CS-040
Muhammad Hummad
CS-055
Syed Moiz Ali
CS-060
INTRODUCTION:
An induction motor is an AC circuit in which all
electromagnetic energy is transferred by inductive coupling from a
primary winding to a secondary winding, the two windings being
separated by an air gap.
The induction motors derives its name from the AC voltages
are induced in the rotor circuit by the rotating magnetic field of the
stator. In many ways induction in this motor is similar to the
induction between primary and secondary winding of the
transformer. Large motors and permanently mounted motors that
drive loads at fairly constant speed are often induction motors.
Its characteristic features areo Simple and rugged construction
o Low cost and minimum maintenance
o High reliability and sufficiently high efficiency
o Needs no extra starting motor and need not be synchronized
An induction motor has basically two parts.
1. Stator
2. Rotor
Stator
Rotor
The stator and the rotor are electrical circuits that
perform as electromagnets.
STATOR
CONSTRUCTION:
The stator is the stationary electrical part of the motor.
The stator is laminated iron core. Coils are placed in the slots
to form a three or
WINDINGS:
single
phase
The stator windings are connected windings.
directly to the power source. 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.
ROTOR
CONSTRUCTION:
The rotor is the rotating part of the electromagnetic
circuit. The most common type of rotor is the "squirrel cage"
rotor. The rotor consists of a stack of steel laminations with
evenly spaced conductor bars around the circumference.
Squirrel Cage Rotor:
Rotor is from laminated iron core with slots. Aluminum bars are
molded in the slots instead of a winding. One or 2 are attached to the
shaft in the sides of rotor to cool the circuit.
Wound Rotor:
An alternate design called the wound rotor is used when
variable speed is required. In this case the rotor has the
same no. of poles as the stator and the windings are made up
of
wire.
Compared to squirrel cage rotors, wound motors are
expensive and require maintenance of the slip rings and
brushes.
Induction Motor Construction:
Basic Principles:
An AC current is applied in the
stator armature which generates a
flux in the stator magnetic circuit.
This flux induces an emf in the
conducting bars of rotor as they
are “cut” by the flux while the
magnet is being moved.
A current flow in the rotor circuit
due to the induced emf, which in
term
produces
a
force.
Principles of Operation
The basic idea behind the operation of an induction machine is
quite simple. Detailed mathematical understanding of the interaction of
magnetic fields and resultant torque is more complex. In many cases,
understanding the qualitative ideas and then applying a circuit model is
sufficient. The qualitative description is provided here, together with a
more mathematical description for those who prefer that type of
approach. The mathematical description also includes some important
definitions that are required in order to develop a circuit mode.
Qualitative Description
The three-phase stator winding is connected to a three-phase
supply. Currents flow in the stator winding, producing a rotating mmf
and flux density
The stator flux density rotates at synchronous speed:
The magnetic field passes conductors on the rotor and induces a voltage
in those conductors. Since the conductors are short circuited, current
flows in the rotor conductors. The rotor currents produce a second
rotor magnetic field, which acts to oppose the stator magnetic field and
also rotates at synchronous speed.
With two magnetic fields rotating at constant speed, a torque is
induced:
The rotor flux density will lag the stator flux density (flux density
lags current by 90° electrically), therefore the torque will be in the same
direction as the rotation of the magnetic fields.
The torque accelerates the rotor until synchronous speed is
reached, at which time there is no relative motion between the
conductors and the stator flux density. Since the relative velocity is zero,
the induced voltage, rotor currents and flux density fall to zero and
torque is also zero
Types of Induction Motors:
There are basically two types of induction motors.
1. Single Phase Induction Motors.
2. Three Phase Induction Motors.
Single Phase Induction Motors:
The single phase induction motors are frequently used in
the whole world. They are highly reliable and economical.
These types of machines are used in our daily purposes
like in washing machines, fans, mixers etc.
Three Phase Induction Motors:
The three phase induction motors are the most widely
used electric motors in industry. They run at constant speed
from no load to full load. These motors are simple, low
priced, easy to maintain and can be manufactured with
characteristics to suit most industrial requirements.
Housing
Motor
Uses of Induction Motors:
Squirrel cage induction motor with relatively low
resistance is used for fans, Centrifugal pumps.
Squirrel cage induction motor with relatively high
resistance is used in elevators, in thermal power plants and
others.
Sources:
http://en.wikipedia.org/wiki/Induction_motor
http://www.ece.ualberta.ca/~knight/electrical_machines/induction/basics/principles.html
http://www.engineeringtoolbox.com/electrical-motor-slip-d_652.html