INTRODUCTION:
An induction motor is an AC circuit in which all
electromagneticenergy is transferred byinductive couplingfrom 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 are
oSimple and rugged construction
oLow cost and minimum maintenance
oHigh reliability and sufficiently high efficiency
oNeeds no extra starting motor and need not be synchronized
An inductionmotor 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 single phase windings.
WINDINGS:
The stator windings are connected
directly to the power source. Stator 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.
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.
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.
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 OperationThe 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 densityThe 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
dailypurposes like in washing machines, fans, mixers etc.
Three Phase Induction Motors:
The three phase induction motors are the most widely used electricmotors 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.