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8/3/2019 3Ph I.M Introduction
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Instructional Objectives
Construction
Types of rotor
Operating PrincipleEffect of slip in rotor
Torque equation- Starting & Running
Torque-Slip Characteristics
By
P.Anitha,Lecturer
EEE Dept
Three-phase induction motor
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Type of Electric Motors
Electrical current reverses direction
Two parts: stator and rotor Stator: stationary electrical component
Rotor: rotates the motor shaft
Speed difficult to control
Two types
Synchronous motor
Induction motor
AC Motors
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AC motors
Alternating current (AC) motors use an
electrical current, which reverses its direction
at regular intervals.
An AC motor has two basic electrical parts: a
"stator" and a "rotor". The stator is in the
stationary electrical component. The rotor is
the rotating electrical component, which inturn rotates the motor shaft.
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The main advantage of DC motors over AC
motors is that speed is more difficult to control
for AC motors. To compensate for this, AC motors can be
equipped with variable frequency drives but
the improved speed control comes together
with a reduced power quality.
Disadvantages of AC motors
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Type of AC Motors
There are two types of AC motors:
synchronous and induction.
The main difference between the synchronous
motor and the induction motor is that the rotor
of the synchronous motor travels at the same
speed as the rotating magnetic field.
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Constant speed fixed by system frequency
DC for excitation and low starting torque:
suited for low load applications
Can improve power factor: suited for high
electricity use systems
Synchronous speed (Ns):
AC MotorsSynchronous motor
Ns = 120 f / P
F = supply frequency
P = number of poles
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AC MotorsSynchronous motor
A synchronous motor is an AC motor, whichruns at constant speed fixed by frequency ofthe system.
It requires direct current (DC) for excitationand has low starting torque, and synchronousmotors are therefore suited for applicationsthat start with a low load, such as aircompressors, frequency changes and motorgenerators.
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AC MotorsSynchronous motor
Synchronous motors are able to improve thepower factor of a system, which is why theyare often used in systems that use a lot of
electricity. This motor rotates at a synchronous speed,
which is given by the following equation Ns = 120 f / P
Where:
f = frequency of the supply frequency
P= number of poles
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Induction motors are the most common motors used
for various equipments in industry.
Their popularity is due to their simple design, they are inexpensive (half or less of the cost of a DC motor)
High power to weight ratio (about twice that of a DC motor)
are simple, rugged, low-cost and easy to maintain.
They run at essentially constant speed from zero-to-full load.
can be directly connected to an AC power source
AC MotorsInduction motor
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AC MotorsInduction motor
An induction motor has two main electricalcomponents as shown in the figure
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Construction
Frame
- Outer part of the body To support the stator core & windings
To protect the inner parts
Serve as a ventilating housing
Stator core
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Construction
The iron core has cylindrical
shape and is laminated with
slots.
The iron core on the figure haspaper liner insulation placed in
some of the slots.
In a three-phase motor, the
three phase windings are placed
in the slots.
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Construction
Stator or primary or field winding
- Stator winding is 3 ph winding supplied
from 3 ph source.- 3 ph can be connected in either star or
delta depending upon the method of
starting
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Stator and rotor magnetic circuit
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AC MotorsInduction motor
Rotor-Induction motors use two types of rotors:
A squirrel-cage rotor consists of thick conducting
bars embedded in parallel slots. These bars are short-
circuited at both ends by means of short-circuitingrings.
A wound rotor has a three-phase, double-layer,
distributed winding. It is wound for as many poles as
the stator. The three phases are wired internally andthe other ends are connected to slip-rings mounted on
a shaft with brushes resting on them.
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Squirrel cage rotor
Squirrel cage rotor consists of copper bars,slightly longer than the rotor, which are pushedinto the slots.
The ends are welded to copper end rings, sothat all the bars are short circuited.
In small motors, the bars and
end-rings are die-cast in
aluminum to form an
integral block.
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Squirrel cage rotor.
This rotor has a laminated iron core with slots, and is mountedon a shaft.
Aluminum bars are molded in the slots and the bars are shortcircuited with two end rings.
The bars are slanted on a small rotor to reduce audible noise.
Fins are placed on the ring that shorts the bars. These finswork as a fan and improve cooling.
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Squirrel cage rotor
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wound rotor Most motors use the squirrel-cage rotor because of the
robust and maintenance-free construction.
However, large, older motors use a wound rotor with
three phase windings placed in the rotor slots.
The windings are connected in a three-wire wye.
The ends of the windings are connected to three slip rings.
Resistors or power supplies are connected to the slip ringsthrough brushes for reduction of starting current and speed
control
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wound rotor
The external resistors are mainly usedduring start-up under normal runningconditions the windings short-circuited
externally.
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AC MotorsInduction motor
Stator.The stator is made up of a numberof stampings with slots to carry three-phase windings. It is wound for a definite
number of poles. The windings aregeometrically spaced 120 degrees apart
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Operating Principle
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Operation Principle
The three-phase stator is supplied by balanced threephasevoltage that drives an ac magnetizing current through eachphase winding.
The magnetizing current in each phase generates a
pulsating ac flux.
The flux amplitude varies sinusoidally and the
direction of the flux is perpendicular to the phase
winding.
The total flux in the machine is the sum of the three fluxes.
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The summation of the three ac fluxes results in a rotating flux,which turns with constant speed and has constant amplitude.
The rotating flux induces a voltage in the short-circuitedbars of the rotor. This voltage drives current through the bars.
The induced voltage is proportional with the difference ofmotor and synchronous speed.
Consequently the motor speed is less than the
synchronous speed
The interaction of the rotating flux and the rotor current
generates a force that drives the motor. The force is proportional with the flux density and the rotor
bar current
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The figure shows the three components
of the magnetic field at a phase angle of
60. Each phase generates a magnetic field
vector.
The vector sum of the component
vectors a
, b
, c
gives theresulting rotating field vector
rot,
The amplitude is 1.5 times the
individual phase vectoramplitudes,and rot rotates
with constant speed.
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Induction motor-Points to be remembered
The speed of flux cutting is the difference
between the magnetic field speed and the rotor
speed.
The two speeds can be calculated by using the
radius at the rotor bar location and the
rotational speed.
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Induction motor-Points to be remembered
The voltage and current generation in the rotor bar require a
speed difference between the rotating field and the rotor.
Consequently, the rotor speed is always less than the
magnetic field speed.
The relative speed difference is the slip, which is calculated
using
P
fN
N
NNs
S
S
S
120
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Induction Motors: Active PowerFlow
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Induction Motors: Active PowerFlow
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Torque/Speed Curve
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Complete Torque-SpeedCharacteristics
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THANK YOU