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Induction Motors
Amit Mishra
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Topics to be DiscussedTopics to be Discussed
Induction Motors : Applications.
Construction :
Stator.
Rotor : Squirrel cage rotor.
Wound rotor.
The emf generated in the rotor.
Torque generated on the rotor.
Slip of Induction Motor.
Frequency of Rotor Currents.
Speed of Rotation of Rotor-Field.
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Rotor emf.
Rotor Impedance.
Rotor Current.
Differences from a Transformer.
Power Flow Diagram.
Starting Torque.
Torque-Slip Characteristic Curve. Three Modes of Operation :
(1) Motor action (0 < s 1).
(3) Generator action (s
< 0). Condition for Maximum Torque.
Maximum Torque.
Equivalent Circuit.
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Induction Motors A synchronous motor has limited practical
applications like control instrumentation, lightdisplays, textile, engineering and allied industries.
An alternative is the asynchronous motor which we
usually call induction motor.
Most motors that we see around are induction
motors.
Induction motors
are more rugged, need less maintenance,
are less expensive.
Are efficient but less than synchronous motors.
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Applications of Induction Motors
Three-phase induction motors are used for
high power and industrial applications such
as
lifts, cranes, pumps, exhaust fans, lathes etc.,
Single-phase induction motors find use in
domestic electric appliances such as
fans, refrigerators, washing machines, pumps,hair-driers, etc.
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Construction
Two Main Parts
Stator
Rotor
Squirrel Cage Rotor
(Mostly used)
Phase Wound Rotor
The statorof an induction motor is similar to
that of a synchronous machine.
When three-phase currents flow through the
stator windings, a magnetic flux is produced that
rotates at synchronous speed.
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Induction motor components
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Stator-Rotor Assembly
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Stator of a large induction motor.
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Squirrel cage rotor
This rotor has a laminated iron core with slots, andis mounted on a shaft.
Aluminum bars are molded in the slots and the bars
are short circuited with two end rings.
The bars and the shorting rings look like a
squirrel cage.
Fins are placed on the ring that shorts the bars.
These fins work as a fan and improve cooling.
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Squirrel cage rotor
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Rotor bars (slightly skewed)
End ring
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1. It helps in reduction of magnetic hum, thus keeping the
motor quiet.
2. It also helps to avoid Cogging, i.e. locking tendency of the
rotor. The tendency of rotor teeth remaining under the stator
teeth due to the direct magnetic attraction between the two.
3. Increased rotor resistance due to comparatively lengthier
rotor conductor bars.
4. Increased slip for a given torque.
Reasons for Having Skewed Rotor
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Wound rotor
Most motors use the squirrel-cage rotor because ofthe robust and maintenance-free construction.
However, large motors use a wound rotor with threephase windings placed in the rotor slots.
The windings are connected in a three-wire wye.
The ends of the windings are connected to three sliprings.
Resistors or power supplies are connected to theslip rings through brushes for reduction of startingcurrent and speed control
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Slip-ring rotor assembly
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The emf generated in the rotor.
Assume that the stator flux rotates anticlockwise.
With respect to this flux, the rotor conductors move inclockwise direction.
The emfs in conductorsA andB are induced.
This is why this type of motor is called an inductionmotor.
In fact, an induction motor is like a transformer with itssecondary winding short-circuited.
The only difference is that in an induction motor, thesecondary winding is free to rotate.
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Torque generated on the rotor
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Fleming's left hand rule (forelectric
motors) shows the direction of the thrust ona conductorcarrying a current in a
magnetic field.
The left hand is held with the thumb, index
fingerand middle fingermutually at right
angles. It can be recalled by rememberingthat "motors drive on the left, in Britain
anyway."
The First finger represents the direction of
the magnetic Field. (north to south)
The Second finger represents the direction
of the Current (the direction of the current isthe direction of conventional current; from
positive to negative).
The Thumb represents the direction of the
Thrust or resultant Motion.
Flemings left hand rule
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Current-carrying conductorsA and B experience
force Fas shown (Flemings left hand rule).
Torque is generated to rotate the rotor in
anticlockwise direction.
Torque keeps on generating till there is relative
motion between the rotating field and the rotorconductors.
Rotor accelerates, but can never attain the
synchronous speed.
Why ?
Ans. : If it does, the induced voltage and hence
current would be zero. As a result, the torque would
become zero.
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Motor on Load
On putting load on the shaft, the rotor slows down.
The relative speed increases.
The induced emf in the rotor-conductors increases
and hence the torque exerted on the rotor
increases.
The speed adjusts itself to get sufficient torque.
Obviously, the speed has slightly reduced.
Note that unlike a synchronous machine, the
induction motor has field on the stator and armature
on the rotor.
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Slip of Induction Motor
The difference between the synchronous speed Nsand the actual speed Nof the rotor is known as slip
speed.
The normalized slip speed, or simply the slip s isexpressed as
pu or %s
s s
N NN
s N N
(
! !
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For a given slip s, the rotor speed is given as
(1 )s N N s!
When standstill(that is, it is not running), the
rotor speed Nis zero.
Hence s = 1.
The value ofs can never be zero.
Why ?
Ans. : Torque generated would be zero.
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Is it possible to make the slip s have a
negative value ?
Ans. : Yes, if the rotor is made to rotate by a
prime-mover at a speed higher than the
synchronous speed. The negative slip corresponds to the
generatoraction.
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Frequency of Rotor Currents
When standstill, It is like a transformer.
fr= f
When running, fr depends upon the relative
speed or slip-speed.
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120r
s
f
N N P !
120But ; Dividing the two,s
fN
P
!
ors r r
s
N N f f s
N f f
! !
r f s f @ !
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