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7/29/2019 83269343 3 Phase Induction Motors
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Content
•Introduction
•Construction
•Rotating Magnetic Field
•Principle of operation
•Induction motor speed
•The Slip
•Torque
•Power losses in Induction machines
•Modes of induction motor
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Introduction
Three-phase induction motors are the mostcommon and frequently encountered machines in
industry
- simple design, rugged, low-price, easy maintenance
- wide range of power ratings: fractional horsepower to
10 MW
- run essentially as constant speed from no-load to full
load
- Its speed depends on the frequency of the power source
• not easy to have variable speed control
• requires a variable-frequency power-electronic drive for
optimal speed control
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Construction
An induction motor has two main parts- a stationary stator
• consisting of a steel frame that supports a hollow, cylindrical core
• core, constructed from stacked laminations (why?), having a
number of evenly spaced slots, providing the space for the stator winding
Stator of IM
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Construction
- a revolving rotor • composed of punched laminations, stacked to create a series of rotor
slots, providing space for the rotor winding
• one of two types of rotor windings
• conventional 3-phase windings made of insulated wire (wound-rotor ) »
similar to the winding on the stator • aluminum bus bars shorted together at the ends by two aluminum rings,
forming a squirrel-cage shaped circuit (squirrel-cage)
Two basic design types depending on the rotor design
- squirrel-cage: conducting bars laid into slots and shorted at both
ends by shorting rings.- wound-rotor: complete set of three-phase windings exactly as the
stator. Usually Y-connected, the ends of the three rotor wires are
connected to 3 slip rings on the rotor shaft. In this way, the rotor
circuit is accessible.
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Construction
Squirrel cage rotor
Wound rotor
Notice the
slip rings
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Construction
Cutaway in a
typicalsquirrel cage
IM.
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CONSTRUCTION OF
SQUIRREL CAGE ROTOR
The squirrel cage is made of hollow copper rods
soldered to two disks making a cage, which is free
to rotate. A solid plate of copper is being held just
above a coil surrounding an iron core.The rotor consists of a cylindrical laminated core
with parallel slots for carrying the rotor conductors
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Rotating Magnetic Field Balanced three phase windings, i.e.
mechanically displaced 120 degrees
form each other, fed by balanced
three phase source
A rotating magnetic field with
constant magnitude is produced,
rotating with a speed
Where f e is the supply frequency and
P is the no. of poles and n sync is called
the synchronous speed in rpm
(revolutions per minute)
120 e sync
f n rpm
P
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Synchronous speed
P 50 Hz 60 Hz
2 3000 3600
4 1500 1800
6 1000 1200
8 750 900
10 600 720
12 500 600
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Rotating Magnetic Field
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Rotating Magnetic Field
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Principle of operation
This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings
Due to the fact that the rotor windings are short circuited, for
both squirrel cage and wound-rotor, and induced current
flows in the rotor windings The rotor current produces another magnetic field
A torque is produced as a result of the interaction of those
two magnetic fields
Where ind is the induced torque and B R and BS are the magnetic
flux densities of the rotor and the stator respectively
ind R skB B
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Induction motor speed
At what speed will the IM run?- If rotor runs at the synchronous speed, which is the
same speed of the rotating magnetic field, then the rotor
will appear stationary to the rotating magnetic field and
the rotating magnetic field will not cut the rotor. So, noinduced current will flow in the rotor and no rotor
magnetic flux will be produced so no torque is
generated and the rotor speed will fall below the
synchronous speed
- When the speed falls, the rotating magnetic field will
cut the rotor windings and a torque is produced
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Induction motor speed
So, the IM will always run at a speed lower thanthe synchronous speed
The difference between the motor speed and the
synchronous speed is called the Slip
Where n slip
= slip speed
n sync= speed of the magnetic field
nm = mechanical shaft speed of the motor
slip sync mn n n
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The Slip
sync m
sync
n n s
n
Where s is the slip
Notice that : if the rotor runs at synchronous speed
s = 0
if the rotor is stationary
s = 1
Slip may be expressed as a percentage by multiplying the above
eq. by 100, notice that the slip is a ratio and doesn’t have units
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Torque
While the input to the induction motor is electrical power, its output is mechanical power and for that we
should know some terms and quantities related to
mechanical power
Any mechanical load applied to the motor shaft willintroduce a Torque on the motor shaft. This torque is
related to the motor output power and the rotor speed
and.out
load
m
P N m
2
/60
mm
nrad s
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Induction Motors and Transformers
Both IM and transformer works on the principle of induced voltage
- Transformer: voltage applied to the primary windings
produce an induced voltage in the secondary windings
- Induction motor: voltage applied to the stator windings produce an induced voltage in the rotor windings
- The difference is that, in the case of the induction
motor, the secondary windings can move
- Due to the rotation of the rotor (the secondary windingof the IM), the induced voltage in it does not have the
same frequency of the stator (the primary) voltage
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Power losses in Induction machines
Copper losses- Copper loss in the stator ( P SCL) = I 1
2 R1
- Copper loss in the rotor ( P RCL) = I 22 R2
Core loss ( P core
)
Mechanical power loss due to friction and windage
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MODES OF OPRATION
INDUCTION MOTOR
It acts like as generator and break
• when the rotor run faster
than its
synchronous speed aninduction motor
runs as a generator called a
induction generator.
O S O O A O
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•In induction motor if we change any of the stator the
direction of the revolving flux as well as torque reversed
thus Appling break of the motor during this plugging
period the motor acts as break.
MODES OF OPRATION
INDUCTION MOTOR
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