Download Chapter 2 Polyphase induction motor File

Chapter 2
Poly phase Induction Motor
Prepared by:- Jaydeepsingh Chauhan
Electrical Department
SVBIT Gandhinagar
Induction Motor
•Most popular motor today in the low and medium horsepower range
•Very robust in construction
•Speed easily controllable using V/f or Field Oriented Controllers
•Have replaced DC Motors in areas where traditional DC Motors
cannot be used such as mining or explosive environments
•Of two types depending on motor construction: Squirrel Cage
or Slip Ring
•Only Disadvantage: Most of them run with a lagging power factor
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induction motor
Construction of Induction Motor
Stator
Squirrel Cage Rotor
Cut section Of Induction Motor
Slip ring Rotor
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induction motor
AC Machine Stator
‘b’ phase axis
1200
1200
‘a’ phase axis
1200
‘c’ phase axis
4
induction motor
Squirrel Cage Rotor
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induction motor
Slip Ring Rotor
•The rotor contains windings similar to stator.
•The connections from rotor are brought out using slip rings that
are rotating with the rotor and carbon brushes that are static.
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induction motor
Currents in different
phases
of
AC
Machine
t
t
01
12
Amp
t0
t1
1 Cycle
7
t2
t3
t4
time
induction motor
a
Fc
RMF(Rotating Magnetic Field)
b’
c’
1.5
Fa
F
1
Fa
c
b
t = t0= t4
F
Fc
0.5
Fb
a’
0
Fb
-0.5
t = t0= t4
-1
-1.5
-93
F
Fb
a
c’
8
Fc
Fb
b’
c
a’
t = t1
113
216
Space angle () in degrees
a
c’
b
10
a
b’
Fa
F
c
b
Fc
t = t2
a’
b’
c’
c
b
t = t3
induction motor
Fc
a’
F
Fb
Torque Production in an Induction Motor
•In a conventional DC machine field is stationary and the current carrying
conductors rotate.
•We can obtain similar results if we make field structure rotating and current
carrying conductor stationary.
•In an induction motor the conventional 3-phase winding sets up the rotating
magnetic field(RMF) and the rotor carries the current carrying conductors.
•An EMF and hence current is induced in the rotor due to the speed
difference between the RMF and the rotor, similar to that in a DC motor.
•This current produces a torque such that the speed difference between the
RMF and rotor is reduced.
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induction motor
Slip in Induction Motor
•However, this speed difference cannot become zero because that would stop generation of the
torque producing current itself.
•The parameter slip ‘s’ is a measure of this relative speed difference
ns  n  s  
s

ns
s
ns 
120 f1
; p # of poles
p
where ns,s,f1 are the speeds of the RMF in RPM ,rad./sec and supply frequency
respectively
n, are the speeds of the motor in RPM and rad./sec respectively
•The angular slip frequency and the slip frequency at which voltage is induced in the rotor
is given by
 2  s , f 2  sf1 , E2 s
10
N2
s
E1
N1
N1  Stator turns N2  Rotor turns
induction motor