41

Describe the relation between rotor copper loss and
rotor input in the three phase induction motor.


The rotor copper loss is equal to slip times rotor input in the three
phase induction motor.

42

Describe the relation between gross rotor output and
rotor copper loss in the induction motor.


Gross rotor output / Rotor copper loss
= ( 1 – s ) /
s
Where s = Slip

43

Why the shaft torque is always less than the gross
torque in the induction motor?


Shaft torque
 The shaft torque is always less than the gross torque in the induction
motor because there is some loss due to friction and windage in the rotor.

44

Explain : Synchronous watt in the induction motor


Synchronous watt
 The power developed across the air – gap of the induction motor is
called as synchronous watt.

45

What is difference between powers developing in the
three phase induction motor and that of DC motor?


Back emf in the Induction motor
 The power developed in the armature of the dc shunt motor is equal to
E_{b* }I_{a }
Where E_{b
}= Back emf and
I_{a }= Armature current
 The rotor induced emf in the induced emf decreases from ( E_{2 }to
sE_{2 }) during rotation of the induction motor.
 The difference
between E_{2} and sE_{2 }is equal to ( 1 – s ) E_{2}
is called as back emf in the induction motor.

46

What is direction of the flux in the sector
induction motor?


The direction of the flux in the sector induction motor is
continuously from one end to the other end.

47

Write the equation of the synchronous speed and
linear synchronous speed.


 The synchronous speed is given by
N_{s} = 120 × f / p
f
= Supply frequency
P = Number of poles
 The linear synchronous speed is given by
V_{S} = 2 × Ï‰
× f
Ï‰ = Width of pole – pitch
and
f =
Supply frequency

48

Give the practical example of rotor is stationary
and stator is fixed in three phase induction motor.


Magnetic levitation train

49

Why the no load current in the three phase induction
is much higher than that of transformer?


No load current
 The core of the transformer consists of low reluctance path therefore
magnetizing current is very small resulting no load current is small.
 The flux passes through stator to rotor in the three phase induction
motor through air gap which consists of very high reluctance path therefore
the magnetic current and hence no load current is 40 to 50% of the full load
current.

50

What is speed of rotating field of rotor with
respect to the stator magnetic field?


The rotating field of the rotor rotates at synchronous speed with
respect to the stator magnetic field.

51

What is mechanical load on the induction motor at
unity value of slip?


Mechanical load at unity slip
 The mechanical load in the induction motor can be represented by R_{2
}{ ( 1 / s ) – 1 }
Where s = Slip and R_{2 }= Rotor resistance of the induction
motor
 When the slip is equal to unity then mechanical load is zero.

52

What is condition for maximum power output in the
induction motor?


Condition for maximum output
 The induction motor transfers maximum power when the load resistance
is equal to leakage impedance of the motor.

53

Which losses are known as no load losses in the
induction motor?


No load losses
 When the induction motor run on no load following losses occurs :
stator copper loss, stator core loss and loss due to friction and windage.

54

Which losses are known as fixed losses? Why it is
called fixed losses?


Fixed losses
 The stator core losses and friction and windage losses are
collectively known as fixed losses.
 As its name suggests that its value does not change with change in
load current.

55

Why the circle diagram of the induction motor is
semi circle?


The equivalent circuit of the induction motor consists of fixed
reactance and variable resistance.
 Fixed reactance as referred to stator side X_{01}
 Fixed resistance as referred to stator side R_{01}
 Variable resistance as referred to stator side R_{L}^{’}
 When supply voltage V is given to induction motor, it consists of
series circuit with constant reactance and variable resistance.
The stator current vector is circle with diameter V / X_{01}
I = V / √ ( R^{2}
+ X_{01}^{2 })
= ( V / X_{01 }){ X_{01 }/ √
( R^{2} + X_{01}^{2 }) }
= ( V / X_{01}) sin Î¦
 As we concerned only positive stator current in the induction motor,
the locus of the stator current is semi circle.

56

Give reason : The input power of the three phase
induction motor during no load is W1  W2 ( Difference between two wattmeter
reading ).


Induction motor on no load
 When three phase power is measured by two watt meter method, the
wattmeter reading is directly proportional to
W_{1 }Î± cos ( 30 + Î¦ )
_{ }W_{2 }Î± cos ( 30 – Î¦ )
Where Î¦ is power
factor angle
 The no load power factor of the induction motor is very low i.e. less
than 0.5 ( Power factor angle > 60^{0} ) resulting one watt meter
shows negative reading.

57

Which losses do not depend upon load current?


Stator core loss, friction and windage loss

58

Which test to be performed on the induction motor in
order to draw circle diagram of the induction motor?


Test requires for Circle diagram
 No load test
 Block rotor test
 Stator resistance test

59

Why starter is necessary to start induction motor?


Function of starter
 The working of the induction motor is similar to transformer with
short circuit secondary.
 When the rated voltage is applied to the induction motor, it will draw
heavy current during starting period.
 The function of the starter is to limit starting current. The overload
relay in the starter circuit protects motor against overload.

60

Describe the methods of starting induction motor.


Types of starter
 Primary resistor
 Star – delta starter
 Auto transformer starter

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