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UNIT 4
INDUCTION MACHINES
1. State the main constructional difference between cage induction motor and slip
ring induction motor.
The squirrel cage motor has the following advantages as compared with wound rotor
machine:
i.
No slip rings, brush gear, short circuiting devices, rotor terminals for starting
rheostats are required. The Y-Δ starter is sufficient for starting.
ii.
It has a slightly higher efficiency.
iii.
It is cheaper and rugged in construction.
iv.
It has better space factor for rotor slots, a short over hang and consequently a
smaller copper loss.
v.
It has bare and rings, a larger space for fans and thus the cooling conditions are
better.
Disadvantages: the greatest disadvantage of cage rotor is that it is not possible to
insert resistance in the rotor circuit for the purpose of increasing the starting torque. The
cage motor has a smaller starting torque and larger starting current as compares with
wound rotor motor.
2. What are the different losses in an induction motor?
a. Constant losses (core loss, Mechanical loss)
b. Variable loss (Copper loss in stator and rotor winding)
3. What is dispersion coefficient in an induction motor?
The dispersion coefficient σ can be defined as

4.
I
Magnetizin g Current
 m
Ideal short circuit current I sc
Give the methods to reduce harmonic torques in an induction motor.
a. Chording
b. Integral slot windings
c. Skewing
5. What are the effects of change of air gap length in an Induction motor.
Additional R cannot be included-squirrel cage rotor Can be included-wound rotor
6. What is the effect of change of air gap length in an induction motor?
Increase in air gap length result in
i) Decrease in power factor ii) Decrease in pulsation losses, iii) Noise iv) Decrease in
unbalanced pull v)Improve cooling
7. List the main parts of a Slip ring induction motor.
i) Rotor core, Rotor winding ii) Slip rings iii) frame iv) stator core v)stator winding
8. What is the function of end rings in the rotor of cage induction motor?
End rings are provided to short circuit the rotor bars at the ends.
9. What are the merits of slip ring induction motor over squirrel cage induction
motor?
a. It Is possible to insert resistances in the rotor circuit for the purpose of increasing
the starting torque.
b. The starting current is low when compared to the cage induction motor.
10. How does the external resistance of slip ring induction motor influence the
motor performance?
The starting torque of a slip ring motor is increased by improving its power factor by
adding external resistance in the rotor circuit from the star connected rheostat, the
resistance being progressively cut out as the motor gathers speed. ii) Adding of external
resistance, however increases the rotor impedance and so reduces the rotor current.
11. What is the overload capacity in 3 phase induction motor?
It is the full rated capacity above which Induction motor attains instability and below
which Induction motor works stability.
12.What are the materials used for slip rings and brushes in induction motor?
Slip-rings : Brass or phosphor bronze
Brushes : Metal graphite which is an alloy of copper and carbon.
13.Name the materials used to insulate the laminations of the core of induction
motor.
The materials used to insulate the laminations are kaolin and varnish.
14.What are the factors to be considered for the choice of specific magnetic
loading?
The choice of specific magnetic loading depends on power factor, iron loss and overload
capacity.
15. What are the factors to be considered for the choice of specific electric
loading?
The choice of specific electric loading depends on copper loss, temperature rise, voltage
rating, and overload capacity.
16. How the induction motor can be designed for best power factor?
For best power factor the pole pitch, ζ is chosen that ζ=
17. What are the types of slots preferred in induction motor?
Semi enclosed slots are preferred for induction motor. It results in less air gap
contraction factor giving a small value of magnetizing currents, low tooth pulsation loss
and much quieter operation(less noise)
18. What are the advantages and disadvantages of large air gap length in
induction motor?
Advantage: A large air gap length results in higher overload capacity, better cooling,
reduction in noise and reduction in unbalanced magnetic pull.
Disadvantage: A disadvantage of large air gap length is that it results in high value of
magnetizing current.
19.List out the methods to improve the power factor of induction motor.
The power factor of the induction motor can be improved by reducing the magnetizing
current and leakage reactance. The magnetizing current can be reduced the length of air
gap. The leakage reactance can be reduced by reducing the depth of stator and rotor
slots, by providing short chorded winding and reducing the overhang in stator winding.
20. What is crawling and cogging?
Crawling is a phenomenon in which the induction motor runs at a speed lesser than sub
synchronous speed.
Cogging is a phenomenon in which the induction motor refuses to start.
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