This document discusses three-phase induction motors. It describes their advantages over other motors, including being very simple, rugged, reliable, and low cost. It then discusses the construction of three-phase induction motors. The construction includes a stationary stator and a rotating rotor. The stator contains windings that produce a rotating magnetic field. The document describes two types of rotors: squirrel cage and wound (slip ring). Squirrel cage rotors are the most common due to their simple and rugged design. Wound rotors allow adding resistance to the rotor circuit for starting.

1. Sri Venkatesa Perumal College
of Engineering and Technology
Puttur
AC MACHINES LECTURER NOTES
III YEAR V SEMESTER
20PC0210
BY
MUNISEKHAR SADU
ASSOCIATE PROFESSOR
DEPARTMENT OF EEE
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2. 2
DO YOU KNOW
• What is meant by Faraday’s laws of Electromagnetic
Induction
• What types of Materials can be employed in the
construction of Electromagnetic machines
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3. 3
Objectives
On completion of this period, you would be able
to know :
• Advantages of three phase induction motors over other
motors
• Construction of three-phase induction motors
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4. 4
ADVANTAGES OF 3-Ø I.M
• Very simple, extremely rugged and almost unbreakable
construction especially squirrel cage type
• Low cost and highly reliable
• Has sufficiently high efficiency
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5. 5
.
• No brushes required in normal running condition. hence
frictional losses are reduced
• It has reasonably good power factor.
• Requires minimum maintenance
• It starts from rest and needs no extra starting motor.
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6. 6
3-Phase Induction Motors
• Based on the construction, 3-phase induction motors
are classified as:
• Squirrel cage type IM
• Slip-ring or wound rotor type or Phase wound type IM

7. 7
CONSTRUCTION OF 3-Ø I.M
An induction motor consists of essentially two parts
• A stationary part called STATOR
• A rotating part called ROTOR
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8. 8
Blow up view of 3-Phase IM
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Fig(1)

9. 9
.
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Fig(2)

10. 10
STATOR
• The stator of an induction motor is in principle, the
same as that of a synchronous motor or generator.
• It is made up of a number of stampings which are slotted to
receive the stator windings
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11. 11
.
• The stator carries a 3-phase winding and is fed from a
three phase supply.
• Stator is wound for a definite number of poles the exact
number of poles being determined by the requirements
of speed.
• Greater the number of poles, lesser the speed and vice-
versa
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12. 12
.
• The stator windings when supplied with three phase
currents produce a Rotating Magnetic Field which
revolves at synchronous speed (given by Ns =120f/p)
• This revolving magnetic flux induces an emf in the rotor
by mutual induction.
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13. 13
Squirrel Cage Rotor
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fig(3)

14. 14
SQUIRREL CAGE (only rotor winding)

15. 15
.
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Slip ring rotor or phase wound rotor
Fig(4)

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ROTOR CONSTRUCTION
Two types of rotor construction are employed
• One type is Squirrel Cage Construction, also called as
squirrel cage rotor.
• 3- ø IM employing Squirrel Cage Rotor is called as 3- ø
squirrel cage IM.
• The other type of rotor is Phase Wound Rotor or just
wound rotor.
• IM’s employing this rotors are called slip ring IM’s
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17. 17
SQUIRREL CAGE ROTOR
• Almost 90% of IM’s are squirrel type, because this type
of rotor has the simplest and most rugged construction.
• It is almost indestructible.
• The rotor consists of a cylindrical laminated core with
parallel slots for carrying the rotor conductors.
• Rotor conductors are not wires but consists of heavy
bars of copper or aluminum or alloy.
• One bar is placed in each slot, rather the bars are
inserted from the end when semi closed slots are used.
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18. 18
SQUIRREL CAGE ROTOR cont…
• The rotor bars are brazen or electrically welded or bolted
to two heavy and stout short circuiting end rings, thus
giving a squirrel cage appearance.
• It should be noted that the rotor bars are permanently
short circuited on themselves.
• Hence it is not possible to add any external resistance in
series with the rotor circuit for starting purposes.
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19. 19
.
• The rotor slots are usually not parallel to the shaft but
are purposely given a slight skew.
This is useful in two ways
(a) It helps in reducing the locking tendency of the rotor
i.e. the tendency of the rotor teeth to remain under the
stator teeth due to direct magnetic attraction between
the two.
(b) It helps to make the motor run quietly by reducing
the magnetic hum.
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20. 20
Phase wound rotor
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External star
connected
rheostat
b1b2b3=brushes S1S2S3=sliprings
Fig.5

21. 21
PHASE WOUND ROTOR
• This type of rotor is provided with 3-phase, double layer,
distributed winding consisting of coils as used in
alternators.
• The rotor is wound for as many poles as the number of
stator poles.
• Rotor is always wound for 3-phase even when the stator
is wound two-phase.
• The three phases are starred internally.
• The other three winding terminals are brought out and
connected to three insulated slip rings mounted on the
shaft with brushes resting on them.
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22. 22
PHASE WOUND ROTOR cont….
• These three brushes are further externally connected to a
three phase star connected rheostat.
• This makes possible for the introduction of additional
resistance in the rotor circuit during the starting period for
increasing the starting torque of the motor.
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23. 23
.
• when running under normal conditions, the slip rings are
automatically short circuited by means of a metal collar which
is pushing along the shaft and connect all the rings together.
• The brushes are automatically lifted from the slip rings to
reduce the frictional losses and wear & tear
• Hence it is seen that under normal running conditions, the
wound rotor is short circuited on itself just like the squirrel
cage rotor.
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24. 24
SUMMARY
We have discussed the following
• Advantages of three phase induction motors over other
motors
• Classification and construction of three-phase induction
motors
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25. 25
Quiz
1. One of the following is an advantage of 3-
Phase induction motor
a) Cheap
b) Robust
c) Reliable
d) All of the above
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26. 26
.
2. Stamping are provided for the stator of 3-
Phase induction motor to reduce
a) Hysteresis losses
b) Eddy current losses
c) Copper losses
d) None of the above
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Quiz

27. 27
Quiz
3). The slip-ring rotor winding looks like
a). Squirrel cage
b). 2-phase winding
c). 3-phase winding
d). None of the above

28. 28
ASSIGNMENT
1.Where the Squirrel cage Induction Motors employed
2.What are the advantages of Slip ring induction motors
over Squirrel cage Induction Motors
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29. 29
Frequently Asked Questions
1. Classify 3-Phase induction motors based on construction.
2. Why Rotor conductors are slightly skewed ?
3. What are the advantages of Slip Ring IM over a Squirrel
Cage IM?
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