THIS PPT MAINLY DISCUSS ABOUT DOUBLE CAGE INDUCTION MOTOR AND IT'S CONSTRUCTION AND THIS PPT MAINLY CONTAINS THE DIFFERENT CHARACTERISTICS OF THE MACHINE IN VARIOUS FACTORS ANTHE PLOTTINGS ARE ALSO WE CAN SEE IN THIS PPT SO IT IS SO USEFUL TO ELECTRICAL MACHINES STUDENTS

1. Department of Electrical & Electronics Engineering
Raghu Engineering College (Autonomous)
CASE STUDY
ON
DOUBLE CAGE INDUCTION
MOTOR
BACHELOR OF TECHNOLOGY
IN
ELECTRICAL AND ELECTRONICS ENGINEERING
Under the Supervision of
Mr.M.SRIKANTH
Assistant Professor
Mr P.Eswar sai 18981A0237
Mr P.Mohan 18981A0238
Ms P.Supriya 18981A0239
Mr.P.Nitish 18981A0240
Mr.P.Sanjay Kumar 18981A0241
By
Head of the department
Dr.P.SASI KIRAN
Professor
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2. CONTENTS
 Need for Deep-Bar and Double-Cage rotor
 Deep-Bar rotors
 Torque-Slip relationship
 Double-Cage rotors
 Equivalent circuit diagram
 Applications
 Conclusion
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3. DOUBLE CAGE INDUCTION MOTOR
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4. WHY WE NEED IT………
• Conventional squirrel cage motors suffer from low starting torque
and High strarting Current.
• It has a double cage or two Rotor windings or cages are
used.
• This arrangement is used for obtaining high starting torque at a low value of
starting current.
• The stator of a double cage rotor of an induction motor is same as that of a
normal induction motor.
• In the double cage rotor of an induction motor there are two layers of the bars.
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5. INTRODUCTION :-
• Induction motor has two types on basis of
construction.
i. Wound rotor
ii. Squirrel cage rotor
In wound rotor we limit current by using
external resistances through slip rings. But
squirrel rotor is short circuited so cannot add
resistance to it.
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7. In wound rotor induction motors these conditions are
met by connecting external resistance in rotor circuit at
the time of starting & resistances are cut out in steps as
the motor attains its normal speed.
In squirrel cage motor this is not feasible as the
conductors are short circuited by end rings. In order to
attain the above desired conditions following types of
rotors are used:-
Deep bar rotor.
Double cage rotor.
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8. DEEP BAR ROTOR
 On the other hand, the bottom layer ‘C’ links
maximum flux, therefore its leakage
inductance is maximum.
 At starting the rotor frequency is equal to
the supply frequency. The bottom layer
element ‘C’ offers more impedance to the
current flow then upper layer ‘A’.
 Therefore the maximum current flows
through the top layer and minimum
through bottom layer. This unequal current
distribution causes the effective rotor
resistance to increase.
 Figure below shows a cage rotor with deep and narrow bars. A bar may be
assumed to be made up of number of narrow layers connected in parallel.
 It is seen that the top most layer element ‘A’ is linked with minimum leakage flux
and therefore its leakage inductance is minimum.
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9.  The slip-torque curve for deep-bar rotor is shown
below along with the deep-bar stator. Torque/Slip curve
Deep-Bar rotor.
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10. DOUBLE CAGE ROTOR CONSTRUCTION:-
 The outer cage bars have a smaller cross-
sectional area than the inner bars and are
made of high resistivity materials like brass etc.
 An induction motor with two rotor windings or cages is used for obtaining high
starting torque at low current.
 The stator of a double cage induction motor is similar to that of ordinary induction
motor. In double cage rotor there are two layers of bars as shown:
 The inner cage bars are made up of low
resistance material like copper. Thus the
resistance of outer cage is greater than that
of inner cage.
 There is a slit between the top and bottom
slot. This increases premeance for leakage
flux around the inner cage bars.
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11.  At normal speed, the leakage reactance of both the winding becomes negligibly small.
 The rotor current division between the two cages is governed mainly by their
resistances.
 Since the resistance of the outer cage is about 5 to 6 times that of inner cage, most of
the rotor current flows through the inner cage.
 Hence under normal operating speed, torque developed mainly by low resistance inner
cage.
Double cage rotor
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12. TORQUE SLIP CHARACTERISTICS OF A DOUBLE
CAGE ROTOR
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13. DOUBLE SQUIRREL-CAGE ROTOR WORKING
• At starting, the rotor frequency is the same as
that of the line (i.e., 50 Hz),making the
reactance of the lower winding much higher
than that of the upper winding.
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14. AT RUNNING CONDITION…
• As the motor accelerates, the rotor frequency
decreases, thereby lowering the reactance of
the inner winding, allowing it to carry a larger
proportion of the total rotor current.
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15. EQUIVALENT CIRCUIT :-
 The approximate equivalent circuit of double cage rotor induction motor is shown
below. Though the two cages are somewhat coupled magnetically, they can be
treated as independent for simplicity and it gives approximately same results.
 I2ru' and I2rl' are the currents in the upper and lower cages respectively referred to
the stator. R2u'and R2l' are the resistanceof upper and lower cages referred to the
stator whereas X2u’ and X2l’ are leakage reactances of the two cages referred to the
stator of the motor.
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16. ADVANTAGES :-
• It has high efficiency and good speed regulation.
• It gives higher starting torque.
• Lower starting current and are cheaper in cost.
• They are more robust and are explosion proof
since the risk of sparking is eliminated by the
absence of slip ring and brushes.
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17. APPLICATIONS :-
 Used in water pumps for various Irrigational and Industrial
purposes.
 Used in Lathe machines and Drilling machines.
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18. CONCLUSION:-
 It is evident by now that for low starting torque requirement, low-cost ordinary
squirrel-cage construction is employed.
 A deep-bar construction is adopted for higher starting torque applications and
double-cage for still higher starting torque needs.
 For large size motors with stringent starting torque needs, the most expensive
slip-ring construction is used.
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19. REFERENCES :-
 I.J. Nagrath and D.P. Kothari,’Electric Machines’, 4th
edition., 2010, Tata McGraw Hill Education Pvt Ltd.
 B.R. Gupta & Vandana Singhal, “Fundamentals of Electrical
Machines, New Age International, 2009.
 http://www.electrical4u.com/squirrel-cagerotors/(
04/09/2014)
 http://www.electrical4u.com/squirrel-cage-double-
anddeep- bar-rotors/(04/09/2014)
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