DOUBLE CAGE INDUCTION MOTOR AND IT'S CONSTRUCTION

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 18981 A0240 Mr.P.Sanjay Kumar 18981A0241 By Head of the department Dr.P.SASI KIRAN Professor RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 1

2. CONTENTS Need for Deep-Bar and Double-Cage rotor Deep-Bar rotors Torque-Slip relationship Double-Cage rotors Equivalent circuit diagram Applications Conclusion RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 2

3. DOUBLE CAGE INDUCTION MOTOR RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 3

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. RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 4

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. RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 5

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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. RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 7

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. RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 8

9. The slip-torque curve for deep-bar rotor is shown below along with the deep-bar stator. Torque/Slip curve Deep-Bar rotor. RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 9

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 . RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 10

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 RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 11

12. TORQUE SLIP CHARACTERISTICS OF A DOUBLE CAGE ROTOR RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 12

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. RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 13

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. RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 14

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. RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 15

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. RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 16

17. APPLICATIONS :- Used in water pumps for various Irrigational and Industrial purposes . Used in Lathe machines and Drilling machines. RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 17

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 . RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 18

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) RAGHU ENGINEEERING COLLEGE BATCH 8 EM-2 CASE STUDY 19

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