UNIT 2 DC MachinesPART A

1. Give methods adopted to reduce the effect of armature reaction in a DC machine.

2. List the advantages of lap winding in a DC

3. State the relationship between number of armature coils and number of commutator segments in a dc machine.

4. State different losses in a dc generator.

5. What are the main dimensions of a rotating machine?

6. Define specific magnetic loading.

7. Define specific electric loading

8. What is output equation?

9. Write the expressions for output co-efficient

10. List the factors that influences the separation of D and L of a dc machine .

11. What is square pole criterion?

12. In a dc machine what are the limiting values of armature peripheral speed and voltage between adjacent commutator segments .

13. List the various L/ used for separation of D and L in induction motor.

14. What are the factors that affect the size of the rotating machines?

15. What are the factors that decide the choice of specific magnetic loading?

16. What are the factors that decide the choice of specific electric loading?

17. What are the factors to be considered for the selection of number of poles in a dc machine?

18. List the advantages and disadvantages of large number of poles.

19. Why square poles preferred?

20. What is the effect of interpole on main pole?

21. Give the main parts of a d.c motor.

22. Write the guiding factors for choice of number of armature slots of D.C machine.

23. Define brush contact loss.

24. What are the main parts of a DC generator?25. Distinguish between lap and wave winding in a D.C.machine.

26. What are dummy coils?

27. What is meant by equalizer connections?PART- B

1. A 250 kW, 500V, 600 r.p.m. d.c. generator is built with an armature diameter of 0.75m and a core length of 0.3m. The lap connected armature has 720 conductors. Using the data obtained from this machine, determine the armature diameter, core length, number of armature slots, armature conductors and commutator segments for a 350kW, 440V, 720 r.p.m 6pole d.c. generator. Assume a square pole face with ratio of pole arc to pole pitch equal to 0.66. The full load efficiency is 0.91 and the internal voltage drop is 4% of rated voltage.The diameter of commutator is 0.7 of armature diameter. The pitch of commutator segments should not be less than 4mm. The voltage between adjacent segments should not exceed 15Vat no load.2. Explain the design procedure for the shunt field winding of D.C.machine.

3. Derive the output equation of a d.c machine and point out its salient features.4. State and explain the factors which govern the choice of specfic magnetic loading in a dc machine.5. Discuss the factors that influence the choice of number of poles of a dc machine.6. Design a shunt field coil of a dc motor from the following data: Field ampere turns/pole=9000 Mean length of turn=1.4m, Depth of coil=35X10-3m Voltage across field coil=40V Resistivity of wire=0.021 ohm/m and mm2.Thickness of insulating varnish=0.2mm, Power dissipation from total surface of the coil