QUESTION BANK
1. Why do all practical energy conversion devices make use of the magnetic field
as a coupling medium rather than an electric field?
2. Write the applications of single excited and the doubly excited magnetic
system.
3. Explain the following terms with respect to rotating electrical machines
- pole pitch
- chording angle
4. Write the equation which relates rotor speed in electrical and mechanical
radians/second.
5. Why the fields in rotating machines should be quasi-static in nature?
6. Define energy and coenergy.
7. What is the significance of co- energy.
8. Draw and explain the general block diagram of an electromechanical energy
conversion device
9. Derive an expression for the torque in a doubly excited system having salient pole type of stator as well as rotor.
10. With neat sketch explain the multiple excited magnetic field system in electro
mechanical energy conversion systems. Also obtain the expression for field
energy in the system.
11. Explain clearly how a rotating magnetic field is set up around the three phase
AC winding having 120º (electrical) phase displacement each when three phase balanced supply is given to it.
12. Show that the field energy in a linear magnetic system is given by
13. For the electromechanical system shown in fig the air gap flux density under steady operating condition is
B(t) = Bm Sin ωt
Find
(i) The induced coil voltage
(ii) The force of field origin as a function of time and
(iii) The motion of armature as a function of time
14. Derive an expression for the mmf of a 3-phase winding. State the assumption made.
15. Deduce an expression for generated voltage per phase of a 3 phase ac machine with distributed windings.
16. Show that the net electromagnetic torque developed is zero if the rotating electrical machine has different number of poles on its stator and rotor.
17. What is an autotransformer.
18. Define the regulation up and regulation down of a transformer.
19. What is meant by turns ratio in Transformer? Give any two advantages of auto
Transformer.
20. Derive expressions for the current shared by two transformers operating in
parallel, with unequal no load voltages.
21. Draw and explain the no load phasor diagram of a single phase
transformer.
22. Derive the emf equation of single phase transformer.
23. Describe Sumpner’s test and give its limitations.
24. Explain open delta connectionof a 3-phase transformer. What are the applications of this system? Discuss.
25. Explain the division of load between transformers in parallel.
26. What are the possible connections for a 3phase transformer bank? Explain Y-∆ connection.
27. Define ‘ All day efficiency ‘. A transformer has its maximum efficiency of 0.98 at 15KVA unity power factor load. Find the all day efficiency for the following loading cycle:
(i) 2kW at 0.5pf for 12 hours,
(ii) 12kW at 0.8pf for 6 hours and
(iii) 18kW at 0.9pf for 6 hours.
28. Draw the circuit diagrams for conducting OC and SC tests on a single phase
transformer. Also explain how the efficiency and voltage resgulation can be
estimated by these tests.
29. Explain in detail about parallel operation of single phase transformers
30. Develop the exact equivalent circuit of a 1- phase transformer. From this, derive the approximate and simplified equivalent circuits of the transformer. State the various assumption made.
31. Give the equivalent circuit of a transformer and define its various parameters.
32. Describe a test on a 1 –phase transformer that gives its core losses. Discuss the determination of the equivalent circuit parameters which can be obtained from this test.
33. Describe a test on a 1 –phase transformer that gives its ohmic losses. Discuss the determination of the equivalent circuit parameters which can be obtained from this test.
34. In an auto-transformer, the power transfer from primary circuit to the secondary circuit is partly by transformer action and partly by direct conduction. Justify this statement.