Page 868 - Basic Electrical Engineering

14. Why should we have high torque by weight ratio of the moving system of an instrument low?
15. Explain the constructional details and principle of the working of a permanent-magnet
moving-coil instrument.
16. Explain the function of deflecting torque, control torque, and damping torque of a moving coil
instrument.
17. Explain eddy current damping and air friction damping.
18. Explain the equation for deflecting torque of a pmmc instrument.
19. Explain why the pmmc-type instrument have a linear scale while moving iron instrument have
square scale?
20. What are the sources of error in pmmc-type instruments? Explain them.
21. Where do we use shunts and multipliers?
22. How do we extend the range of ammeters and voltmeters?
23. Distinguish between a pmmc-type ammeter and a voltmeter.
24. Explain the working principle of attraction-type moving iron instruments.
25. Distinguish between an attraction-type and a repulsion-type moving-iron instrument.
26. Draw the constructional details and explain the working principle of a repulsion-type moving-
iron instrument.
27. Can we use a pmmc-type instrument for measurement of both dc and ac quantities?
28. Can we use a moving-iron instrument for both ac and dc measurements?
29. Explain the constructional details of a dynamometer-type instrument.
30. What are the sources of error in dynamometer-type instruments?
31. Prove that power in an ac circuit is equal to VI cosφ.
32. Explain how a wattmeter reading will be equal to VI cosφ.
33. Explain how error in measurement may occur due to the connections of current coil and
pressure coil of a wattmeter.
34. Explain the constructional details and working principal of a single-phase induction-type
energy meter.
35. What do you mean by creeping of an energy meter? How can creeping be eliminated?
36. Explain the use CT and PT .
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37. State the advantages of CT and PT.
38. What is meant by ratio error and phase angle error of a current transformer?
39. Explain the basic principle of working of a multimeter.
40. Show how a megger is used for measurement of insulation resistance.

B. Numerical Problems

41. A PMMC-type instrument gives a full-scale deflection of 1 mA and has an internal resistance
of 100 Ω. Calculate the values of shunts required to extend the range of the instrument to 0–
100 mA and 0–500 mA.

[Ans 1.01 Ω; 0.02 Ω]

42. Calculate the value of multiplier resistance so that an instrument of internal resistance of
100Ωand full-scale deflection current of 1 mA can be converted into a 0–100 V range