Electrical Machine Design MCQ PDF Download

21. CRGOS cannot be used in ………. rotating machines.

(a) small
(b) medium
(c) large
(d) any

22. For under frequency operation the eddy current loss

(a) decreases.
(b) increases.
(c) remains unaltered.
(d) cannot be predicted. [A.M.I.E Sec B. Elec. Design 1993]

23. ………… is extensively used for those portions of magnetic circuit which carry steady flux and need superior mechanical properties.

(a) Stainless steel
(b) Cast iron
(c) Grey cast iron
(d) Aluminium

24. What is suitable shape of plunger magnet to give almost flat force-stroke characteristics ?

(a) Flat faced.
(b) Conical.
(c) Stepped.
(d) Tapered.

25. The most common thickness of silicon steel sheets ranges from

(a) 0.35 mm to 0.5 mm.
(b) 0.1 mm to 0.25 mm.
(c) 0.4 mm to 0.75 mm.
(d) 0.5 mm to 1.0 mm.

26. An ideal insulating material should have low

(a) dielectric strength.
(b) dielectric loss.
(c) insulation resistance.
(d) thermal conductivity.

27. The basic considerations in the design of insulation are

(a) mechanical considerations.
(b) electrical considerations.
(c) thermal consideration.
(d) all of the above.

28. Excessive insulation is harmful to a coil because

(a) it increases the cost.
(b) copper losses are increased.
(c) heat transfer becomes poor.
(d) dielectric strength is reduced.

29. Maximum temperature limit for class B insulation is

(a) 105 °C
(b) 120 °C
(c) 130 °C
(d) 155 °C [A.M.I.E. Sec B. Elec. Design 1993]

30. Maximum temperature limit for class F insulation is

(a) 105 °C
(b) 120 °C
(c) 130 °C
(d) 155 °C [A.M.I.E. Sec B. Elec. Design 1994]

31. The class of insulation employed for large rating high voltage alternators is

(a) B
(b) C
(c) E
(d) F

32. Tan δ (dielectric loss angle) increases with the

(a) presence of moisture.
(b) fall in temperature.
(c) variation in voltage.
(d) all of the above.

33. The heat generated in a transformer is dissipated to surroundings mainly by

(a) conduction.
(b) convection.
(c) radiation.
(d) conduction and convection.

34. Heat convected depends on

(a) gravitational constant.
(b) thermal resistivity, density, specific heat.
(c) temperature difference of heated surface and coolant.
(d) power density.
(e) all of the above.

35. Radiation from a plain surface is generally

(a) 12.5 W/m²/°C
(b) 3.0 W/m²/°C
(c) 6.0 W/m²/°C
(d) 9.0 W/m²/°C [A.M.I.E 2005]

36. In case of air natural cooling, the cooling surface increases as the ………. of the linear dimensions.

(a) square
(b) inverse
(c) cube
(d) inverse square

37. Losses in a machine increases as the ……….. of the linear dimensions of the machine.

(a) inverse
(b) inverse square
(c) cube
(d) square

38. The increase in heat dissipation by air blasts is due to increase in

(a) conduction.
(b) convection.
(c) radiation.
(c) any of these.

39. The time taken by the machine to attain ………. of its final steady temperature rise is called the heating time constant.

(a) 0.5
(b) 0.632
(c) 0.85
(d) none of these.

40. Temperature rise-time curve is a/an

(a) straight line.
(b) parabola.
(c) exponential curve.
(d) hyperbola.