MCQs on Electrical Machines


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01․ Which of the following is not a part of transformer?
Commutator.
Radiator.
Tap changer.
Conservator tank.

Commutator is a part of DC machine.

02․ Maximum allowable acidity level of power transformer insulating oil is generally taken as
1 mg of KOH / gm.
0.5 mg of KOH / gm.
0.3 mg of KOH / gm.
2 mg of KOH / gm.

As per IS 1866 of 2000 the maximum allowable acidity level of transformer oil is 0.3 mg of KOH / gm.

03․ Maximum allowable tan - delta (tanδ) of dielectric dispassion factor of power transformer insulating oil is generally taken as
0.5 at 90°C.
2 at 90°C.
3 at 90°C.
1 at 90°C.

As per IS 1866 of 2000 the maximum allowable tan - delta ( tanδ ) of dielectric dispassion factor of power transformer insulating oil is 1 at 90°C.

04․ Minimum allowable resistivity of power transformer insulating oil is generally taken as
2 X 1012 Ω - cm at 27°C and 0.2 X 1012
1 X 1012 Ω - cm at 27°C and 0.1 X 1012
3 X 1012 Ω - cm at 27°C and 0.3 X 1012
4 X 1012 Ω - cm at 27°C and 0.4 X 1012

As per IS 1866 of 2000 the minimum allowable resistivity of transformer oil is 1 X 1012 Ω - cm at 27°C and 0.1 X 1012 Ω - cm at 90°C.

05․ Maximum allowable moisture content of power transformer insulating oil is generally taken as
45 ppm.
35 ppm.
25 ppm.
20 ppm.

As per IS 1866 of 2000 the maximum allowable moisture content of power transformer insulating oil is 35 ppm.

06․ What does the use the of height flux density value in transformer design lead to?
Increase in weight/KVA.
Decrease in weight/KVA.
Reduced copper loss.
Reduced iron loss.

So for a constant voltage input φfT = constant. As T = constant and f = constant so φ  = constant of BmA = Constant. So if Bm increases, area of cross section of core decreases. Hence weight/KVA decreases.

07․ Dissolve gas analysis of transformer oil is carried out to determine
condition of transformer oil.
condition of transformer.
condition of bushings.
condition of bushing stud.

Whenever there is any thermal fault occurs inside the transformer the oil gets decomposed and produces different gases which are dissolved in transformer insulating oil. By quantitative analyzing these dissolved gases one can predict nature of internal fault of transformer and also the internal condition of transformer.

08․ Fufuraldehyde analysis is done to determine the
condition of paper insulation in transformer.
condition of oil in transformer.
condition of lamination on CRGO steel sheet.
condition of cork gaskets used in transformer.

When oil soaked paper insulation in transformer is damaged by heat, some unique oil soluble compounds are realized and dissolved in the oil. These compounds belong to the Furfuraldehyde group. Hence, Fufuraldehyde analysis is done to determine the condition of paper insulation.

09․ Which of the following voltage regulator is preferred for providing large values of load current?
Zener diode shunt regulator.
Transistor series regulator.
Transistor shunt regulator.
None of the above.

Under construction.

10․ In dc machines, the armature windings are placed on the rotor because of the necessity for
electromechanical energy conversion
generation of voltage
commutation
development of torque

Required output from a DC machine is DC which can be collected from the armature. So armature should be placed on the rotor so that commutation action (ac to dc or dc to ac) take place.

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