Ques 101. An inductor is supplied from a sinusoidal voltage source. The magnetic field energy in the inductor changes from the peak value to the minimum value in 10 msec. The supply frequency is (SSC- 2013)
50 Hz
25 Hz
1 Hz
100 Hz
Answer.2. 25 Hz
Explanation:
For a quarter part of wave pulse energy going down to the maximum value to minimum value. i,e
Ques 102. The magnetic field energy in an inductor changes from maximum value to minimum value in 5 msec when connected to an ac source. The frequency of the source (SSC- 2013)
500
200
50
20
Answer.3. 50
Explanation: For a quarter part of wave pulse energy going down to the maximum value to minimum value. i,e T/4 = 5ms T =20ms f = 1/T =1/20ms = 50Hz
Ques 103. Two coupled coils, connected in series, have an equivalent inductance of 16 mH or 8 mH depending on the connection. The mutual inductance between the coils is (SSC- 2013)
12 mH
8 √2 mH
4 mH
2 mH
Answer.4. 2mH
Explanation:
The overall inductance of 2 coil L1 and L2 connected in series with mutual inductance aiding self-inductance L1 with mutual inductance opposing self-inductance L2 then the mutual inductance M is given as
1/2 (L1 – L2)
1/2 (16 – 8)
= 2mH
Ques 104. Which one of the following is a valid value of the coefficient of coupling between two inductors? (SSC- 2013)
1.414
0.9
1.732
17.32
Answer.2. 0.9
Explanation:
The value of the coefficient of coupling is always greater than 0 and less than 1, or 0% and 100% respectively. A coefficient of coupling of 0 would represent no coupling, and 1 would represent perfect coupling.
Ques 105. Given two coupled inductors L 1 and L 2 having their mutual inductance M. The relationship among them must satisfy (SSC-2012)
M > L1L2
M ≤ L1L2
M = L1L2
M > (L1 + L2)/2
Answer.2. M ≤ L1L2
Expalanation:-
The expression for mutual inductance
M = K√L1L2
Now the value of the coefficient of coupling K lies between 0 and 1 i.e Mutual inductance is maximum when K = 1 and mutual inductance is zero when K = 0
So K ≤ 1
∴ M ≤ √L1L2
Ques 106. If the length of a bar of magnetic material is increased by 20% and the cross-sectional area is decreased by 20% then the reluctance is (SSC-2012)
Increased by 50%
Decreased by 33%
Increased by 67%
Remaining same
Answer.1. Increased by 50%
Explanation:-
The reluctance of a uniform magnetic circuit can be calculated as:
S = L/μA
S2/S1 = A1 L2/A2 L1
Since length l2 of the bar is increased by 20% = 120/100 = 1.2 i.e
L2 = 1.2L1
And area is decreased by 20% = 80/100 = 0.8 i.e
A2 = 0.8A1
Ques 107. Two coupled inductors L1 = 0.2 H and L2 = 0.8 H, have a coefficient of coupling K = 0.8, The mutual inductance M is (SSC-2012)
0.16 H
0.02 H
0.32 H
0.24 H
Answer.3. 0.32 H
Expalantion:-
The expression for mutual inductance
Ques 108. A coil with a certain number of turns has a specified time constant. If the number of turns is doubled, its time constant would (SSC-2012)
Become doubled
Get halved
Remain Unaffected
Become Fourfold
Answer.4. Become Fourfold
Explanation:-
The self-inductance of a solenoid is given as
L = μN2A/I = L1
where
N is the number of turns of the solenoid
A is the area of each turn of the coil
l is the length of the solenoid
and μ is the permeability constant
so, if the number of turns was to be doubled the self-inductance would be
L2 = u (2N)2A/l
or
L2 = 4L1
it would be quadrupled or increase fourfold.
Ques 109. The iron loss per unit frequency in a ferromagnetic core, when plotted against frequency, is a (SSC-2012)
Constant
Straight-line with positive slope
Straight-line with a negative slope
Parabola
Answer.2. Straight-line with positive slope
Explanation:-
Both hysteresis loss and eddy current loss give rise to heat in a magnetic circuit. The two losses are usually taken together and are called ‘iron loss’.
Pi = PE + PH
Pi = f2B2max + f B1.6max
Iron losses thus vary with both frequency and magnetic flux density. The power transformer is likely to be fed at the constant frequency and at the constant voltage, which means that magnetic flux density is almost constant. Thus iron losses in a transformer are constant from no load to full load.
Ques 110. The mutual inductance between two closely coupled coils is 1 H. If the turns of one coil is decreased to half and those of the other is doubled, the new value of the mutual inductance would be (SSC-2012)