31. A laminated steel ring is wound with 200 turns. When the magnetizing current varies between 5 and 7 A, the magnetic flux varies between 760 and 800 Wb. Calculate the inductance of the coil.
A. 40H
B. 4 H
C. 4000H
D. 0.004 H
Answer: C
From the formula of incremental inductance, we know that:
L = (Change in flux/Change in current) × Number of turns
Substituting the values from the given question, we get L = 4000 H.
32. Calculate the number of turns in an inductor with a ferromagnetic core when the inductance is 4000 H, the current changes from 5A to 7A, and the flux changes from 760 to 800 Wb.
A. 100
B. 200
C. 300
D. 400
Answer: B
From the formula of incremental inductance, we know that:
L = (Change in flux/Change in current) × Number of turns
Substituting the values from the given question, we get N = 200.
33. Calculate the change in current in an inductor having an inductance of 4000H, a number of turns is 200, and the flux changes from 760 to 800 Wb.
A. 2 A
B. 4 A
C. 6 A
D. 8 A
Answer: A
From the formula of incremental inductance, we know that:
L = (Change in flux/Change in current) × Number of turns
Substituting the values from the given question, we get a change in current = 2A.
34. Calculate the initial current in an inductor having an inductance of 4000 H, a number of turns is 200, and the flux changes from 760 to 800 Wb. Current changes to 7A.
A. 10 A
B. 2 A
C. 3 A
D. 5 A
Answer: D
From the formula of incremental inductance, we know that:
L = (Change in flux/Change in current) × Number of turns
Substituting the values from the given question, we get a change in current = 2A.
Change in current = final current − initial current.
2 = 7 − initial current.
Initial current = 5A.
35. Calculate the change in flux of an inductor having an inductance of 4000 H, a number of turns is 200, and the current changes from 5A to 7A.
A. 20 Wb
B. 40 Wb
C. 60 Wb
D. 80 Wb
Answer: B
From the formula of incremental inductance, we know that:
L = (Change in flux/Change in current) × Number of turns
Substituting the values from the given question, we get a change in flux = 40 Wb.
36. Calculate the final flux in an inductor having an inductance of 4000 H, the number of turns is 200, and the current changes from 5A to 7A. The initial flux is 760 Wb.
A. 200 Wb
B. 400 Wb
C. 600 Wb
D. 800 Wb
Answer: D
From the formula of incremental inductance, we know that:
L = (Change in flux/Change in current) × Number of turns
Substituting the values from the given question, we get a change in flux = 40 Wb.
Change in flux = final flux − initial flux.
Thus final flux = 800 Wb.
37. In a pure inductive circuit, the power factor is?
A. Maximum
B. Minimum
C. 0
D. Infinity
Answer: C
In a pure inductive circuit, the current is lagging by 90 degrees from the voltage. The power factor is the cosine of the angle between the voltage and the current.
If the angle between the voltage and the current is 90, then cos 90° = 0. Hence, the power factor is zero.
38. Among the following, which is the right formula for growth in an inductive circuit?
A. VL = V(1 − e − tR/L)
B. VL = (e − tR/L)
C. VL = (1 − e − tR/L)
D. VL = V(e − tR/L)
Answer: D
The correct formula for growth in an inductive circuit is VL = V(e − tR/L). As the time increases, the voltage decreases.
39. The charging time constant of a circuit consisting of an inductor is the time taken for the voltage in the inductor to become __________ % of the initial voltage.
A. 33
B. 63
C. 37
D. 36
Answer: C
We know that: V = V0(1 − e − tR/L).
When time constant = t, we have: V = V0(1 − e − 1) = 0.37 × V0.
Hence the time constant is the time taken for the charge in an inductive circuit to become 0.37 times its initial charge.
40. What is the time constant of an inductive circuit?
A. LR
B. R/L
C. 1/LR
D. L/R
Answer: D
The time constant in an inductive circuit is the time taken for the voltage across the inductor to become 63 percent of its initial value. It is given by Time constant = L/R.
