Quiz: Electrical Engineering 23 June 2021

Quiz: Electrical Engineering
Exam: DFCCIL
Topic: Miscellaneous

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. In the salient pole of 3-phase synchronous machine the rotor will have
(a) Larger diameter, shorter axial length
(b) Smaller diameter, larger axial length
(c) Larger diameter, larger axial length
(d) Smaller diameter, shorter axial length

Q2. In a two-value capacitor motor, the capacitor used for running purposes is
(a) Air capacitor
(b) Paper spaced oil filled type
(c) Ceramic type
(d) A.c electrolyte type

Q3. A meter has a square law scale. For 2 A current the deflection in 90°. For a deflection of 45°, the current is
(a) 1 A
(b) 1.5 A
(c) 1.414 A
(d) 1.61 A

Q4. In which mode the JFET can operate?
(a) Depletion mode only
(b) Enhancement mode only
(c) Saturation mode only
(d) Noise mode only

Q5. What is the use of higher flux density in the transformer design?
(a) For increasing the weight/KVA
(b) For decreasing the weight/KVA
(c) For reducing iron losses
(d) For improving insulation

Q6. A series RLC circuit has a resonant frequency of 1 kHz and a quality factor Q = 100. If each of the R, L and C is doubled from its original value, the new Q of the circuit is:
(a) 25
(b) 50
(c) 100
(d) 200

Q7. The current through 2 Ω branch is:

(a) 5/13 A
(b) 3/13 A
(c) 4/13 A
(d) None of these

Q8. A negative sequence relay is commonly used to protect
(a) An alternator
(b) A transformer
(c) A transmission-line
(d) A bus-bar

Q9. For enhancing the power transmission in along EHV transmission line, the most preferred method is to connect a
(a) Series inductive compensator in the line
(b) Shunt inductive compensator at the receiving end
(c) Series capacitive compensator in the line
(d) Shunt capacitive compensator at the sending end

Q10. An open loop system represented by the transfer function

(a) Stable and of minimum phase type
(b) Stable and of non-minimum phase type
(c) Unstable and of minimum phase type
(d) Unstable and of non-minimum phase type

SOLUTIONS

S1. Ans.(a)
Sol. 3-phase synchronous machine rotor construction:
• Salient pole rotor: larger diameter, shorter axial length
• Cylindrical pole rotor: smaller diameter, larger axial length

S2. Ans.(b)
Sol. Two-value capacitor motor also known as capacitor start capacitor run induction motor.
In a two-value capacitor motor:
At the starting, the two capacitors are connected in parallel.
Starting capacitor is short time rated and is almost electrolytic.
The run capacitor is long time rated and is made of oil filled paper.

S3. Ans.(c)
Sol. For square scale: Ө α I^2
∴(I_2^2)/(I_1^2 )=Ө_2/Ө_1
⇒I_2=√(Ө_2/Ө_1 )×I_1=(45/90)^(1/2)×2=√2=1.414 A

S4. Ans.(a)
Sol. JFET only works in the depletion mode, whereas MOSFETs have depletion mode and enhancement mode.

S5. Ans.(b)
Sol. The value of flux density in the core determines the core area.
High value of flux density gives smaller core area, so saving in iron cost. Also, small core area provides reduced mean turn of winding which gives reduction in copper cost. But
higher flux density increases iron losses resulting high temperature rise.
i.e., B_m α 1/A

S8. Ans.(a)
Sol. A negative sequence relay is commonly used to protect an alternator.
A relay which protects the electrical system from negative sequence component is called a negative sequence relay or unbalance phase relay. The negative sequence relay protects the generator(alternator) and motor from the unbalanced load which mainly occurs because of the phase-to-phase faults.

S9. Ans.(c)
Sol. For enhancing the power transmission in along EHV transmission line, the most preferred method is to connect a Series capacitive compensator in the line.
As we know, P α 1/X, where; X=X_L-X_C for series capacitive compensation

S10. Ans.(b)
Sol.
Open loop system stability is depending only on pole locations
All poles are in LHS ⇒ system is stable.
There is one zero on right half of s-plane so system is non – minimum phase

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