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DFCCIL’21 EE: Daily Practice Quiz. 11-Sep-2021

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EXAM DATES: 26 SEPT TO 30 SEPT

               DFCCIL’21 EE: Daily Practices Quiz 11-Sep-2021

Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 08 min.

Q1. Inductance of bundled conductor line in comparison to inductance of the line with one conductor per phase is…
(a) More
(b) Less
(c) Not comparable
(d) None of the above

Q2. An over excited synchronous motor will take current at…
(a) Leading pf
(b) Lagging pf
(c) Unity pf
(d) None of the above

Q3. Which of the following is not suitable for the measurement of low resistances?
(a) Ammeter voltmeter method
(b) Potentiometer method
(c) Kelvin’s double bridge method
(d) Wheatstone bridge

Q4. In a communication circuit employed to turn-off an SCR, satisfactory turn-off is obtained when
(a) Circuit time constant > device turn-off time
(b) Circuit time constant < device turn-off time
(c) Circuit turn-off time > device turn-off time
(d) Circuit turn-off time < device turn-off time

Q5. Find the stability for the following transfer functions?
G(s)=10/s(s+2) .
(a) Critically stable
(b) Marginally stable
(c) Stable
(d) Unstable

Q6. Secondary breakdown does not occur in
(a) MOSFET but occurs in BJT
(b) Both MOSFET and BJT
(c) BJT but occurs in MOSFET
(d) None of the above

SOLUTIONS

S1. Ans.(b)
Sol. L=2×10^(-7) ln⁡(GMD/GMR)
L α 1/GMR
By using bundle conductor GMR increases as self GMR increases, inductance of conductor decreases more than single conductor.

S2. Ans.(a)
Sol. An over excited synchronous motor will take current at leading power factor.
For over excited synchronous motor…
E_f cos⁡〖δ>V〗
Reactive power is negative

S3. Ans.(d)
Sol. Wheatstone bridge is used for measurement of medium resistance.

S4. Ans.(c)
Sol. The circuit should not turn-off before making off the thyristor. i.e., Circuit turn-off time > device turn-off time.

S5. Ans.(c)
Sol. characteristics equation: 1+ G(s)H(s)=0.

∴1+10/s(s+2) =0⇒s^2+2s+10=0.

Routh array table:

1 10
2 0
10 0

So. No sign changes. Hence, it is stable.

S6. Ans.(a)
Sol. Secondary breakdown is a failure mode in BJT because it has Negative Temperature Coefficient. its negative resistance (current concentration) occurs under high-voltage and high-current conditions. MOSFET has Positive Temperatures Coefficient so no secondary breakdown in it.

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