Quiz: Electrical Engineering 21 June 2021

Quiz: Electrical Engineering
Exam: AE/JE
Topic: Miscellaneous

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

Q1. A voltage source having an open-circuit voltage of 150 V and internal resistance of 75 Ω, is equivalent to a current source of
(a) 2 A in parallel with 75 Ω
(b) 2 A in series with 75 Ω
(c) 2 A in series with 37.5 Ω
(d) 1 A in parallel with 150 Ω

Q2. …………. opposes the change in circuit voltage.
(a) Resistance
(b) Inductance
(c) Capacitance
(d) Inductive reactance

Q3. Which one of the following is the best method of measurement of temperature of hot bodies radiating energy in visible spectrum?
(a) Thermocouple
(b) Barometer
(c) Thermopile
(d) Optical pyrometer

Q4. The brake test for the determination of efficiency of a DC machine is………….
(a) A regenerative method
(b) A direct method
(c) An indirect method
(d) None of the above

Q5. The results of a ‘slip test’ for determining direct-axis(X_d) and quadrature-axis(X_q) reactance of a star-connected, salient pole alternator is given below:
Phase values: V_max=100 V; V_min=96 V; I_max=12 A; I_min=10 A.
Hence the two reactances will be:
(a) Xd=8.33 Ω and Xq=9.6 Ω
(b) Xd=9.6 Ω and Xq=8.33 Ω
(c) Xd=10 Ω and Xq=8 Ω
(d) Xd=8 Ω and Xq=10 Ω

Q6. A 100 MVA, 11 KV, 3-phase, 50 Hz, 8-pole synchronous generator has an inertia constant H equal to 4 seconds. The stored energy in the rotor of the generator at synchronous speed will be:
(a) 100 MJ
(b) 12.5 MJ
(c) 800 MJ
(d) 400 MJ

Q7. For a 15-bus power system with a 3-voltage controlled bus, the size of the Jacobian matrix is
(a) 11 × 11
(b) 12 × 12
(c) 22 × 22
(d) 25 × 25

Q8. If the Op-Amp in the figure is ideal, the output voltage Vo will be equal to

(a) 1 V
(b) 6 V
(c) 14 V
(d) 17 V

Q9. A dc series motor drawing an armature current of I_a is operating under saturated magnetic conditions, torque developed in the motor is proportional to
(a) 1/I_a
(b) 1/I_a^2
(c) I_a^2
(d) I_a

Q10. Fusing factor of fuse is always
(a) More than 1
(b) Less than 1
(c) Infinity
(d) Zero

SOLUTIONS

S1. Ans.(a)
Sol.
Using source transformation (i.e., voltage source to current source transformation)
Isc=Voc/Ri =150/75=2 A
So, given circuit is equivalent to a current source of 2 A in parallel with 75 Ω.

S2. Ans.(c)
Sol.
Capacitor resists a sudden change in the voltage.

S3. Ans.(d)
Sol.
When temperature being measured is high and there is no physical contact with the process to be measured, the thermal radiation methods like optical pyrometer are used.

S4. Ans.(b)
Sol.

Brake test is also known as direct method of testing of dc machines.
It is suitable for small dc machines.

S5. Ans.(c)
Sol.
From slip test: X_d=V_max/I_min =100/10=10 Ω
And X_q=V_min/I_max =96/12=8 Ω

S6. Ans.(d)
Sol.
Inertia constant(H)=(stored KE in MJ at synchronous speed)/(machine rating in MVA)
∴Stored energy=4×100=400 MJ

S7. Ans.(d)
Sol.
Number of bus(n)=15
Voltage controlled bus(m)=3
∴size of Jacobian matrix=(2n-m-2)×(2n-m-2)=25×25

S9. Ans.(d)
Sol.
We know Torque Tα ΦIa, As the motor is operating under saturation condition so torque will be independent of flux i.e., T α Ia.

S10. Ans.(a)
Sol.
Fusing factor=(minimum fusing current)/(current rating of fuse)
Fusing factor of fuse is always >1.

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