Quiz: Electrical Engineering 30 Sep 2020

Quiz: Electrical Engineering
Exam: UPPSC-AE
Topic: Miscellaneous

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

Q1. In the shown figure

1. If both ammeter and voltmeter are ideal, then reading of ammeter is zero, reading of voltmeter is 10 V
2. If both ammeter and voltmeter are ideal, then reading of ammeter is 10 A, reading of voltmeter is 0 V
3. If ammeter is non ideal, voltmeter is ideal, then reading of ammeter is 0 A, reading of voltmeter is 10 V
4. If ammeter is ideal, voltmeter is non ideal, then reading of ammeter is less than 10 A, reading of voltmeter is less than 10 V
Which of these statements are correct?
(a) 1,2 and 3 only
(b) 2,3 and 4 only
(c) 1, 3 and 4 only
(d) All of the above

Q2. A galvanometer having a coil resistance of 100 Ω gives a full-scale deflection, when a current of 1 mA is passed through it. The value of the resistance, which can convert this galvanometer into ammeter giving a full-scale deflection for a current of 10 A is:
(a) 2 Ω
(b) 0.1 Ω
(c) 3 Ω
(d) 0.01 Ω

Q3. The Charging reactance of 50 km length of line is 1500 ohm. What will be the charging reactance for 100 km length of the line?
(a) 600 Ω
(b) 3000 Ω
(c) 750 Ω
(d) 1500 Ω

Q4. Turn off time of an SCR is measured from the instant:
(a) Anode current becomes zero
(b) Anode voltage becomes zero
(c) Gate current becomes zero
(d) Anode voltage and current become zero at the same time

Q5. The number of address lines required to address a memory size of 16K is:
(a) 16
(b) 14
(c) 12
(d) 15

Q6. Skin effect occurs when a conductor carries current at………frequencies.
(a) Medium
(b) High
(c) Low
(d) Very low

Q7. The distribution or winding factor is defined in an alternator in terms of
(a) Power factor
(b) Electromotive force
(c) Power
(d) Current

Q8. A 3 phase, 50 Hz induction motor draws 50 kW from the mains. If the stator losses are 2 kW and rotor emf oscillates at 100 oscillations per minute the rotor copper losses will be:
(a) 1.8 kW
(b) 1.2 kW
(c) 1.9 kW
(d) 1.6 kW

Q9. Slip of an induction machine is 0.04 and the stator supply frequency is 150 Hz. What will be the frequency of the rotor induced emf?
(a) 0.06 Hz
(b) 6 Hz
(c) 16 Hz
(d) 4 Hz

Q10. The crawling in an induction motor is caused by
(a) Improper design of the machine
(b) Low voltage supply
(c) High loads
(d) Harmonics developed in the motor

SOLUTIONS
S1. Ans.(c)
Sol. If both are ideal, voltmeter has infinite resistance and hence, no current flows through the circuit. Thus, ammeter will read zero and voltmeter will read 10V.
If ammeter is non ideal but voltmeter is ideal, current through circuit will be 0 and hence ammeter will read 0A and voltmeter 10V.
If voltmeter is non ideal, finite current will flow through the circuit and hence there will be some voltage drop across the resistor. Therefore, Voltmeter will read less than 10V. Due to finite resistance of voltmeter, current will be less than 10A and hence ammeter will read less than 10A.

S2. Ans.(d)
Sol. Galvanometer is a very sensitive instrument therefore it cannot measure heavy currents. In order to convert a Galvanometer into an Ammeter, a very low shunt resistance is connected in parallel to Galvanometer. Value of shunt is so adjusted that most of the current passes through the shunt.
If resistance of galvanometer is Rg and it gives full-scale deflection when current Ig is passed through it. Then,
V = Ig Rg
V = 1 mA × 100 Ω
Let a shunt of resistance (Rs) is connected in parallel to galvanometer. If total current through the circuit is I.
I = 10 A = Is + Ig
V = Is Rs = (I – Ig) Rs
(I – Ig) Rs = Ig Rg
(10 – 10-3) Rs = 100 × 10-3
Rs ≈ 0.01 Ω

S3. Ans.(c)
Sol. charging reactnce(X_C ) α 1/L
∴X_C2/X_C1 =L1/L2
⇒X_C2=1500×50/100= 750 Ω

S4. Ans.(a)
Sol. Turn off time of an SCR is measured from the instant when anode current becomes zero.

S5. Ans.(b)
Sol. memory size=16K=16×1024 bytes=2^4×2^10=2^14 bytes
So, the number of address lines required to address a memory size of 16K is 14.

S6. Ans.(b)
Sol. The non-uniform distribution of electric current over the surface or skin of the conductor carrying a.c is called the skin effect. In other words, the concentration of charge is more near the surface as compared to the core of the conductor.
Factors affecting skin effect
Frequency – Skin effect increases with the increase in frequency.
Diameter – It increases with the increase in diameter of the conductor.
The shape of the conductor – Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
Type of material – Skin effect increase with the increase in the permeability of the material (Permeability is the ability of material to support the formation of the magnetic field).

S7. Ans.(b)
Sol. The distribution or winding factor is defined in an alternator in terms of electromotive force(emf).
Kd=(phasor sum of coil emfs)/(arithmetic sum of coil emf)

S8. Ans.(d)
Sol. Slip (s)= fr/fs =(100/60)/50
= 1/30
Rotor input = stator input – stator loss
= 50-2 = 48 kw
Rotor copper loss = S × Rotor input
= 1/30×48
= 1.6 kw

S9. Ans.(b)
Sol. Rotor frequency (fr)= Sf
= 0.04×150
= 6 HZ

S10. Ans.(d)
Sol. Crawling is associated with squirrel cage induction motor due to which motor has a tendency to run at very low speed compared to its synchronous speed. The fundamental runs at synchronous speed and harmonics 3^rd,5^th,7^th may rotate in backward or forward direction at Ns/3,Ns/5,Ns/7 speed respectively.
Hence harmonics torque developed along with fundamental.
5^th Harmonics = 120°×5= 600°= -120°, so will run in backward direction with speed (Ns/S) and 7th Harmonics = 120×7 = 840°= +120°, so will run in forward direction with speed Ns/7.

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