Quiz: Electrical Engineering 24 Apr 2021

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

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

Q1. A minimum-phase system with no zeros has a phase angle of -270⁰ at gain crossover frequency. The system is……
(a) Stable
(b) Unstable
(c) Marginally stable
(d) Conditionally stable

Q2. Humans are more vulnerable to electric shock current at?
(a) 40 Hz
(b) 45 Hz
(c) 48 Hz
(d) 50 Hz

Q3. A 3-phase synchronous motor draws 200 A from the line at unity power factor at rated load. Considering the same line voltage and load, the line current at a power factor of 0.5 lagging is:
(a) 200 A
(b) 100 A
(c) 400 A
(d) 150 A

Q4. The function of starter in electric motor is
(a) Starting purpose
(b) Minimize high starting current
(c) Avoid the over load
(d) All of the above

Q5. Interpole winding is connected in
(a) Parallel with armature
(b) Series with armature
(c) Series with main poles
(d) Parallel with main poles

Q6. A 4-pole, 50 Hz induction motor operates at 4 % slip. The frequency of emf induced in the rotor will be
(a) 50 Hz
(b) 25 Hz
(c) 2 Hz
(d) 2.5 Hz

Q7. Slip of an induction motor ……………. when the rotor starts gaining speed.
(a) Increases
(b) Decreases
(c) Remains same
(d) Increase or decrease

Q8. Current chopping usually occurs in
(a) Oil circuit breaker
(b) Minimum OCB
(c) MCB
(d) Air blast circuit breaker

Q9. A coil of resistance 20 Ohms and inductance 10 mH is connected in a series with a 0.01 mF capacitor. Find the value of current (in A) at resonance, if the circuit is connected across 20 V supply.
(a) 10
(b) 1
(c) 14.14
(d) 1.414

Q10. In an R-C-L series circuit, during resonance, the impedance will be ………….
(a) Zero
(b) Minimum
(c) Maximum
(d) None of these

SOLUTIONS
S1. Ans.(b)
Sol. Phase angle at gain-crossover frequency, ϕ=-270⁰
∴PM=180-|ϕ|=180-|-270|=-90⁰
A negative phase margin (PM) indicates that system is unstable.

S2. Ans.(a)
Sol. Lower the frequency, lower will be the impedance of the human body which will allow high current through human body and cause severe effects on human body.
Among given option 40 Hz is the lowest frequency.

S3. Ans.(c)
Sol. P=VI cos⁡ϕ
For same line voltage and load, P=constant
i.e., I cos⁡〖ϕ=constant〗
∴I1 cos⁡ ϕ1=I2 cos⁡ϕ2
∴200×1=I_2×0.5
⇒I2=400 A

S4. Ans.(d)
Sol. starter are used to protect the motor from over load and minimize starting current to a safe value.

S5. Ans.(b)
Sol. Interpole winding is connected in series with armature.

S6. Ans.(C)
Sol. The frequency of emf induced in the rotor: f_r=sf=0.04×50=2.0 Hz

S7. Ans.(b)
Sol. Slip(s)=(Ns-Nr)/Ns
So, when the rotor starts gaining speed, due to increase of rotor speed, slip of an induction
motor decreases.

S8. Ans.(d)
Sol. Current chopping usually occurs in Air blast circuit breaker.
Current Chopping in circuit breaker is defined as a phenomenon in which current is forcibly interrupted before the natural current zero.

S9. Ans.(b)
Sol. In case of series Resonance Z = R
and I = V/Z=V/R = 20/20
= 1 A

S10. Ans.(b)
Sol. In an RLC series resonant circuit,
Z = minimum and denoted by Z_min=R
i.e., In Case of series Resonance.
(i) X_L=X_C
(ii) Z = minimum & Z = R
(iii) Pf = unity (i.e., Resistive circuit)

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