DFCCIL’21 EE: Daily Practices Quiz 13-July-2021

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

Q1. A 200/400 V ratio transformer has a secondary winding resistance of 0.5 Ω. The secondary winding resistance as referred to primary is about:
(a) 0.5 Ω
(b) 0.125 Ω
(c) 2 Ω
(d) 1 Ω

Q2. In a capacitor start single phase induction motor, the current in the
(a) Supply lines lead the voltage
(b) Starting winding lags the voltage
(c) Main winding leads the voltage
(d) Starting winding leads the voltage

Q3. An induction motor when started on load, it does not accelerate up to full speed but runs at 1/7th of the rated speed. The motor is said to be
(a) Locking
(b) Plugging
(c) Crawling
(d) Cogging

Q4. If X_L= 100 Ω and R = 100 Ω in a series RL circuit, then the magnitude of total impedance will be:
(a) 141.4 Ω
(b) 14.14 Ω
(c) 100 Ω
(d) 200 Ω

Q5. Which is the most severe fault?
(a) phase to phase fault
(b) phase to earth fault
(c) Three phases to ground fault
(d) Phase to phase to earth fault

Q6. Which of the following fault occurs least frequently in a power system?
(a) Single line to ground (L – G) fault
(b) Double line faults
(c) LLG faults
(d) LLLG faults

SOLUTIONS
S1. Ans.(b)
Sol. Turn ratio=N_1/N_2 =V_1/V_2 =200/400=1/2
The secondary winding resistance as referred to primary=R_2^’=R_2×(N_1/N_2 )^2=0.5×(1/2)^2=0.5/4=0.125 Ω.

S2. Ans.(d)
Sol. In the capacitor start inductor motor two winding is used, the main winding and the starting winding. With starting winding, we connect a capacitor so the current flowing in the capacitor leads the applied voltage by some angle 𝟇.

S3. Ans. (c)
Sol. Running stably at a low speed around 1/7th of normal motor speed is known as crawling.

S4. Ans.(a)
Sol. Impedance(Z)=√(R^2+X_L^2 )=√((100)^2+(100)^2 )=√2×100=141.4 Ω

S5. Ans.(c)
Sol. LLLG or three-phase to ground fault is most severe and in this short-circuit current value is maximum.
The severity of the fault.
LLLG > LLL > LLG > LL > LG

S6. Ans.(d)
Sol. frequency of fault:
SLG > LL > LLG > LLL > LLLG

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