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DFCCIL’21 EE: Daily Practice Quiz. 20-Sep-2021

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EXAM DATES

26 SEPT TO 30 SEPT

DFCCIL’21 EE: Daily Practices Quiz

DFCCIL’21 EE: Daily Practices Quiz 20-Sep-2021

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

Q1. In V/f control method of an Induction motor…
(a) The maximum torque decreases
(b) The motor speed increases
(c) The maximum torque is constant
(d) The maximum torque increases

Q2. Maximum torque in 3-phase induction motor varies as:
(a) f
(b) 1/f
(c) 1/f^(2 )
(d) 1/f^3

Q3. How many 200 W/220V incandescent lamps connected in series would consume the same total power as a single 100 W/220V incandescent lamp?
(a) 1
(b) 2
(c) 3
(d) Not possible

Q4. The surge impedance of 400 km long over-head transmission line is 300 Ω. For a 200 km length of the same transmission line surge impedance will be
(a) 300 Ω
(b) 150 Ω
(c)75 Ω
(d) 600 Ω

Q5. In a dynamometer type wattmeter, the pressure coil connected across the load terminal is
(a) Highly inductive
(b) Highly capacitive
(c) Highly resistive
(d) Non inductive

Q6. A parallel RLC circuit resonates at a frequency of 2 MHz. At a frequency 2.2 MHz, the nature of circuit impedance will be
(a) Resistive
(b) Capacitive
(c) Inductive
(d) None of the above

SOLUTIONS

S1. Ans.(c)
Sol. In V/f control method, V/f is constant as 𝟇 is constant.
In this method, T_m α(V/f)^2 i.e.T_m=constant.
By varying voltage and frequency, speed can be changed.

S2. Ans.(c)
Sol. T_m α (V/f)^2
⇒T_m α 1/f^2 .

S3. Ans.(b)
Sol. For 200 W/220V incandescent lamp, R=V^2/P=220^2/200=242 Ω
And for 100 W/220V incandescent lamp, R_t=V^2/P=220^2/100=484 Ω
According to question, for series connection: R_t=n×R
⇒484=n×242
∴n=2

S4. Ans.(a)
Sol. The surge impedance is independent of the length of the transmission line. Hence it will be same as before i.e., 300Ω.

S5. Ans.(c)
Sol. In a dynamometer type wattmeter, the pressure coil connected across the load terminal is highly Resistive.

S6. Ans.(b)
Sol. For series resonant circuit, nature of circuit is capacitive below resonant frequency and inductive above resonant frequency and vice-versa for parallel resonant circuit.

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