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DFCCIL’21 EE: Daily Practices Quiz 28-July-2021

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

Q1. A Brake test on DC motor is confined to
(a) Small horse power motors
(b) Variable speed motors
(c) High speed motors
(d) Open frame type motors

Q2. Star-star power transformer is protected by current transformer having
(a) Star/star connections
(b) Delta/delta connection
(c) Star/delta connections
(d) Delta/star connections

Q3. Tellegen’s theorem is based on the principle of law of _________________.
(a) Conservation of charge
(b) Conservation of mass
(c) Conservation of velocity
(d) conservation of energy

Q4. The RMS value of a half – wave rectified alternating current is 20A. its value for full-wave rectification will be: –
(a) 40/π A
(b) 80/π A
(c) 40/√2 A
(d) 40 A

Q5. If x(t)=t^n u(t), then the Laplace transform of x(t) is:
(a) nǃ/s^(n+1)
(b) nǃ/s^n
(c) n/s^(n+1)
(d) nǃ/s^(n-1)

Q6. The input – output relationship of a system is given by
r(t) = (d^2 c(t))/dt² + 3 (dc(t))/dt + 2 c(t)
where r(t) and c(t) are input and output respectively. The transfer function of the system is equal to
(a) 1/((s^2+ s +2))
(b) 1/((s^2 + 3s + 2))
(c) 2/(s^2+3s+2)
(d) 1/((s^2 + 5s + 3))

SOLUTIONS

S1. Ans.(a)
Sol. Brake Test:
It is direct test useful to find efficiency of small DC Motor.

S2. Ans.(b)
Sol. CTs are connected to every phase of the system. In order to nullify the phase shift of the Power transformer (PT), The Current transformers (CT) connected in a reverse configuration. The star-star power transformer is protected by the current transformer having delta/delta connections.

S3. Ans.(d)
Sol. Tellegen’s theory is based on conservation of energy.
ie.∑_(K=1)^n▒〖P_k=0 〗
ie,∑_(K=1)^n▒〖I_k×V_K=0 〗

S4. Ans.(c)
Sol. for HWR: I_rms=I_m/2
⇒ 20 = I_m/2
⇒ I_m=40A
For FWR: I_rms= I_m/√2=40/√2 A

S5. Ans.(a)
Sol. L[t^n u(t)]=nǃ/s^(n+1)

S6. Ans. (b)
Sol. r(t) = (d^2 c(t))/dt² + (3dc(t))/dt + 2c(t)
Taking Laplace transform
R(s) = s² C(s) + 3sC(s) + 2C(s)
(C(s))/(R(s)) = 1/(S^2+3s+2)

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