DFCCIL’21 EE: Daily Practices Quiz 19-July-2021

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

Q1. The short circuit test in a transformer is used to determine……
(a) Eddy current
(b) Hysteresis loss
(c) Copper loss at any load
(d) Iron loss at any load

Q2. Two coils with self-inductances 1 H and 2 H having a mutual inductance of 1 H between them carry currents of 2 A and √2 A respectively. The energy stored in the field, in joules, is
(a) 2(1+√2)
(b) 3(1+√2)
(c) 2(2+√2)
(c) 3(2+√2)

Q3. Ferranti effect happens in a transmission line when line is
(a) short and unloaded
(b) long and unloaded
(c) short and loaded
(d) none of the above

Q4. What is the ratio of starting torque and maximum torque of a 3-phase, 50 Hz, 4 pole induction motor for maximum torque at 1200 rpm?
(a) 0.5
(b) 0.6
(c) 0.421
(d) 0.384

Q5. If the gain of the open-loop system is doubled, the gain margin
(a) Becomes half
(b) Gets doubled
(c) Becomes one-fourth
(d) Is not affected

Q6. Match the following

SOLUTIONS
S1. Ans.(c)
Sol. SC test of transformer:
Used to determine copper loss or ohmic loss
LV side short circuited
Instruments kept on HV side
Also determines equivalent resistance and leakage reactance

S2. Ans.(c)
Sol. ACQ: L_1=1 H,L_2=2 H & M=1 H
& i_1=2 A,i_2=√2 A
Energy stored=[1/2 L_1 i_1^2+1/2 L_2 i_2^2+Mi_1 i_2 ]J
∴E=[1/2×1×4+1/2×2×2+1×2×√2]=4+2√2=2(2+√2) joules

S3. Ans.(b)
Sol.
Ferranti effect occurs for medium and long transmission line.
Due to Ferranti effect, receiving end voltage is more than sending end voltage.ie.V_R>V_S
It occurs at light loaded or no-load condition.

S4. Ans.(d)
Sol. N_s=120f/P=(120×50)/4=1500 rpm
∴s=(N_s-N_r)/N_s =(1500-1200)/1500=0.2
∴T_st/T_m =2/(s+1/s)=2/(0.2+1/0.2)=2/5.2=0.384

S5. Ans.(a)
Sol. Gain margin is calculated at phase cross over frequency. At phase cross-over frequency phase of G(jω) is -180⁰.
GM=1/X
If X becomes doubled then the GM becomes half.

S6. Ans.(c)
Sol. Bridges used for measurement of inductance:
Anderson bridge (Q<1).
Maxwell bridge (1<Q<10).
Hay bridge (Q>10).

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