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

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

Q1. When one transformer is removed from △-△ bank of a 30 KVA transformer, the capacity of the resulting 3-phase transformer corresponding input line voltage?
(a) 17.3 KVA
(b) 20 KVA
(c) 11.5 KVA
(d) 25.9 KVA

Q2. In an induction motor, what is the ratio of rotor copper loss and rotor input?
(a) s
(b) 1/s
(c) (1-s)
(d) s/(1-s)

Q3. The AC bridge used for measurement of dielectric loss of capacitor is
(a) Anderson bridge
(b) Schering bridge
(c) Wien bridge
(d) Hay’s bridge

Q4. In a transformer, Low voltage windings are placed nearer to the core in the case of concentric windings because
(a) it reduces hysteresis loss
(b) it reduces eddy current loss
(c) it reduces insulation requirement
(d) it reduces leakage fluxes

Q5. Reactance relay is used for protection in:
(a) Long transmission lines
(b) Short transmission lines
(c) Medium transmission lines
(d) Both long transmission lines and short transmission lines

Q6. FET is a ___________ controlled device.
(a) current
(b) resistance
(c) impedance
(d) voltage

SOLUTIONS
S1. Ans.(a)
Sol. Capacity of V-V connection=1/√3×rating of △-△connection
=1/√3×30=17.32 KVA

S2. Ans.(a)
Sol. The rotor input power P2, rotor copper loss Pc, gross mechanical power developed Pm and slip has below-mentioned relation.
P_2: P_c : P_m = 1∶ s∶ (1-s)
Using the formula,
The total mechanical power is developed in the rotor is equal to the (1-s) times of the rotor input.
The total copper loss is equal to the slip s times of the rotor input.
Total mechanical power is equal to the difference between rotor input to rotor copper loss.

S3. Ans.(b)
Sol. Schering bridge is used for measurement of dielectric loss of capacitor.

S4. Ans.(c)
Sol. In concentric arrangement of placing of HV and LV winding, less insulation is required when LV winding is placed near the core which is at ground potential. The HV winding is placed around the LV.
If high voltage winding is placed near the core, the more insulation will be required and the more insulation led to increase in the size and cost of the transformer.

S5. Ans.(b)
Sol. Relay used for different configuration of transmission lines……
Short transmission line: reactance relay
Medium transmission line: impedance relay
Long transmission line: mho relay

S6. Ans.(d)
Sol. Properties of FET: –
(i) Unipolar device
(ii) Voltage controlled
(iii) current flow due to only majority charge carries
(iv) Zero offset voltage, so better than chopper.

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