Quiz: Electrical Engineering 10 Mar 2021

Quiz: Electrical Engineering
Exam: SSC JE
Topic: Miscellaneous

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. If a 3-phase induction motor is running at super synchronous speed for self- excitation then the machine
(a) Draws reactive power from the mains
(b) Draws active power from the mains
(c) Feeds reactive power to the mains
(d) Feeds active power to the mains

Q2. A 3-phase induction motor has a starting torque of 200 Nm when switched on-directly to supply. If an auto-transformer with 50 % tapping is used for starting, the starting torque would be
(a) 50 Nm
(b) 100 Nm
(c) 200 Nm
(d) 400 Nm

Q3. The emf induced per phase in the rotor winding of 3-phase induction motor is 200 V at standstill. Under full-load condition, this emf would be normally
(a) 200 V
(b) 100 V
(c) 8 V
(d) 0.4 V

Q4. A 2KVA transformer has iron loss of 100 watts and full-load copper loss of 150 watts. The maximum efficiency of the transformer would occur when the total loss is-
(a) 300 W
(b) 250 W
(c) 200 W
(d) 175 W

Q5. A 500 KVA transformer has a percentage resistance of 25% and % reactance of 5% when delivering 400 KVA at 0.8 Pf lagging. Its percentage voltage drop is:
(a) 2.4%
(b) 2.5%
(c) 4%
(d) 5%

Q6. A dynamometer type wattmeter consists of __
(a) only potential coil
(b) potential and current coils
(c) only current coil
(d) no coils

Q7. What is a Q meter?
(a) quality meter
(b) quantity meter
(c) instrument
(d) detector

Q8. Which of the following statement is true for MHO relay?
(a) Voltage controlled overcurrent relay
(b) Voltage restrained directional relay
(c) Directional restrained over voltage relay
(d) Directional restrained over current relay

Q9. In residential wiring in practice for earthing the following method is adopted……
(a) Resistance earthing
(b) Neutral earthing
(c) Pipe earthing
(d) Plate earthing

Q10. The current from neutral to ground connection is 12 A. calculate the zero-phase sequence component current in each phase:
(a) 2 A
(b) 4 A
(c) 6 A
(d) None of the above

SOLUTIONS

S1. Ans.(a)
Sol. If a 3-phase induction motor is running at super synchronous speed for self- excitation then:
Slip is negative
It goes in to generating region
To provide the reactive power for the machine, it will draw reactive power from a source.

S2. Ans.(a)
Sol. For 50 % tapping, x=0.5
∴T_auto=x^2×T_DOL=(0.5)^2×200=50 Nm

S3. Ans.(c)
Sol. At standstill, s=1
∴E_2^’=sE2=1×200=200 V
For full-load condition, slip varies from 2% to 5 %.
∴E_2^’=sE2=(0.02 to 0.05)×200
So possible values lie between: 4 V to 10 V.
So, option© is correct.
NOTE: At standstill, induction motor slip s is 1, hence the frequency of induced EMF in the rotor of the induction motor is same as that of supply frequency and reduces with increase in speed (due to the reduction in slip).

S4. Ans.(c)
Sol. → for maximum efficiency
Iron loss = copper loss
Total loss = 2 × iron loss
= 2 × 100
= 200 watts

S5. Ans.(c)
Sol. % voltage drop = (%)Icosϕ/If +(%×)Isinθ/If
For lagging; (+) sign
For leading; (-) sign
[Icosϕ/If ×V/V]=kW/(KVA rating)
[Isinϕ/If ×V/V]=KVAR/(KVA rating)
∵ % voltage drop = (%)kW/(KVA rating)+(%X)KVAR/(KVA rating)
=((2.5)×320)/500+(5×240)/500
= 4%

S6. Ans.(b)
Sol. Dynamometer type wattmeter is used for the measurement of A.C. as well as D.C. power. It consists of a fixed coil forming the current coil whereas the moving coil forms the potential coil.

S7. Ans.(c)
Sol. Q meter is basically an instrument that is used for the measurement of electrical properties of capacitors and coils. It is also used as a laboratory instrument.

S8. Ans.(b)
Sol. MHO relay:
Voltage restrained directional relay
Less affected by power surge
Used for long transmission line

S9. Ans.(d)
Sol. In residential wiring in practice for earthing, plate earthing is adopted.

S10. Ans.(b)
Sol. The zero-phase sequence component current in each phase:I_N/3=12/3=4 A.

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