Quiz: Electrical Engineering 22 Mar 2021

Quiz: Electrical Engineering
Exam: SSC JE
Topic: Miscellaneous

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

Q1. The primary and secondary windings of a transformer are wound on the top of each other in order to reduce….
(a) Iron losses
(b) Copper losses
(c) Winding resistance
(d) Leakage reactance

Q2. If a dynamometer type wattmeter is connected in an ac circuit, the power indicated by the wattmeter will be:
(a) Average power
(b) Peak power
(c) Instantaneous power
(d) Volt ampere product

Q3. Three wattmeter method of power measurement can be used to measure power in
(a) Balanced circuits
(b) Unbalanced circuits
(c) Both balanced and unbalanced circuits
(d) None of the above

Q4. A Lissajous pattern on an oscilloscope has 5 horizontal tangencies and 2 vertical tangencies. The frequency of the horizontal inputs is 100 Hz. The frequency of the vertical input will be:
(a) 400 Hz
(b) 4000 Hz
(c) 250 Hz
(d) 5000 Hz

Q5. The reactive power generated by a synchronous alternator can be controlled by
(a) Changing the prime mover input
(b) Changing the terminal voltage
(c) Changing the field excitation
(d) Changing the alternator speed

Q6. For measuring current at high frequency, we should use
(a) moving iron instrument
(b) electrostatic instrument
(c) thermocouple instrument
(d) none of the above

Q7. The resistance in the circuit of the moving coil of a dynamometer wattmeter
should be
(a) almost zero
(b) low
(c) high
(d) none of the above

Q8. A synchronous generator has its effective internal impedance Z_s=10 Ω and the resistance ra=1 Ω. Its generated voltage emf Ef and the terminal voltage Vt are both 500 V. The maximum power output is
(a) 22500 W
(b) 30000 W
(c) 45500 W
(d) 50000 W

Q9. In a synchronous machine if the field flux is ahead of the armature field flux in the direction of rotation, the machine operating is
(a) Synchronous generator
(b) Synchronous motor
(c) Asynchronous generator
(d) Asynchronous motor

Q10. A squirrel cage induction motor having a rated slip of 4 % on full load has a starting torque same as that of full load torque. The starting current is
(a) Five times full-load current
(b) Equal to full-load current
(c) Twice the full-load current
(d) Four times full-load current

SOLUTIONS
S1. Ans.(d)
Sol. The primary and secondary windings of a transformer are wound on the top of each other in order to reduce leakage reactance.

S2. Ans.(a)
Sol. If a dynamometer type wattmeter is connected in an ac circuit, the power indicated by the wattmeter will be average power.

S3. Ans.(c)
Sol. Three wattmeter method of power measurement can be used to measure power in both balanced and unbalanced circuits.

S4. Ans.(c)
Sol. f_V/f_H =(number of horizontal tangencies)/(number of vertical tangencies)=5/2
∴f_V=5/2×100=250 Hz.

S5. Ans.(c)
Sol. Changing the field excitation is governed reactive power.

S6. Ans.(c)
Sol. Thermo couple instruments can be used for both AC as well as DC applications. These instruments are very useful and accurate to measure current and voltages at very high frequencies.

S7. Ans.(c)
Sol. Moving coil is voltage coil and must have high resistance.
Generally, a high resistance is connected in series with the moving coil to limit the current through it. By limiting the current, the moving coil is made lightweight, which in turn increases the sensitivity of the instrument.

S8. Ans.(a)
Sol. Pm=(Ef Vt)/Zs -(Ef^2)/(Zs^2 )×ra=(500)^2/10 (1-1/10)=22500 W

S9. Ans.(a)
Sol. synchronous generator

S10. Ans.(d)
Sol. Tst/Tfl =(Ist/Ifl )^2 sfl
ACQ: Tst=T_fl
∴I_st/I_fl =√(1/sfl =1/(4/((100)))=25)=5

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