SSC Junior Engineer Electrical Exam 2010

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1. The value of voltage across the diode in figure given below is?




... Answer is C)
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2. The temperature co-efficient of an intrinsic semiconductor is ?




... Answer is C)
Intrinsic semiconductors
In the intrinsic semiconductor the valence band is full, but the conduction band is empty at very low temperatures. The energy gap between the two bands is so very small that electrons can jump across it by the addition of thermal energy alone or even light energy of a suitable wavelength. Heating the specimen or shining a light on it maybe sufficient to cause electrical conduction. The conductivity increases with temperature as more and more electrons are liberated. Intrinsic semiconductors have negative temperature coefficients of resistance.



Extrinsic semiconductors
These are semiconductors to which a very small amount of impurity has been added, this process is called doping. Doping with an impurity can have quite marked effects on the electrical properties of the material. The addition of one impurity atom in one hundred million will increase the conductivity of germanium by twelve times at 300 K.

3. A 4 pole 1200 rpm DC lap wound generator has 1520 conductors. If the flux per pole is 0.01 weber, the emf of generator is ?




... Answer is B)
EMF Of generator is given by

E = PΦZN/60A

Φ = flux per pole in Weber

P = number of poles

Z = total number of armature conductor

N = rotation speed of the armature in revolution per minute (r.p.m)

A = number of parallel paths (Number of parallel paths in lap winding, A = P)

E = (4 x 0.01 x 1520 x 1200)/(60 x 4)

= 304 V.

4. In a 3-phase induction motor starting torque will be maximum when ?




... Answer is B)
when slip s = R2 / X2, the torque will be maximum and this slip is called maximum slip Sm and it is defined as the ratio of rotor resistance to that of rotor reactance.


NOTE: At starting S = 1, so the maximum starting torque occur when rotor resistance is equal to rotor reactance.

5. The ratio of resistances of a 100 W, 220 V lamp to that of a 100 W, 110 V lamp will be at respective voltages ?




... Answer is A)
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6. Two sinusoidal equations are given as– ?




... Answer is A)
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7. The moderator used in fast breeder reactor is– ?




... Answer is D)
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8. The ratio of the puncture voltage to the flashover voltage of an insulator is– ?




... Answer is D)
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9. Bucholtz relay cannot be used on– ?




... Answer is D)
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10. An ammeter is obtained by shunting a 30 Ω Galvanometer with 30 Ω resistance. What additional shunt should be connected across it to double the range? ?




... Answer is A)
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11. Swamping resistance is used to compensate error due to– ?




... Answer is D)
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12. Which of the following is of high importance in case of induction heating ?




... Answer is B)
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13. If four 10µF capacitors are connected in parallel, the net capacitance is– ?




... Answer is B)
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14. Variation in dc excitation of a synchronous motor causes variation in– ?




... Answer is D)
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15. The earth’s potential is taken as– ?




... Answer is D)
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16. The value of diversity factor is– ?




... Answer is B)
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17. The resistance welding process requires a– ?




... Answer is A)
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18. The tarrif most suitable for large industrial consumers is– ?




... Answer is C)
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19. Advantage of transmitting power at high voltage is – ?




... Answer is D)
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20. A FET is essentially a– ?




... Answer is B)
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21. Differential relays are used to protect the equipment against– ?




... Answer is A)
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22. Skin effect exists only in– ?




... Answer is D)
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23. For a 3-phase, 4-pole, 50 Hz synchronous motor the frequency, no. of poles, and the load torque are all halved. The motor speed will be– ?




... Answer is C)
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24. The making current of 3 -phase breaker with rating 2000 MVA, 33 kV will be– ?




... Answer is D)
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25. A transformer is working at its full load and its efficiency is also maximum. The iron loss is 1000 watts. Then, its copper loss at half of full load will be– ?




... Answer is A)
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26. Dielectric heating is also called– ?




... Answer is A)
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27. Voltage drop is the main consideration while designing a– ?




... Answer is B)
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28. Laboratory wattmeters are– ?




... Answer is D)
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29. An electric motor may give noise due to– ?




... Answer is D)
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30. Which of the following motor is used in household refrigerator ?




... Answer is D)
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31. The lighting arrester is conducted– ?




... Answer is B)
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32. If Rg in the circuit shown in figures 1 is variable between 20 Ω and 80 Ω then maximum power transferred to the load RL will be ?




... Answer is C)
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33. The knowledge of diversity factor helps in computing ?




... Answer is D)
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34. Distribution transformers are designed to have maximum efficiency nearly at– ?




... Answer is B)
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35. Electronic switching are becoming more and more popular because of ?




... Answer is D)
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36. The controlling torque in gravity controlled meter is proportional to ?




... Answer is B)
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37. The condition for a maximum power output from dc motor is ?




... Answer is B)
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38. Time constant of the network shown in figure is ?




... Answer is D)
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39. A 2 kVA transformer has iron loss of 150 W and full load copper loss of 250 W. The maximum efficiency of the transformer will occur when the total loss is ?




... Answer is C)
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40. While starting synchronous motor its field winding should be ?




... Answer is D)
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41. A 4- pole, 3-phase induction motor is running at 4% slip at full load. If the speed of the motor is 750 rpm, the supply frequency is ?




... Answer is B)
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42. An oscillator uses ?




... Answer is A)
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43. Permeance is analogous to ?




... Answer is A)
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44. A wire has a resistance10 Ω. It is stretched by one-tenth of its original length. Then its resistance will be ?




... Answer is B)
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45. Potential transformers are used ?




... Answer is A)
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46. To increase the range of a voltmeter ?




... Answer is C)
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47. A three phase power transformer is provided with star-delta connections. In order to protect against fault, the connection for current transformer should be in ?




... Answer is B)
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48. For active region operation of NPN transistor ?




... Answer is B)
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49. The advantage of electric breakingis ?




... Answer is A)
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50. A single phase motor is made self-starting by the addition of ?




... Answer is B)
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