DMRC JE Electrical Question Paper With Explained Solution – 2016

Ques 61. In a uniform electric field, field lines and equipotentials

  1. Are parallel to one another
  2. Intersect at 45°
  3. Intersect at 30°
  4. Are orthogonal

The equipotential surfaces associated with a uniform electric field consist of a family of planes perpendicular to the field lines.

In the case of the uniform electric field, the field lines are straight and equally spaced. Therefore, equipotential surfaces will be parallel planes at right angles to field lines.

 

Ques 62. During forward blocking state, the SCR has

  1. Low current, medium voltage
  2. Low current, large voltage
  3. Medium current, large voltage
  4. Large current, low voltage

silicon controlled rectifier or semiconductor-controlled rectifier is a four-layer solid-state current-controlling unidirectional device (i.e. can conduct current only in one direction).

The silicon control rectifier (SCR) consists of four layers of semiconductors, which form NPNP or PNPN structures, having three P-N junctions labeled J1, J2 and J3, and three terminals.

The SCR has Two state

  1. High current Low Impedance ON state
  2. Low current High Impedance OFF state

Forward Blocking Mode

The SCR is said to be forward-biased when the anode is made positive with respect to the cathode. Due to this forward bias, the junction J1 and J3 are forward biased and J2 is reverse biased. Hence the forward voltage is to be held by junction J2. A very small current flows from anode to cathode. This current is called forward leakage current. This current is of the order of few milliamperes. In the forward blocking mode, the voltage can be increased lit VBO. When the forward voltage reaches VBO, the SCR turns on.  During Forward Blocking Mode SCR acts as a Resistor.

 

Ques 63. Turn-on time for an SCR can be reduced by using a

  1. Rectangular pulse of high amplitude and narrow width
  2. Rectangular pulse of low amplitude and wide width
  3. Triangular pulse
  4. Trapezoidal pulse

SCR turn on Time is reducing by applying Rectangular Pulse of high amplitude and Narrow width

 

Ques 64. The disruptive critical voltage will

  1. Decrease with the increase of moisture content in the air
  2. Increase with the increase of moisture content in the air
  3. Increase with the decrease of moisture content in the air
  4. Decrease with the decrease of moisture content in the air

The critical disruptive voltage is defined as the minimum phase to the neutral voltage at which corona occurs.

The disruptive critical voltage depends on the following factors:

  • The atmospheric condition
  • The condition of the surface of the conductor
  • The conductor configuration

Dust and dirt. Due to the presence of dust and dirt, less voltage gradient is required for sustained discharge. The critical disruptive voltage is reduced due to dust and dirt and therefore the corona loss is more.

Rain, snow, hail, and fog. Bad weather conditions such as rain, storm, hail, etc. reduce the critical disruptive voltage and thus there is more corona loss. Corona loss is less in a fail weather condition.

The diameter of the conductor: With the increase in the diameter of the conductor the critical disruptive voltage is increased.

 

Ques 65. What type of insulator will be used if the direction of the transmission line is to be changed?

  1. Pin-type
  2. Suspension type
  3. Strain type
  4. Shackle type

Strain type insulators are used for handling the mechanical stress at the angle position of the line i.e Dead end, intermediate anchor tower, corner, sharp curve.

These insulators reduce the excessive tension on the line under such abnormal conditions. For low voltage lines below 11 kV shackle insulators are used but for higher voltages strain insulators are used. Assembly of the suspension insulators is used as a strain insulator. The discs of the strain insulators are in a vertical plane instead of the horizontal plane as in the suspension insulators In case of conditions like the crossing of the river, there is excessive tension on the line. In such a case, two or more string of the insulators is used in Parallel.

 

Ques 66. Which statement is correct

  1. SF6 gas is toxic
  2. SF6 gas is lighter than air
  3. SF6 gas is yellow in color
  4. SF6 gas has the pungent smell

Pure SF6 gas is colorless, odorless, tasteless and non-toxic, it is chemically stable and non-flammable.

SF6 gas, when used in electrical equipment, gets decomposed when electrical discharges occur. During the Arc extinction process SF6 is decomposed into atoms, electrons, and ions. These atomic components do not recombine completely to form the original SF6 gas on cooling. They form low molecular gaseous sulfur fluorides and compounds with the contact metals principally SF4 and SF2, together with a small amount of  S2, F2, S, and F. Some of the decomposition products are toxic and harmful to personnel working with the equipment.

 

Ques 67. One coulomb of electrical charge is contributed by how many electrons?

  1. 6.24 × 1018
  2. 1.6 × 1019
  3. 1019
  4. None of these

The charge possed by the proton is equal to 1.602 × 10-19 whereas the charged possed by the electron is equal to -1.602 × 10-19. The symbol Q often denotes charge and the SI unit of electric charge is the coulomb (C). The charge on one electron is 1.602 × 10-19, so the one-coulomb charge is defined as the charge possessed by the total number of ( 1/ 1.602 × 10-19) electrons i.e. 6.24 x 1018 number of electrons.

 

Ques 68. A dynamic system with input x(t) and output y(t) is represented by the following:

  1. y(t)= ax(t) +bx2(t)
  2. y(t)= ax sin(t)
  3. y(t)= f(x(t),t)
  4. None of these

If the impulse response of the discrete time LTI system is not identically zero then the system is called Dynamic system or system with memory.

The Dynamic system is represented  by
 y(t) = f(x(t),t)

For example, the systems defined below are dynamic or memory systems.
y(t)=x(t-1)
y(t)=x(t)+x(t+2

 

Ques 69. Three equal resistors each equal to R ohm are connected as shown in fig. The equivalent resistance between points A and B is:

  1. R
  2. 3R
  3. 2R/3
  4. R/3

In the given Figure All the Three resistors are in parallel to each other therefore their equivalent resistance is
1/Req= 1/R + 1/R + 1/R = 3/R

Req= R/3

 

Ques 70. The time constant of the circuit shown in fig. is:

  1. RC/3
  2. 4RC/3
  3. 2RC/3
  4. 8RC/3

Since 3 capacitors are in series, therefore, its equivalent resistance

1/C + 1/C + 1/C = C/3

Now C/3 is in parallel with a capacitor C hence

C + C/3 = 4C/3

Now the equivalent circuit is shown as below

Since both the resistance are in series

R + R = 2R

The time constant of the RC circuit is

τ = RC = 2R × 4C/3 = 8RC/3

Ques 71. If the current in the armature of d.c series motor is reduced to 5%, the torque of the motor will become:

  1. 50% of the Previous Value
  2. 25% of the Previous Value
  3. 150% of the Previous Value
  4. 100% of the Previous Value

In DC series Motor, the torque is directly proportional to the square of the armature current

T ∝ Ia2

So if the armature current is reduced to 5% then torque is

T = 52

T = 25%

 

Ques 72. If the excitation of an alternator operating in parallel with other alternators decreased, its:

  1. Power factor becomes more leading
  2. Output Kw will change
  3. Power factor becomes more lagging
  4. Power factor becomes unity

  • When the excitation of the alternator is decreased below normal excitation then reactive power will change and active power output (W or KW) of the alternator will remain unchanged.
  • The under-excited alternator delivers leading current to the infinite bus bar.
  • It is because the leading current produces an adding m.m.f to increase the under excitation.
  • Similarly, an overexcited alternator operates at lagging power factor and supplies lagging reactive power to an infinite bus bar.

Conclusion:

From both of the above condition i.e whether the excitation is increased or decreased its power factor change

 

Ques 73. The output signal of a common-collector amplifier always:

  1. Larger than the input signal
  2. In phase with the input signal
  3. Out of phase with the input signal
  4. Exactly equal to the input signal

In common-collector configuration, the collector terminal of a transistor is connected as a common terminal between input and output as shown in fig. The base and collector terminals are used to apply the input signal whereas the output signal is obtained. The CC configuration is also commonly known as the emitter follower or the voltage follower.

This is because the output signal across the emitter is almost the replica of the input signal with little loss. In other words, the output voltage follows the input voltage and hence the voltage gain will be a maximum of one.

Ques 74. A current is said to be alternating when it changes in:

  1. Magnitude
  2. Direction
  3. Both magnitude and direction
  4. None of these

An alternating current (AC) is an electric current of which magnitude and direction vary, unlike direct current, whose direction remains constant.

Ques 75.  Hysteresis loss in a transformer depends upon:

  1. Frequency
  2. Supply voltage
  3. Square of the frequency alone
  4. Square of the voltage alone

Hysteresis losses are also known as Iron Loss or Core Loss and it is always constant.

Hysteresis loss is due to the reversal of magnetization of transformer core whenever it is subjected to alternating nature of magnetizing force.

Hysteresis Losses is given by

Hysteresis Loss = Kh × BM1.67 × f × v watts

where
Kh = Hysteresis constant depends upon the material
Bm = Maximum flux density
f = frequency
v = Volume of the core

Hence the Hysteresis losses are directly proportional to the supply frequency.


For SSC JE  Conventional Solved Paper 2017 CLICK HERE

For SSC JE  Conventional Solved Paper 2016 CLICK HERE

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