DMRC JE Electrical Question Paper With Explained Solution – 2016

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.

A 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 junctions 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 by V_BO. When the forward voltage reaches VBO, the SCR turns on.  During forward Blocking Mode SCR acts as a Resistor.

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

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.

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 the 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 are used in Parallel.

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.

The charge possed by the proton is equal to 1.602 × 10^-19 whereas the charge 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.

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

R_eq= R/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

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%

• When the excitation of the alternator is decreased below normal excitation then reactive power will change and the active power output (W or KW) of the alternator will remain unchanged.
• The under-excited alternator delivers a 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 a lagging power factor and supplies lagging reactive power to an infinite bus bar.

Conclusion:

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

In a 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.

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

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 the transformer core whenever it is subjected to the alternating nature of magnetizing force.

Hysteresis Loss is given by

Hysteresis Loss = K_h × B_M^1.67 × 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.