NMRC JE ELECTRICAL SOLVED PAPER 2017

PNP TRANSISTOR

If a thin layer of N-type Silicon is sandwiched between two layers of P-type silicon. This transistor is referred to as PNP.Consider a PNP transistor shown in Fig. biased in the forward active mode, that is emitter-base junction is forward biased (V_EB is +ive) and the collector-base junction is reverse biased (V_CB is —ive). Due to the externally applied voltages V_EB and V_CB, currents flow in the transistor. In PNP transistor hole is the majority charge carrier.

In a PNP transistor, the P region emitter is biased in forward direction because it emits or injects the charge carriers into the transistor. The collector is biased in reverse direction because its function is to collect or attract the charge carriers. The middle electrode which has a common region to the two out of electrodes (emitter and collector) is called as the base. The forward biased junction is called the emitter junction.Similarly, the reverse biased junction is called as the collector junction.

As soon as biasing is applied the majority charge carriers (holes) of P region (emitters) flow (i. e. diffuse) to the N region (base) as they are repelled by the positive terminal of the battery. At the same time, the electrons of N region are attracted by the positive electrode. Hence the potential barrier at the emitter junction reduces forward emitter biasing. Thus in PNP transistor with emitter forward biased, the majority of the holes are able to drift across the base (which is made very thin) without combining with the electrons present there. However, a small number of holes (about five percent) are lost in the base (N region) due to combination with electrons, which result in a small base current.

The remainder (more than 95 percent) cross into collector region to constitute collector current Ic. In this way, almost the entire emitter current flows in the collector circuit. It may be noted that current conduction within PNP transistor is by holes. However, in the external connecting wires, the current is still by electrons.

In three phase system, the order in which the voltages attain their maximum positive value is called Phase Sequence. The order in which the phase voltages of a 3-phase system attain their peak or maximum positive values is called the phase sequence of the system. The phase sequence RYB normally means that the red phase is followed by yellow phase, which is followed by the blue phase. When the voltage of the red phase is at its positive maximum value, the voltage of the Y-phase will be 120° behind its positive maximum value and that of B-phase 240° behind its positive maximum value as shown in fig.

CRGO is cold rolled grain oriented steel and as the name suggests, its grains(crystals) are aligned in the direction of rolling. when the magnetic flux passes through it, the magnetic domains (the region in grains with aligned magnetic moment) get aligned in one direction causing the minimum resistant path to flux causing lesser hysteresis loss.

Other advantages of high silicon steel are

1. Low copper loss due to lower grain resistance.
2. High magnetic permeability.
3. Lower thickness of lamination.

Load factor is the ratio of average load to maximum demand during the certain period of time (e.g. day/month/year) is called load factor.

Load factor = Average Demand/Maximum Demand

Load factor is inversely proportional to maximum demand.Higher load factor means steady consumption or supply which means less maximum demand thus the initial and running cost will be low. Therefore the cost per unit power generation will be less. 

The slip in an induction motor is the difference between the main flux speed and their rotor speed and it is expressed as

s = (Ns – N)/Ns

Moderator in Nuclear Power Station

In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235 or a similar fissile nuclide.

• Neutrons produced from the fission process are ejected at one-twentieth of the speed of light and are termed fast neutrons.
• These fast neutrons have more probability of being captured by fertile material U-238 and are far less effective in causing the fission of U-235.
• U-238 absorbs the fast neutron to such an extent that the neutrons produced are absorbed before they can reach a U-235 nucleus.
• Further, fast neutrons try to escape from the reactor core. To increase the chances of fission of U-235 (fissile material), either slow neutrons also called thermal neutrons are required or percentage of U-235 is increased in the fuel.
• Thus, to improve the utilization of these fast neutrons generated from the previous fission reaction, their speed is reduced by using moderator for the next fission reaction in the thermal reactor.
• Lower velocity of neutrons provides the better opportunity for the fission of U-235 without enrichment. The absorption properties of U-238 are very much reduced when slow neutron is used.
• The function of moderators depends on the law of mechanics and neutrons are slowed down as a result of an elastic collision.
• If a neutron collides with a nucleus of equal mass it will loose all its energy and comes to standstill.
• If it collides with a nucleus of heavier mass there will be little change in its velocity and velocity will be reversed.
• The speed of neutrons is reduced by colliding them with the nuclei of other material which is lighter, does not capture the neutrons but scatters them.
• Each such collision causes loss of energy, and the speed of the fast-moving neutrons is reduced. The best moderating materials are of low mass number and with no or little tendency to capture neutrons.
• Graphite, light water (H₂O), heavy water (D₂O) and beryllium are generally used as the moderator.

Active Region: The transistor is said to operate in active region when the emitter junction is biased in forward direction and the collector junction in the reverse direction. The transistor is used as an amplifier when it is biased in an active region.When the emitter junction is forward biased, the current in the junction flows from emitter through the base to collector and is positive when entering the emitter.

This active region of operation in a transistor is the most sensitive region of operation in a transistor. The variation of output collector current depends on the input emitter current variations. Even though the magnitude of the output current is small compared to that of the input current, the variation is very sensitive.

The value of earthing is dependent upon the soil resistivity of that particular area. The earth resistance which we are measuring is not the resistance of the electrode. actually, it is the resistance of the conductor and its associated with soil. since charcoal contains carbon which is a perfect conductor it serves the purpose. The mixture has to be watered on a regular basis in order to increase its conductivity. Hence better the conductivity of the soil or lesser its resistance, more beneficial it is. When the fault occurs the current has to dissipate to the soil, if the soil has higher resistivity then the dissipation will not happen properly.

The fault occurring between a few and some hundred the kilometers from the breaker is termed as the short-line fault.

SF6 gas circuit breakers are widely used for EHV applications today, as Sulphur hexafluoride(SF6) gas is an electronegative gas having excellent dielectric and arc quenching properties resulting in many advantages such as compactness and less maintenance of EHV circuit breaker

The excellent insulating properties of SF6 gas make it possible to design circuit breakers with smaller overall dimensions, shorter contact gaps, which help in the construction of outdoor breakers with fewer interrupters, and the evolution of metal-enclosed SF₆ gas-insulated switchgear.

The basic requirement in an arc extinction is not primarily the dielectric strength but the rate of recovery of dielectric strength. In the SF₆ gas, the dielectric strength is quickly regained.

The behavior of the arc during current zero and immediately afterward, is of decisive importance for the reliable interruption of the current. In SF₆ gas, the diameter of an arc is relatively small. As a consequence, the thermal time constant of the arc (which is a function of the square of the arc radius), approaching current zero, is almost 100 times smaller than in air. The dielectric strength of the break increases rapidly after current zero so that the breaker is able to control even extreme rates of rise of the transient recovery voltage (TRV), in case of short line faults.

A three-phase induction motor consists of three-phase winding as its stationary part called the stator. The three-phase stator winding is connected in star or delta.  The windings are supplied by a balanced three-phase ac supply. The three-phase winding made on the stator slots is R – R’, Y -Y’, B – B’ which are displaced 120° electrical.

The three-phase currents flow simultaneously through the windings and are displaced from each other by 120° electrical. Each alternating .phase current produces its own flux which is sinusoidal. So all three fluxes are sinusoidal and are separated from each other by 120°. If the phase sequence of the windings is R-Y-B, then mathematical equations for the instantaneous values of the three fluxes Φ_R, Φ_Y, Φ_B can be written as,

Φ_R = Φmsin(ωt)

Φ_Y = Φmsin(ωt – 120°)

Φ_B = Φmsin(ωt – 240°)

The characteristic impedance, also called surge impedance of a uniform transmission line, usually written Z. is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The unit of characteristic impedance is ohm.

${Z_o} = \sqrt {\dfrac{{R + j\omega L}}{{G + j\omega C}}}$

where

R is the resistance per unit length, considering the two conductors to be in series,
L is the inductance per unit length,
G is the conductance of the dielectric per unit length,
C is the capacitance per unit length,
j is the imaginary unit, and
ω is the angular frequency.

A lossless line is defined as a transmission line that has no line resistance and no dielectric loss. This would imply that the conductors act like perfect conductors and the dielectric acts like a perfect dielectric. For a lossless line, R and G are both zero, so the equation for characteristic impedance derived above reduces to

[latex display=”true”]{Z_o} = \sqrt {\dfrac{L}{C}}$

For a lossless line terminated in Z₀, there is no loss of current across the line, and so the voltage remains the same along the line.