UPPCL JE Electrical question paper with Explanation 2018

The ratio of field current required to produce rated voltage on open-circuit to the field current required to circulate rated current on short-circuit in armature while the machine is driven at synchronous speed is called short-circuit ratio (SCR) of a synchronous machine.

SCR = Field current for rated voltage at no load ⁄ field current for rated armature current when shorted

SCR is just reciprocal of per unit synchronous reactance Xs of the machine i.e SCR = 1/Xs. The value of synchronous reactance depends upon saturated conditions of the machine whereas, SCR is specific and defined at the rated voltage.

Significance of SCR

Smaller is the value of SCR, larger is the value of synchronous reactance which limits the short circuit current to the smaller value. But it causes difficulty during parallel operation of the machines owing to the smaller value of synchronizing power.

The short circuit ratio is the measure of the alternator’s sensitivity to load changes. Machines with higher SCRs are larger in size, weigh more and also cost more. Their voltage regulation is smaller than those of machines with smaller SCRs. Conversely, alternators with lower SCRs have larger synchronous reactances and higher voltage regulations. Hence, they require quick acting; field control devices to maintain a fairly constant output voltage

The larger value of SCR increases the stability of the machine and improves its voltage regulation.

Usually, the SCR of high-speed non-salient pole alternators lies between 0.5 and 0.75 whereas it lies between 1.0 and 1.5 for low-speed salient pole type alternators.

Therefore, the salient pole type alternators are more stable than non-salient pole type alternators.

The process of bringing the motor to rest within the predetermined time is known as braking.

A good braking system must have the following features:

1. Braking should be fast and reliable.
2. Force exerted by it should be controllable
3. The equipment to stop the motor should be in such a way that the kinetic energy of the rotating parts of the motor should be dissipated as soon as I brakes are applied.

Braking applied to bring the motor to rest position is of two types and they are:

1. Electric braking.
2. Mechanical braking.

Mechanical braking

In this process of braking, the kinetic energy of the rotating parts is dissipated in the form of heat by the brake shoes of the brake lining that rubs on a wheel of vehicle or brake drum.

Electric braking

In this process of braking, the kinetic energy of Be rotating parts of the motor is converted into electrical energy which in turn is dissipated as heat energy in a resistance or in sometimes, electrical energy is returned to the supply. Here, no energy is dissipated in brake shoes.

TYPES OF ELECTRIC BRAKING

Electric braking can be applied to the traction vehicle, by any one of the following methods, namely:

1. Plugging
2. Rheostatic braking.
3. Regenerative braking.

Regenerative braking

• In regenerative braking, the motor, instead of being disconnected from the supply, remains connected to the supply and returns the braking energy to the supply line. So, the wastage of kinetic energy in the plugging and rheostatic braking methods is prevented in the regenerative braking. This method the drives cannot be brought to the standstill. It reduces the speed to a minimum permissible value.
• In the case of a d.c. series motor, increase in excitation results decrease in speed. As such it is not possible to get e.m.f. more than voltage. It is not possible to make field current more than the armature current. Hence regeneration braking with series motors is not possible. But can be used with traction motors with some special arrangements.
• In one method of regenerative braking with a single series motor, the motor is provided with a main series field winding and a few auxiliary field windings.

• For normal running, all the field windings are connected in parallel as shown in above Fig.
• During braking period the auxiliary windings are connected in series with each other and this series connected windings are connected in parallel with the series motor, and then, connected across the supply.
• The machine acts as a compound generator, slightly differential. Such an arrangement is quite stable. A change in the line voltage causes a corresponding change in excitation which produces a change in the induced emf. Thus, the change in line voltage is compensated.

Plugging

• Plugging It is also called reverse current braking.
• The connections of the motor are reversed, reversing the direction of torque, thereby bring it to a quick stop. For this, either the direction of field current or the direction of the armature current is reversed.

• The field circuit usually has a large time constant due to the high value of inductance, so the time taken to bring the field current to zero is large. Hence, it is usual to reverse the armature current.
• On reversing the armature current the supply voltage to the armature and back emf developed in it are additive. So, this condition is worse than the starting condition as the applied voltage to the armature is approximately double (V+E = 2V) to the supply voltage.
• Hence, an external resistance is inserted in the armature circuit simultaneously with the reversal of the armature current. Braking torque can be regulated by varying the magnitude of this resistance.

The disadvantages of this method are:

1. The kinetic energy of the motor is dissipated in the external resistance in the form of heat. So, this method of braking is inefficient.

2. The braking in this method fails in case of failure of the supply.

Rheostatic braking

• Rheostatic braking is also called as dynamic braking
• In rheostatic braking, the armature is cut off and the armature and the motor are connected in series with a variable resistance. The field is still energized.
• The motor, during braking, works as a generator feeding the resistance.

• The kinetic energy of the motor is converted to electrical energy during the period of braking until finally, the motor stops.
• The electrical energy is dissipated in the resistance in the form of heat.
• Care must be taken that the direction of current through the field winding does not change.
• The external resistance should be of a value less than that of the critical resistance, which is a condition for a self-excited generator to build up.
• The magnitude of the braking torque can be regulated by varying the value of external resistance.

The disadvantage of the method is that the kinetic energy of the motor is dissipated as heat in the resistance and it is a wastage. The armature current and so the magnetic field changes with the change in speed. Therefore, the speed-torque curve is not linear in the case of the series motor.

Protection Against Lightning

Proper protection must be provided to the power stations, substations, overhead transmission lines from the direct lightning strokes while the electrical apparatus must be protected from indirect strokes in the form of travailing waves. the lightning strokes can cause severe damage than other types of switching surges or transients. Hence protection against lightning strokes is an important consideration in the system design.

Protection of Generating Stations and Substations Against Direct Strokes

Generally the generate stations are housed In big buildings while the substations are housed in switchyard or outdoor. To protect a structure against direct strokes, three requirements must be fulfilled.

1. Interception
2. Conduction
3. Dissipation.

These involve:

(a) an object in good electrical connection with the earth to attract the leader stroke.

(b) a path joining this object to earth of a low impedance.

(c) a low resistance connection with the earth.

For INTERCEPTION OF LIGHTNING STROKE, the upper portion of a metal structure, or a separate metallic system (often called shield) mounted on the structure may be used. A particular shield configuration is in the form of masts or Overhead ground wires. This provides good shielding to the effect that out of 1000 strokes on the shield or shielded object, 999 will terminate on the shield and only one on the protected object. This is called 0.1% exposure. Such is the effect of good shielding.

For CONDUCTION OF LIGHTNING CURRENT the requirements are:

(i) low resistance

(ii) low reactance, since in the long circuitous line the rapid rate of rise of lightning current may produce high voltage due to high inductance.

(iii) Sufficient clearance from any other conducting object.

Drift velocity:-  The average velocity of the charge carrier in an electric field is known as drift velocity.

Mobility:- The electron mobility characterizes how quickly an electron can move through a metal or semiconductor when pulled by an electric field. The mobility of a charge carrier is defined as the drift velocity acquired by a  per unit applied the electric field.

or

It may also be defined as the ratio of average drift velocity of the charge carriers to the applied electric field. The mobility of a charge carrier increases with the increase of electrical conductivity of the specimen. Electron mobility is almost always specified in units of cm²/(V·s (volt second))

An oscillator is a system consisting of active and passive circuit elements to produce a sinusoidal or other repetitive waveforms at the output without the application of an external input signal. It transforms the dc power from the supply source to the ac power in the load. Thus the function of an oscillator id opposite to that of a rectifier that converts ac power into D.C power. Oscillators find applications in radio and TV transmitters and receivers, and in the laboratory teat equipment.

Depending on the type of the output waveform, oscillators are classified as sinusoidal (or harmonic) oscillators and relaxation oscillators.

If the generated waveform is sinusoidal or nearly so with a definite frequency, the oscillator is said to be a sinusoidal oscillator. If the output waveform is non-sinusoidal (such as square or sawtooth waveforms), the oscillator is termed a relaxation oscillator.

NOTE:-

The Colpitts oscillator is an electronic oscillator that use a combination of inductors (L) and capacitors (C) to produce an oscillation at a certain frequency.

A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can generate a large range of frequencies.

A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a precise frequency.

In this circuit the emitter terminal is common to both the input i. e. base-emitter circuit and the output i.e. the collector-emitter circuit and is grounded. The input (or emitter) circuit is forward biased by connecting the positive of the emitter-base battery V_BB to the emitter terminal and the output (or collector) circuit is reverse biased by connecting the negative of the collector-emitter battery V_CC to the collector terminal. So, output resistance is more than the input resistance.

Out of the three transistor connections, the common emitter circuit is the most efficient. It is used in about 90 to 95 percent of all transistor applications. The main reasons for this are :

• The transistor circuit will have a moderately high input impedance
• Low output impedance
• Moderately high current gain
• Moderately high voltage gain and very good and wide range of frequency response and hence find dominant applications in voltage, current and power amplifiers.

High current gain

In common emitter connection, I_c is the output current and I_B is the input current. The collector current here is expressed as:

Ic = βI_B + I_CEO
Where β = Current Gain
I_CEO = Collector-Emitter current

As the value of β is very large, therefore, the output current I_C is much more than the input current I_B. This increases the current gain effectively. The current gain of CE arrangement ranges from 20 to
500.

High Voltage and Power Gain

As we have seen above, CE arrangement has the high current gain. This is turn, increases the voltage and power gain of the CE circuit. In comparison to CB and CC circuits, the common emitter connection has the highest voltage and power gain. For this reason, the CE transistor connection is often used for amplifying purposes.

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The fault in the power system is generally categorized into two type

1. Symmetrical fault
2. Unsymmetrical fault

Symmetrical Fault

The symmetrical faults are often known as balanced faults. In case of balanced faults, all three transmission lines are affected equally, and the system remains in the balanced condition. These types of faults are rare in the power system, and it contributes 2 % to 5 % of the total fault. These faults are easy to analyze. The symmetrical faults are classified as three lines to ground fault (LLLG) and three lines fault (LLL). The connection diagrams of symmetrical faults are shown in Fig. If the fault impedance, Z_f = 0, then the fault is known as a solid or bolted fault.

Unsymmetrical Faults

The faults in the power system network which disturb the balanced condition of the network are known as unsymmetrical faults. The unsymmetrical faults are classified as the single line to ground faults (SLG), double line to ground faults (DLG 10%)  and line to line faults (LL 15%). More than 90 % of faults occur in a power system are the single line to ground faults. The connection diagrams of different types of unsymmetrical faults are shown in Fig.

Fermi level is the energy which any electron possesses at 0° Kelvin. For P-type semiconductor, the Fermi-level lies above the valence band.  For N-type semiconductor, the Fermi level lies below the conduction band.

For the n-type semiconductor, Fermi level goes down with an increase in temperature and with further increase in temperature it will come at the center of the forbidden energy gap. Thus at a higher temperature that n-type semiconductor behaves like an insulator.

For p-type semiconductor, Fermi level goes up with an increase in temperature and with further increase in temperature it will come at the center of the forbidden energy gap. Thus at higher temperature p-type semiconductors behave like an insulator.

Fermi-Dirac Function f(E)

Fermi-Dirac function f(E) gives the probability that a quantum state with energy E (electron volt) is occupied by an electron. It is also called Fermi-Dirac probability function. For a metal or semiconductor, we have

where,

E = Energy possessed by an electron in eV

k = Boltzmann constant

T= Absolute Temperature (in Kelvin)

E_F = Fermi level for crystal, eV

At T= 0 Kelvin: (E = E_F) condition not possible; hence, we get the following two condition

1. If E > E_f

This indicates that there are no electrons available at 0 Kelvin With energies greater than E_F (E > E_F).

This indicates that at 0° Kelvin electrons exist with energy E < E_f.

This indicates that when the temperature is other than 0° Kelvin then the chances of electrons existing with energy E = E_F is 50 percent.

The main points are:

1. In metals, f(E) = 1
2.  In the semiconductor, if the probability of an electron existing is f(E), then the probability of whole exit existing is 1 − f(E).
3. Fermi-Dirac function is used to find the probability of an electron existing as a function of energy level.
4. Fermi level E_F represents the energy state with 50 percent probability of being filled if no forbidden band exists.
5. At T = 0°, if E >> E_F,  then f(E) = 0. Thus, there is zero probability of finding an occupied quantum state of energy greater than E_f at 0°K.(ii) If E << E_F‘ then f(E) = 1. Thus, all quantum levels with energy less than E_f are occupied at T= 0°K.

The relative permeability or the dielectric constant of silicon is  11.7

Common-Collector Configuration

In common-collector configuration, the collector terminal of a transistor is connected as 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, There is no phase inversion between input and output in the emitter follower circuit.The output voltage is in phase with the input voltage and hence the voltage gain will be a maximum of one. The common-collector configuration is used primarily for impedance-matching purposes since it has a high input impedance and low output impedance, opposite to that of the common-base and common- emitter configuration.