DMRC JE Electrical 2014 Paper With Solution and Explanation

In electronics and telecommunications, a transmitter or radio transmitter is an electronic device that produces radio waves with an antenna. The transmitter is the key component of any radio broadcast.  It takes your broadcast signal, encodes it, and transmits as radio waves that can be picked up by any receiver.

• The modulator adds information to the carrier wave. In the case of FM (frequency modulation), the modulator either slightly increases or decreases the frequency of the carrier wave.
• The antenna sends and receives radio signals.
• The amplifier increases the power of the wave. More powerful amplifiers allow for a larger broadcast area.
• The oscillator creates the alternating current, a carrier wave, that the transmitter sends through the antenna.

The bandwidth with negative feedback increase by the factor (1 + Aβ) and gain decrease by the same factor.

The amount of feedback (1 + Aβ) is also called the de-sensitivity factor.

Noise Reduction: Negative feedback when used in amplifiers increases the signal the noise ratio or causes a reduction in noise.

Non-linear Distortion: Reduction Application of negative feedback to amplifiers causes the amplifier characteristics to be less non-linear or more linearized and the distortion gets reduced by a factor of 1/(1 + Aβ).

Increase in Input Impedance: Negative feedback increases the input impedance of amplifiers which is generally preferred in multistage amplifiers to reduce the loading effect.

In star connection, line and phase currents are the same but line voltage is  √3 times the phase voltage. In the delta connection, line and phase voltages are same but the line current is √3 times the phase current. However, the power in both connections is same, that is

√3V_LI_L cosφ.

1. As Primary in Star connected: Line voltage on the Primary side = √3 X Phase voltage on Primary side. So
2. Phase voltage on the Primary side = Line voltage on the Primary side / √3
3. Transformation Ratio (K) = Secondary Phase Voltage / Primary Phase Voltage
4. As Secondary in delta connected: Line voltage on Secondary side = Phase voltage on the Secondary side.
5. Secondary Phase Voltage = K X Primary Phase Voltage.
6. Secondary Phase Voltage = K X Primary Phase Voltage. =K X (Line voltage on Primary side / √3)
7. Secondary Phase Voltage = (K/√3 ) X Line voltage on Primary side

Advantages of Star-Delta Connection

The star-delta transformer has the following advantages:

1. Since primary is star connected. Less number of turns are required in the primary which makes it economical for high voltage step-down power transformers.
2. Less insulation is required
3. The available neutral on the primary side can be earthed to avoid distortion.
4. Delta doesn’t allow the flow of 3rd harmonics. It is possible to handle the large unbalanced load.
5. Since secondary is connected in Delta, this type of transformer does not suffer from unbalanced.
6. The main use of this connection is at the substation end of the transmission line where the voltage is to be stepped down.

The star-delta transformer has the following disadvantage:

1. Since the secondary voltage is not in phase with the primary, it is not possible to make it parallel with star-star and delta-delta transformers.

Zener diodes are similar to conventional diodes when they are forward biased When they are reverse biased, no conduction takes place until a specific value of reverse breakdown voltage (or “zoner” voltage) is reached. The Zener is designed so that it will operate in the reverse breakdown region of its characteristics curve.

A properly doped junction diode that has sharp reverse breakdown voltage is called a zener diode.

The reverse breakdown of Zener voltage or avalanche breakdown depends upon the amount of doping. If the diode is heavily doped depletion layer will be thin and consequently, the breakdown of the junction will occur at a lower reverse voltage. On the other hand, a lightly doped diode has a higher breakdown voltage.

When used as a voltage regulator the zoner diode is reverse biased so that it will operate in the breakdown region. In this region, changes in current through the diode have little effect on the voltage across it. The constant-voltage characteristic of a Zener diode makes it desirable for use as a regulating device.

From the VI Characteristics of Zener diode, we can see that the Zener diode has a region in its reverse bias characteristics of almost a constant voltage regardless of the current flowing through the diode  This voltage across the diode called Zener voltage V_Z remains nearly constant even with changes in current through the diode caused by variations in the supply voltage or load. This ability of the diode to regulate the source voltage can be best utilized in stabilizing a voltage source against supply or load variations. The diode will continue to regulate until the diode current falls below the l_Zmin ) value, a minimum value that keeps the diode in the reverse breakdown region.

The resistor R_S is connected in series with the Zener diode to limit the current flow with the output from the voltage source V_in being connected across the combination while the stabilized output voltage V_O is taken across the Zener diode. The Zener diode is connected across the load with its cathode terminal connected to the positive rail of the dc supply so it is operating in its reverse-biased condition. The series resistor R_S is so selected as to limit the current flowing in the circuit and hence the Zener diode well within maximum value l_Zmax.

When the load resistance R_L is very high, there is minimum current flowing through R_L (no-load current I_L = minimum or 0) and all the circuit current passes through the Zener diode which is l_Zmax) and dissipates maximum power in Zener. Care must be taken when selecting the appropriate value of resistance that the Zener maximum power rating is not exceeded under this “no-load” condition.

When the load resistance R_L is very low there is maximum current flowing through R_L (full-load current I_L = maximum) and only a small current passes through the Zener diode which is l_Zmin. Care again must be taken when selecting the appropriate value of resistance so that the stabilization of the voltage is effective and the Zener current must stay above this value operating the diode within its breakdown region at all times. In order to achieve this, both RS and RL should be properly selected.

For e.g, A Zener diode that breaks down at 5V will have 5V across it even if you apply 12v to it. The remaining 7v will be across the resistor in series (Rs). So, any load(resistance RL) parallel to the Zener diode will have that constant 5v across it. Hence you get a constant voltage source.

Draft in a heating system refers to the pressure difference which causes a current of air or gases to flow through a combustion chamber, flue, or chimney. A natural draft is a draft obtained without any mechanical means. It is the heat of the combustion processes that create the differential pressure causing the draft. The mechanical draft is created by fans that either force or pull the air or combustion products through the system.

Basically, there are two types of the draft fan

1. Forced Draft (FD)
2. Induced Draft (ID)

Forced Draft fans suck air from the atmosphere, for providing the required quantity of hot air to the furnace for smooth and uniform combustion of fuel. they handle cold air. So they need low maintenance, consume less power (as pointed out, cold air has high density) and therefore their operating costs are low. The number of FD fans used is generally less than the number of ID fans.  FD fan is placed before the boiler & forces the air to the boiler.

Induced Draft produces pressure lower than the atmospheric pressure in the system or we may say that the ID fan will produce the negative pressure in the furnace to remove the flue gases from the furnace via electrostatic precipitators and to push the flue gases to the chimney. The counterflow (conventional) type of induced-draft tower has the fan located at the top.

Things to Rember to understand the question.

CURRENT RATINGS

Latching Current (I_L)

The latching current is the minimum value of anode current to trigger or turn ON the thyristor from its OFF state to ON state even after the trigger pulse is removed. To trigger an SCR, the anode current must be build-up to the latching current before the gate pulse is removed.

 Holding Current (I_H)

The holding current is the minimum value of the anode current to hold the thyristor in the ON state. During turn OFF, the anode current should be below the holding current. Usually, the value of holding current is in milliamperes.

Gate Current (Ig)

The gate current is required at the gate of the thyristor to turn ON. There are two types of gate current: minimum gate current I_gmin and maximum gate current I_gmax.  I_gmin is the minimum value of gate current which is required at the gate to trigger the thyristor from its OFF state to ON state and its value depends on the rate of rise of current. I_gmax is the maximum value of gate current that can be applied to the device for turn-on without damaging the gate. The turn-ON time of the thyristor reduces with the increase of gate current.

The thyristor is a four-layered semiconductor that has three terminals. Current passes from the anode to the cathode when the device is turned on. It is turned on (triggered) by applying a small current, the gate current, through the gate to the cathode and when the anode current is above a specified parameter value called the latching current. On removal of the gate current, a thyristor will stay on if the anode current is above the device. Thus from the above discussion, it is clear that the gate current is independent of the anode current.

In conduction mode, anode current has to be limited only by the load impedance. If the load impedance is increased gradually, the anode current gradually falls. If the anode current reduces to a value below the holding current,

Brass is a metallic alloy that is made of copper and zinc.

63% Tin, 37% Lead is eutectic and melts at a single point, used often used in electronics.

As we know that in the series circuit the current remains the same and in the parallel circuit the voltage remains the same.

WHEN THE CELL ARE CONNECTED IN SERIES

Now consider the series circuit in which all the cells are connected in series. If there is “n” number of cells then the total EMF is nE.

Now all the battery has total internal resistance therefore if n cell is connected in series than the total internal resistance will be “Rn”

Now total current in the circuit is I = nE/nR = E/R

WHEN THE CELL ARE CONNECTED IN PARALLEL

When n number of cells are connected in parallel then total EMF is equal to E (since in parallel circuit voltage remains same).

Internal resistance in parallel = R’ = R/n

Total current in paralle circuit = I = nE/R

Now consider 3 cell each of 2V having internal resistance of 0.1 ohm

Current in series = I = E/R = 2/0.1 = 20A

Current in parallel = I = nE/R = 60A

Hence current capacity increase in the parallel.

A capacitor opposes the change in voltage. Assume initially the capacitor has no charge which means both plates are neutral (each plate has an equal number of protons and electrons). If a battery is connected to a parallel plate capacitor, the positive terminal of the battery forcibly attracts the electrons from the capacitor plate A and the negative terminal of the battery will send the electrons to capacitor plate B.

Initially, as time t is very small current is large(by I=Q/t) but when time passes the plate connected to the positive terminal becomes more positive and the plate connected to the negative terminal becomes more negative. Hence a steady state is reached after a certain amount of time(NOT INSTANTLY AFTER CONNECTING WITH BATTERY) when capacitor voltage becomes equal to the battery voltage. As we know current flows only when there is a potential difference, here the potential difference is zero at a steady state and so the capacitor acts as an open circuit in a steady state.

In order to change the voltage in a capacitor, you have to put a charge into the capacitor because of Q=CV. If you want to change voltage instantaneously (meaning change voltage in zero time), you have to put all the required charges in zero time. That requires a very, very, very big current to move the charge in zero time. It is kind of difficult.

Similarly, using the capacitor equation, I=C*dV/dt, we can see that to make an instantaneous change in the voltage (dt approaches zero), I approach infinity.

Well, think of an empty bucket (capacitor); and you need to fill the bucket with water (water is the charge, and the water level is voltage). You can fill it with your pipe (the water flow rate is current). The bigger the pipe (the larger the flow rate or current), the faster you fill the bucket.,..but it will take time. In order to fill the bucket in zero time, you need a big pipe that carries an infinite flow rate.

1. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
2. Plant cost for AC transmission (circuit breakers, transformers, etc.) is much lower than the equivalent DC transmission
3. From power stations, AC is produced so it is better to use AC than DC instead of converting it.
4. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
5. It is easier to convert AC to DC rather than the other way around.
6. AC can be stepped up and back down with transformers quite easily whereas DC cannot as transformers rely on a constantly changing EMF(electromotive force).
7. If the DC was left at the supply voltage over a very large distance the volt drop would be too great and the wires would have to be enormously thick (and expensive) in order to carry the required current.
8. Modern technology has brought about ways to step up the voltage of DC with solid-state circuitry and is becoming a possibility for the future, but it is still expensive and not quite as efficient or reliable as the good old transformer.

Consumer’s Perspective:

1. All the household and industrial systems are wired for AC rather than DC. It would be highly impractical to convert every piece of equipment to DC.
2. Fairly high voltage DC can be quite dangerous if you were to grab a wire with damaged insulation it becomes very hard to release the hand from it as the muscles clamp uptight.
3. This isn’t to say that grabbing an AC bare wire would be any good for you either but you have more of a chance of releasing yourself from it.
4. The power generation is not possible at high de. voltage levels duel to commutation problems.
5. The transformer works on a.c. and not d.c. supply. So the d.c. voltage levels cannot be stepped up or lowered as per the requirement. Hence transmission at high voltage is not possible. This is the biggest disadvantage of a d.c. system.
6. Obtaining a.c. from d.c. is not easy in practice.
7. The d.c. generators and motors need a lot of maintenance and their construction cost is also more than a.c. machines of the same capacity.
8. The limitations of d.c. switches, as well as circuit breakers, cause problems in d.c. system.