DMRC JE Electrical 9th- April- 2018 Paper With Solution and Explanation

Ques 11. An impulse turbine is used in

  1. Low head Plant
  2. Very Low head Plant
  3. Medium head plant
  4. High head plant

A turbine is a hydraulic machine that converts hydraulic energy into mechanical energy.

Turbines are classified as given below:

  1. Based on the action of water flowing through the turbine runner
    1. Impulse (Pelton wheel turbine)
    2. Reaction turbines (Francis and Kaplan turbines).
  2. Based on the main direction of the flow of water in the runner
    1. Tangential flow (Pelton wheel turbine),
    2. Radial flow, axial flow (Kaplan turbine)
    3. Mixed flow turbine (Modern Francis turbine).
  3. Based on the head and quantity of water
    1. High head and low discharge (Pelton wheel turbine)
    2. Medium head and discharge (Modern Francis turbine)
    3. Low head and high discharge turbines (Kaplan turbine).
  4. Based on the specific speed
    1. Low (Pelton wheel turbine)
    2. Medium (Francis turbine)
    3. High specific speed turbines (Kaplan turbine).
  5. Based on the axis of disposition
    1. Horizontal axis
    2. Vertical axis turbine.

The impulse or Pelton wheel is generally used in plants with heads higher than 850 feet (260 meters), although some installations have lower heads.

It works on the principle of impulse Where the kinetic energy of a fluid jet is used to exert a force on a set of moving blades. The total enthalpy drop takes place in the nozzle or in the fixed blades. Thus, steam pressure remains constant while it flows through the moving blades. Kinetic energy is converted into mechanical power  Examples of such types of turbines are De-Laval, Curties, Rateau etc.

img.7 1

The impulse turbine has two principal characteristics:

  1. It requires nozzles so that the pressure drop of steam takes place in the nozzles. The steam enters the turbine at a high velocity. The pressure in the turbine remains constant because the whole of the pressure drop takes place in the nozzles.
  2. The velocity of the steam is reduced as some of the kinetic energy in the steam is used up in producing work on the turbine shaft.

 

Ques 12. For the circuit given below find the current through 1Ω resistance

img.8

  1. 1.43 A
  2. 2.36 A
  3. 1.23 A
  4. 2 A

Since the two voltage source is of equal polarity hence “additive” therefore

V = V1 + V2 = 10 + 2 = 12 V

Similarly, the resistance 5Ω and 1Ω are connected in series therefore equivalent resistance become

R = R1 + R2 = 5 + 1 = 6Ω

Now the equivalent circuit become as shown in the figure

img.9

Applying Nodal analysis at node A we get

$\begin{array}{l}\dfrac{{V – 12}}{6} + \dfrac{V}{2} + \dfrac{{V – 4}}{2} = 0\\\\V – 12 + 3V + 3V – 12 = 0\\\\V = 3.42\\\\{\text{As the 5ohm and 1ohm are connected}}\\{\text{in series therefore the current is same}}\\\\{\text{Hence current through 1 ohm resistance is}}\\\\I = \dfrac{{V – 12}}{6}\\\\I = \dfrac{{3.42 – 12}}{6}\\\\{\rm{I = – 1}}{\rm{.43A i}}{\text{.e 1}}{\text{.43 (I is leaving the Node)}}\end{array}$

 

Ques 13. For the circuit shown in the figure Find I

img.6 1

  1. 1 A
  2. 0 A
  3. 6 A
  4. 3 A

Since in all the three branches the current is going in the same direction, therefore, all the current will add up.

2 + 1 + 3 = 6A

img.7

Now in the next branch 5A is coming in and 3 A is going out

Hence total current = 6 + 5 – 3 = 8A

img.8 1

In last Branch, 2A is going out hence the remaining current is

8 = I + 2
or I = 8 -2 = 6A

 

Ques 14. The type of armature winding used for high current rated DC machines is

  1. Asymmetrical winding
  2. Series-Parallel Winding
  3. Lap winding
  4. Wave Winding

Armature Winding

  •  An armature is that part of the DC machine where EMF is induced.
  • Armature coils are wound on the armature core and placed inside the armature slots.

Two methods can be used to wound armature coil:

  1. Lap winding
  2. Wave Winding

Lap winding: Here, the end of one coil is connected to the beginning of the next coil. If connections are made this way the coils look as if they are superimposed on each other and then given a push in one direction as shown in Fig.

img.10

Lap wound armatures are constructed with relatively few turns of large wire. They are commonly used in machines that are intended to operate on low voltage and high currents, such as starter motors in automobiles, streetcars, and trolleys. Lap wound armatures have their windings connected in parallel with each other.

Lapwave winding

Wave winding is another type of armature winding. In this winding, the end of one coil is connected to the starting of another coil of the same polarity as that of the first coil.

 

Wave wound armatures are intended for use in high voltage, low current machines, such as high voltage generators. The armature windings are connected in series. in a generator, the voltage produced in each winding combines, to increase the total output voltage. In a motor, the voltage applied to the circuit is divided across each winding.

The lap winding contains more number of parallel paths and provides a large current. Hence, this winding is applied to the generators which are to deliver more current. Wave winding is more suitable for small generators, especially these are meant for 500-600 V circuits.

The main advantage of wave winding is that it gives more emf than lap winding for a given number of poles and armature conductors, whereas, the lap winding would require a large number of conductors for the same emf. This will result in a higher cost of winding and less utilization of space in the armature slots.

Moreover, in wave winding, equalizing connections are not necessary, whereas in lap winding these are required definitely. It is because, in the wave winding the conductors of the two paths are distributed in such a way that they lie under all the poles, therefore, any inequality of pole fluxes affects two paths equally, hence their induced EMFs are equal. But, in lap winding, each parallel path contains conductors which lie under one pair of poles, hence unequal voltages are produced which set up a circulating current causing sparking at the brushes.

Thus, in general practice, wave winding is preferred, the lap winding is only used for low-voltage.

 

Ques 15. Pentavalent Impurity atoms that are connected to intrinsic semiconductors are called

  1. Donor or N-type impurity
  2. Acceptor
  3. Acceptor or P-type impurity
  4. Donor or P-type Impurity

  • When a small amount of pentavalent impurity (having 5 valence electrons) is added to a pure semiconductor, it is known as the n-type semiconductor.
  • An N-Type semiconductor is created by adding pentavalent impurities like phosphorus (P), arsenic (As), or antimony (Sb), bismuth.
  • A pentavalent impurity is called a donor because it is ready to give a free electron to a semiconductor. The impurities are called dopants.
  • The addition of pentavalent impurity provides a large number of free electrons in the semiconductor crystal.
  • The purpose of doing this is to make more charge carriers, or electrons available in the material for conduction.
  • In n-type semiconductors the number of electrons is more than the holes, so electrons are measured as majority charge carriers and holes are referred to as minority charge carriers.

Consider a pure germanium crystal. We know that the germanium atom has four valence electrons. When a small amount of pentavalent impurity like arsenic is added to the germanium crystal, a large number of free electrons become available in the crystal. The reason is simple. Arsenic is pentavalent Eel, its atom has five valence electrons.

img.11

An arsenic atom fits in the germanium crystal in such a way that its four valence electrons form covalent bonds with four germanium atoms. The fifth valence electron of the arsenic atom finds no place in covalent bonds and is thus free as shown in Fig. Therefore, each arsenic atom provides one free electron, yet an extremely small amount of impurity provides enough atoms to supply millions of free electrons.

 

Ques 16.  The minimum value of an actuating quantity at which a relay starts operating is called as

  1. Dropout Value
  2. Operating Time
  3. Reset Time
  4. Pick up Value

Terminologies used in Protective Relaying

The various terminologies used in the protective relaying are,

  1. Protective Relay: It is an electrical relay, which closes its contacts when an actuating quantity reaches a certain preset value. Due to the closing of contacts, the relay initiates a trip circuit of the circuit breaker or an alarm circuit.
  2. Relay Time: It is the time between the instant of fault occurrence and the instant of closure of relay contacts.
  3. Breaker Time: It is the time between the instant at circuit breaker operates and opens the contacts, to the instant of extinguishing the arc completely.
  4. Operating force or torque:- A force or torque which tends to close the contacts of the relay.
  5. Restraining force or torque:- A force or torque which opposes the operating force/torque.
  6. Fault Clearing Time: The total time required between the instant of the fault and the instant of final arc interruption in the circuit breaker is fault clearing time. It is the sum of the relay time and circuit breaker time.
  7. Pickup: A relay is said to be picked up when it moves from the ‘OFF position to the ‘ON’ position. Thus when the relay operates it is said that the relay has picked up.
  8. Pickup Value: It is the minimum value of an actuating quantity at which the relay starts operating. In most of the relays actuating quantity is current in the relay coil and the pickup value of current is indicated along with the relay.
  9. Dropout or Reset: A relay is said to drop out or reset when it comes back Is original position i.e. when relay contacts open from its dosed position. The value of an actuating quantity current or voltage below which the relay reset is called the reset value of that relay.
  10. Reset or dropout- The threshold value of the actuating quantity (current, voltage, out (level) etc.) below which the relay is de-energized and returns to its normal position or state. Consider a situation where a relay has closed its contacts and the actuating current is still flowing. Now, due to some reason, the abnormal condition is over and the current starts decreasing. At some maximum value of the current, the contacts will start opening. This condition is called reset or drop-out. The maximum value of the actuating quantity below which contacts are opened is called the reset or drop-out value.
  11. Operating time:- It is the time that elapses from the instant at which the actuating quantity exceeds the relays pick-up value to the instant at which the relay closes its contacts.
  12. Reset time:- It is the time that elapses from the moment the actuating quantity falls below its reset value to the instant when the relay comes back to its normal (initial) position.
  13. Setting:- The value of the actuating quantity at which the relay is set to operate.

 

Ques 17. A forward bias PN junction will act as a/an:

  1. Amplifier
  2. Open switch
  3. Closed Switch
  4. Attenuator

A PN-junction diode is formed when a p-type semiconductor is fused to an n-type semiconductor creating a potential barrier voltage across the diode junction

Forward Biasing: If the positive terminal of an external battery is connected to the p-type and its negative terminal to the n-type of the PN junction then such a biasing is called forward biasing. Forward biasing reduces potential barrier and hence, depletion layer width decreases. The current is due to majority carries. Current is quite large when applied, with forward voltage. In forward biasing conditions, the PN junction acts as a Closed switch.

Diffiusion capacitance

 

  • An ideal diode acts as a short circuit under forward bias conditions. A practical diode offers small but finite resistance under forward bias conditions.
  • An ideal diode acts as an open circuit under reverse bias conditions. A practical diode offers large but not infinite resistance under reverse bias conditions.
  • A diode conducts current when forward biased and blocks current when reverse biased. This property of the diode is made use of in rectifier circuits.

 

Ques 18. The motor has similar characteristics to the dc machine

  1. Repulsion Motor
  2. Shaded Pole Motor
  3. Universal Motor
  4. Reluctance Motor

The universal motor is one that operates both on A.C and D.C supply. Its principle of operation is the same as that of a de series motor, i.e., force is created on the armature conductors due to the interaction between the main field flux and the flux created by the current-carrying armature conductors. The series wound DC motor is the only type of DC motor that works on AC although the efficiency of the motor is very poor.

Universal Motor

However, the Universal motor is constructed with a few series of fields turns, laminated armature and field circuits, low reluctance magnetic path, increased armature conductors, and commutator segments by using low flux densities. This is done to minimize the adverse effects caused by high field reactance, eddy current, and Hysteresis Losses.

We may consider a typical DC series motor or a DC shunt motor for operation on AC power supplies. It appears that such an operation is possible because reversing the line terminals to a DC motor reverses the current and magnetic flux in both the field and armature circuits. As a result, the net torque of the motor operating from an AC source is in the same direction.

However, the operation of a DC shunt motor from an AC source is impractical because the high inductance of the shunt field causes the field current and the field flux to lag the line voltage by almost 90°. The resulting torque is very low.

A DC series motor also fails to operate satisfactorily from an AC source because of the excessive heat developed by eddy currents in the field poles. In addition, an excessive voltage drop occurs across the series field windings due to high reactance.

To reduce the eddy currents, the field poles can be laminated. To reduce the voltage loss across the field poles to a minimum, a small number of field turns can be used on a low reactance core operated at low flux density. A motor with these revisions operates on either AC or DC and is known as a universal motor. Universal motors in small fractional horsepower sizes are used in household appliances and portable power tools.

Universal motors may be either compensated (distributed field) or uncompensated (concentrated field) types, the latter type being used for the higher speeds anal smaller output ratings (usually not exceeding 200 Watt only.

CONCENTRATED-FIELD UNIVERSAL MOTORS

A concentrated-field universal motor is usually a salient-pole motor with two poles and a winding of relatively few turns. The poles and winding are connected to give opposite magnetic polarity.

DISTRIBUTED-FIELD UNIVERSAL MOTORS

The two types of distributed-field universal motors are the single-field compensated motor and the two-field compensated motor. The field windings of a two-pole, single-field compensated motor resemble the stator winding of a two-pole, split-phase AC motor. A two-field compensated motor has a stator containing the main winding and a compensating winding spaced 90 electrical degrees apart. The compensating winding reduces the reactance voltage developed in the armature by the alternating flux when the motor operates from an AC source.

img.12

The armature of a typical universal motor resembles the armature of a typical D.C motor except that a universal motor armature is slightly larger for the same horsepower output.

Operation

In a series wound motor, the same current flows through field windings and armature, being connected in series with each other when the motor is connected to either DC or AC supply. The magnetic fields developed by the series field winding and armature currents react with each other and hence develop unidirectional torque.

Since the armature and field are in series, the current is the same through them, and there is no time lag or phase displacement in the magnetic fields. The attraction and repulsion forces will nearly be the same if operated on DC or AC. Because these motors are series-wound, they operate at excessive speed in the no-load condition. As a result. they are usually permanently connected by gears to the devices being driven.

 

Ques 19. The Impedance of the circuit is given by Z = 3 + j4. Its conductance will be

  1. 3 ⁄ 4
  2. 3 ⁄ 25
  3. 3 ⁄ 7
  4. 1 ⁄ 3

As we Know that the

$\begin{array}{l}Admittance = \dfrac{1}{{{\mathop{\rm Im}\nolimits} pedance}} = \dfrac{1}{{3 + j4}}\\\\ = \dfrac{1}{{3 + j4}} \times \dfrac{{3 – j4}}{{3 – j4}}\\\\ = \dfrac{{3 – j4}}{{{3^2} + {4^2}}}\\\\ = \dfrac{3}{{25}} – \dfrac{{j4}}{{25}}\end{array}$

Consider the real part then conductance will be = 3⁄25

 

Ques 20. Thevenin’s voltage and resistance for the circuit given below are respectively.

img.13

  1. 12V, 4.17Ω
  2. 10V, 4.17Ω
  3. 30V, 4.17Ω
  4. 12V, 9.17Ω

To determine the voltage Vth the load resistance RL is removed as shown in the figure, Now by applying Node analysis method. The voltage across 5Ω resistance will be our thevenin’s equiavelnt voltage.

img.14

$\begin{array}{l}\dfrac{{{V_1} – 30}}{{15}} + \dfrac{{{V_1} – {V_a}}}{{10}} – 2 = 0\\\\5{V_1} – 3{V_a} = 120 – – – – – – (1)\\\\\dfrac{{{V_a} – {V_1}}}{{10}} + \dfrac{{{V_a}}}{5} = 0\\\\{V_1} = 3{V_a} – – – – – – (2)\\\\{\text{From equ 1 \& 2 we get}}\\\\{{\rm{V}}_a} = 10V\\\\{V_{th}} = 10V\end{array}$

To determine the Thevenin’s Equivalent circuit Resistance Rth, all the voltage source are replaced by the Short circuit and the voltage by the open circuit as shown in the given figure

img.15

The resistance 15Ω and 10Ω are parallel with resistance 5Ω, therefore, the Thevenin’s Equivalent circuit Resistance Rth 

Rth  = 10 + 15 || 5
= (25 × 5) ⁄  (25 + 5)

Rth  = 4.17Ω

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