DMRC JE Electrical 10th- April- 2018 Paper With Solution and Explanation

• The alternator rotor has to maintain a constant speed as it is designed to produce a certain voltage at the desired frequency.
• However, the real power demand is variable and at any instant, it is the duty of the generator to match (supply) this real power demand.
• When an alternator is connected to the grid if the electrical load increases that mean real power increases. This increase in real power takes place by rotor deceleration for short duration of time.
• Hence the active power of an alternator can be varied by changing the prime mover speed.
• The loading on the generator should not exceed the generator rating as it may lead to heating of the stator.

A unilateral circuit is one whose properties or characteristics change with the direction of the operatic For example, in the diode circuit, the characteristic is different in the forward and reverse direction. Thus a unilateral circuit, the properties or the characteristics are unique in a particular direction of operation.

A bilateral circuit is one whose properties or characteristics are the same in either direction. For example, in a resistive circuit, the current through a resistance is not affected by the direction of current flow.

Linear circuit. A linear circuit is one in which the values of the circuit parameters or components, that is, the resistance, capacitance, inductance, gain, etc. do not change with the level of voltage or current in the circuit. A linear circuit obeys the principle of superposition.

Non-linear circuit. A circuit whose parameters change with the application of voltage or current is called a non-linear circuit. Examples of the non-linear behavior of circuit elements and circuits are diodes, transistors in the saturated region, saturated iron core inductors and transformers, modulators, and digital logic circuit

Non-linear circuits are analyzed using approximate numerical methods by electronic circuit simulati~ computer programs such as spice, for accurate results.

After the occurrence of the fault, when the contacts of the CB begins to separate an arc is established in the contact gap.

The two main causes responsible for generating arc between the contacts of a CB are as follows:

Potential difference (PD) between the contacts: When the contacts have a small separation, the PD between them is sufficient to maintain the arc. One way to extinguish the arc is to separate the contacts to such a distance that PD becomes inadequate to maintain the arc. However, this method is impracticable in high voltage systems where separation of many meters may be required.

Ionized particles between contacts: The ionized particles between the contacts tend to maintain the arc. If the arc path is deionized, the arc extinction will be facilitated. This may be achieved by cooling the arc or by bodily removing the ionized particles from the space between the contacts.

Methods of Arc Extinction

There are two methods of extinguishing the arc in CB  viz.

1. High Resistance Method
2. Low resistance Method

In this method, arc resistance is made to increase with time so that the current is reduced to a value insufficient to maintain the arc. Consequently, the current is interrupted or the arc is extinguished. The principal disadvantage of this method is that enormous energy is dissipated in the arc. Therefore, it is employed only in DC CBs and low-capacity AC CBs.

The resistance of the arc may be increased by:

1. Increasing the Length: The resistance of the arc is directly proportional to its length. The length of the arc can be increased by increasing the gap between contacts.
2. Cooling of Arc:– Cooling helps in the deionization of the medium between the contacts. This increases the arc Resistance. Efficient cooling Slav, be obtained by a gas blast directed along the arc.
3. Reducing the cross-section area of an arc:- If the area of the cross-section of the arc is reduced, the voltage necessary to maintain the arc is increased. In other words, the resistance of the arc path is increased. The cross-section of the arc can be reduced by letting the arc pass through a narrow opening or by having a smaller area of contact.
4. Splitting of arc:- The resistance of the arc can be increased by splitting the arc into a number of smaller arcs in series. Each one of these arcs experiences the effect of lengthening and cooling. The arc may be split by introducing some conducting plates between the contacts.

Low Resistance or Current Zero Method

This method is employed for arc extinction in AC circuits only. In this method, arc resistance is kept low until the current is zero where the arc extinguishes naturally and is prevented from restriking in spite of the rising voltage across the contacts. All modern high-power AC CB employ this method for arc extinction.

In an AC system, the current drops to zero after every half cycle. At every current zero, the arc extinguishes for a brief moment. Now the medium between the contacts contains ions and electrons so that it has a small dielectric strength and can be easily broken down by the rising contact voltage known as restriking voltage. If such a breakdown does occur, the arc will persist for another half cycle. If immediately after current zero, the dielectric strength of the medium between contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike and the current will be interrupted. The rapid increase of dielectric strength of the medium near current zero can be achieved by

1. Lengthening of the gap. The dielectric strength or post-zero resistance is directly proportional to the length of the gap between contacts. Therefore, by opening the contacts rapidly, the higher dielectric strength of the medium can be achieved.
2. Increase the pressure in the vicinity of the arc. If the pressure in the vicinity of the arc is increased, the density of the particles constituting the discharge also increases. The increased density of particles causes a higher rate of de-ionization and consequently, the dielectric strength of the medium between contacts is increased.
3. Blast effect. If the ionized particles between the contacts are swept away and replaced by unionized particles, the dielectric strength of the medium can be increased considerably. This may be achieved by a gas blast directed along with the discharge or by forcing oil into the contact space.
4. Cooling. The natural combination of ionized particles takes place more rapidly if they are allowed to cool. Therefore, the dielectric strength of the medium between the contacts can be increased by cooling the arc.

The term electroplating refers to the deposition of a metal onto the surface of another metal, alloy, or any conductor in general, by the process of electrolysis. It consists of a process, by which a metal is deposited on another metal or alloy by passing a direct current through an electrolyte solution containing the metal ions to be deposited.

Therefore, electroplating can be defined as “a process in which a base metal is coated with a thin and uniform layer of another suitable metal by electrolytic deposition”.

The common coating metals used are Zn, Cu. Ni, Cr. Ag, Pt. Au, etc. Electrodeposition is an important and frequently used method in industries for metal coating.

Electroplating is a process of a continuous flow of ions, which is not possible in an Alternating current(due to changing in polarity very frequently) so the continued deposition of cation does not occur.

Process Involve in electroplating

The process involves the electroplating is:-

1. Cleaning Operation
2. Deposition of Metal
3. Polishing and Buffing
4. Periodic Reverse Current Process
5. Polarization
6. Throwing Power
7. Post-Treatment
8. Waste Disposal

Nickel plating is an electrolytic process whereby a thin layer of nickel is deposited on the surface of a metal item by means of a low voltage electric current. If an item to be Electro-plated is iron or steel it is usually copper plated first to enhance the cohesion of the nickel to the article.

Although Chromium plating is also adopted to protect the iron. However, the plated chromium metal is brittle and hence cracks and peels off. Therefore iron is first plated with copper, then with nickel, and finally with chromium. Copper gives good adhesion, nickel affords protection and chromium enhances the appearance. The composite coating is hard and highly resistant to corrosion.

A synchroscope is a device that produces a revolving field proportional to the speed difference between the incoming machine and the system. It is constructed so that the angular phase difference is indicated on the synchroscope dial.

or

A synchroscope indicates when two AC generators are in the correct phase in relation to connecting in parallel and shows whether the incoming generator is running faster or slower than the on-line generator.

The synchroscope measures the difference in voltage and frequency of the two alternators. The pointer of the synchroscope is free to rotate in a 360° arc. The alternator already connected to the load is considered to be the base machine. The synchroscope indicates whether the frequency of the alternator to be parallel to the base machine is fast or slow. When the voltages of the two alternators are in phase, the pointer covers the shaded area on the face of the meter. When the two alternators are synchronized, the synchronizing switch is closed.

The voltage from one phase of the three-phase bus bar system is connected to one set of synchroscope coils. The voltage from the same phase of the incoming alternator is connected to another set of synchroscope coils. A pointer is attached to the synchroscope rotor and rotates over a dial face.

• If the frequencies of the incoming and running generators are different, the synchroscope will rotate at a speed corresponding to the difference.
• It is designed so that if the incoming frequency is higher than the running frequency, it will rotate in the clockwise direction.
• If the incoming frequency is less than the running frequency, it will rotate in the counterclockwise direction.
• When the synchroscope indicates a 0° phase difference, the pointer is at the ” 12 o’clock” position and the two AC generators are in phase. When the pointer stops in a vertically upward position, the frequencies are equal and the voltages are in phase. This means that the alternators are in synchronism and the alternator switch can be closed to parallel both machines.

Testing of Insulation Resistance of Complete Installation to Earth

The principal purpose of testing the insulation is to verify that there are no inadvertent connections between live conductors and between live and Earth before the installation is energized. Tests are required between live conductors (e.g. between phases and between phase(s) and neutral) and between all live conductors and Earth.

 

Insulation resistance of the installation depends on many factors such as atmospheric conditions, humidity, dirt, etc. As such its calculation is not possible, but it can be readily measured. Normally the insulation resistance is quite high and can be measured by an instrument called megger; usually used for the measurement of high resistance. The main object in performing this test is to ascertain whether the complete wiring is sound enough to avoid leakage current. As per the Indian electricity rules, the leakage current of an installation should not exceed 1/5000 of the maximum supply-demand of the consumer. As such the insulation resistance of the complete installation to earth should not be less than 1 MΩ. Insulation resistance test must be conducted on dc voltage of value not less than twice the standard supply voltage. However, it does need not exceed  500 V for medium voltage circuits.

Insulation resistance of the installation to earth is measured by a megger generating dc voltage of about 500 V. Insulation resistance is measured under the following conditions.

(i) Main switch in off position.

(ii) The main fuse to be taken out.

(iii) All other fuses to be provided.

(iv) All lamps must be put in the on position.

(v) All switches must be made on.

(vi) Live and neutral wire to be shorted together.

Testing of Insulation Resistance Between Conductors

After testing the insulation resistance of installation to earth, the insulation resistance between line and neutral should be measured. The major object in conducting this test is again to ensure that the insulation is quite sound between the conductors and there are no chances of an appreciable leakage current between them.

This test should be carried out under the following conditions of installation

(i) Main switch off.

(ii) the Main fuse is withdrawn.

(iii) All other fuses in their position.

(iv) All switches in on position.

(v) All lamps from the holders were removed.

(vi)Two terminals of the meager are connected to the line and neutral wire.

Insulation resistance measured under the above condition by a megger should not be less than 50 MΩ divided by the number of outlets(points + switches). However, it need not be more than 1 MΩ.

RELUCTANCE

Flux in a magnetic circuit also depends on the opposition that the circuit presents to it. Reluctance (R_M), is the opposition a magnetic circuit offers to the formation of magnetic flux.

The opposition depends on the dimensions of the core and the material of which it is made. Like the resistance of a wire, reluctance is directly proportional to length (L) and inversely proportional to cross-sectional area (A). In equation form,

R_M = L/A (Ampere-Weber)

The unit of Reluctance is Ampere-Weber or A/Wb

To change the ‘proportional’ sign into an ‘equals’ sign we must introduce a constant. This constant is called ‘magnetic reluctivity‘ and is directly equivalent to resistivity in an electric circuit. However, it’s far more common to use it’s reciprocal instead, which we call ‘absolute permeability(μ)‘ Which is equivalent to conductivity.

Permeability is a measure of how easy it is to establish the flux in a material. Ferromagnetic materials have high permeability and hence low R_M, while nonmagnetic materials have low permeability and high R_M.

R_M = L/μA

FIELD AND ARMATURE CONFIGURATIONS

There are two arrangements of fields and armatures:

1. Revolving armature and the stationary field
2. Revolving field and stationary armature.

 ADVANTAGES OF ROTATING FIELD IN AN ALTERNATOR

In large alternators, the rotating field arrangement is usually forward due to the following advantages.

1. Ease of Construction: Armature winding of large alternators is complex, the connection and bracing of the armature windings can be easily made for the stationary stator.
2. The number of Slip Rings: If the armature is made rotating, the number of slip rings required for power transfer from the armature to the external circuit is at least three. Also, heavy current flows through brush and slip rings cause problems and require more maintenance in large alternators. Insulation required for slip rings from the rotating shaft is difficult with the rotating armature system.
3. High voltage generation:- Voltages can be generated as high as 11,000 and 13,800 V. These values can be reached because the stationary armature windings do not undergo vibration and centrifugal stresses.
4. High current Rating:-  Alternators can have relatively high current ratings. Such ratings are possible because the output of the alternator is taken directly from the stator windings through heavy, well-insulated cables to the external circuit. Neither slip rings nor a commutator is used.
5. Better Insulation to Armature: Insulation arrangement of armature windings can easily be made from the core on the stator.
6. Reduced Rotor Weight and Rotor Inertia:  Since the field system is placed on the rotor, hence the insulation requirement is less (for low dc voltage). Also, rotational inertia is less. It takes lesser time to gain full speed.
7. Improved Ventilation Arrangement: The cooling can be provided by enlarging the stator core with radial ducts. Water cooling is much easier if the armature is housed in the stator.
8. Direct Output Voltage:- The output terminals are fixed in case of the stationary armature and hence output current can be taken out directly without having it to pass through brush contacts.

Due to the armature is stationary the phenomenon of armature reaction takes place in an alternator

Armature Reaction

When the load is connected to the alternator, the armature winding of the alternator carries a current. Every current-carrying conductor produces its own flux so the armature of the alternator also produces its own flux when carrying a current. So there are two fluxes present in the air gap, one due to armature current while the second is produced by the field winding called main flux. The flux produced by the armature is called armature flux. Hence it is not easy for the armature to carry stator flux due to armature reaction.

Extrinsic Semiconductor

In order to change the properties of intrinsic semiconductors a small amount of some other material is added to it. The process of adding other material to the crystal of intrinsic semiconductors to improve its conductivity is called doping. The impurity added is called dopant. The doped semiconductor material is called extrinsic semiconductors. The doping increases the conductivity of the basic intrinsic semiconductors hence the extrinsic semiconductors are used in practice for the manufacturing various electronic devices such as diodes, transistors, etc.

Depending upon the type of impurities, the two types of extrinsic semiconductors are,

1. N-TYPE
2. P-TYPE

N-TYPE Impurity

The impurity material having five valence electrons is called the pentavalent atom. When this is added to an intrinsic semiconductor, it is called donor doping as each impurity atom donates one free electron to an intrinsic material. Such an impurity is called donor impurity. Examples of such impurities are arsenic, bismuth, phosphorous, Antimony, etc. This crests an extrinsic semiconductor with a large number of free electrons called an n-type semiconductor.

Germanium and Silicon are tetravalent. The impurity atoms may be either pentavalent or trivalent, i.e., from groups V and III of the periodic table. If a small quantity of a pentavalent impurity (having 5-electrons in tt outermost orbit) like Arsenic (As), Antimony (Sb), or Phosphorus (P) is introduced in Germanium, it replaces the equal number of Germanium atoms without changing the physical state of the crystal. Each of the four out of five valency electrons of impurity says Arsenic enters into covalent bonds with Germanium, while the fifth valence electron is set free to moo from one atom to the other as shown in Fig. The impurity is called donor impurity as it donates an electron and the crystal is called an N-type semiconductor.

A small amount of Arsenic (impurity) injects billions of free electrons into Germanium thus it creasing its conductivity enormously. In an N-type semiconductor, the majority carriers of charge are the electrons and holes as minority carriers. This is because when donor atoms are added to a semiconductor, the extra free electrons give the semiconductor a greater number of free electrons than it would normally have.

The direction of rotation of a split-phase motor depends on the magnetic polarity of the start winding. Reversing either the polarity of the “Start” winding, in relationship to the “Run” winding, reverses the direction of rotation of all single-phase alternating current (AC) motors.