← Junior Engineer
Past Questions — 2024
In winter, Solar heating system in India should face towards which direction and kept at an inclination equal to latitude?
A)
South
B)
North
C)
East
D)
West
1. Geographic Location: India is located in the Northern Hemisphere (north of the Equator).
2. Solar Position: In the Northern Hemisphere, the sun is always positioned towards the south of the observer's zenith for most of the year, and specifically during winter, the sun remains low in the southern sky.
3. Maximizing Irradiation: To capture the maximum amount of solar radiation, solar collectors (panels or heating systems) must face the equator. For any location in the Northern Hemisphere, the equator is to the South.
4. Inclination: Setting the inclination equal to the latitude is a general rule of thumb to optimize the average annual energy collection, though for winter specifically, a steeper angle (latitude + 15°) is often used to account for the lower sun angle. However, the directional facing remains constant.
5. Analysis of Options:
- North: Facing north in India would mean the collector faces away from the sun, resulting in minimal energy gain.
- East/West: These directions capture morning or evening sun but not the peak intensity of the midday sun throughout the day.
The AC voltage controller are used in application.
A)
Power generation
B)
Electric heating
C)
Conveyor belt motion
D)
Power transmission
An AC voltage controller (also known as an AC voltage regulator) is a power electronics circuit that converts a fixed-voltage AC source to a variable-voltage AC source with a controllable root-mean-square (RMS) value.
1. Why 'Electric heating' is correct: Electric heating elements are typically resistive loads. AC voltage controllers (using TRIACs or Thyristors) are ideal for controlling the power delivered to resistive loads by adjusting the firing angle, thereby controlling the temperature of the heating element. Common examples include domestic light dimmers and industrial heater controls.
2. Why other options are incorrect:
- Power generation (a): This involves synchronous generators and large-scale electrical machinery, not AC voltage controllers.
- Conveyor belt motion (c): While AC voltage controllers can be used for some small motors, conveyor belts usually require precise speed and torque control, which is typically handled by Variable Frequency Drives (VFDs) that change both voltage and frequency, rather than just the voltage.
- Power transmission (d): This involves high-voltage transformers and HVDC/HVAC systems for long-distance transport, not the local voltage regulation provided by AC voltage controllers.
In the short-circuit test on a transformer, we generally short the-
A)
tertiary winding.
B)
high-voltage winding only.
C)
low-voltage winding only.
D)
low-voltage and high voltage winding.
In a short-circuit (SC) test of a transformer, the objective is to determine the full-load copper losses and the equivalent impedance of the transformer. To perform this test safely and accurately:
1. The Low-Voltage (LV) winding is short-circuited using a thick conductor.
2. A reduced voltage is applied to the High-Voltage (HV) winding.
Reasoning for using the HV side for input and shorting the LV side:
- Measurement Accuracy: The HV side has a higher impedance and lower rated current. This makes it easier to measure the current and voltage using standard instruments without requiring extremely high-current ammeters.
- Safety and Control: Since the SC test requires applying a very small fraction of the rated voltage (typically 2% to 12%) to reach rated current, it is much easier and safer to control and vary a small voltage on the HV side than to apply a very tiny, precise voltage on the LV side.
- Equipment: Standard variable AC sources (variacs) are more suitable for the HV side's voltage range during this specific test.
Therefore, the LV winding is the one that is shorted.
What is the unit of the apparent or complex power?
A)
VA
B)
ohm
C)
Volt
D)
VAR
In electrical engineering, power in an AC circuit is represented as a complex quantity.
1. **Apparent Power (S):** This is the product of the RMS voltage (V) and RMS current (I). Its unit is the Volt-Ampere (VA). It represents the total power delivered to a circuit.
2. **Complex Power:** This is the vector sum of Real Power (P) and Reactive Power (Q), expressed as S = P + jQ. Since it is measured in terms of the total capacity of the system, its unit is also the Volt-Ampere (VA).
Analysis of other options:
- **b. ohm:** This is the unit of electrical resistance or impedance, not power.
- **c. Volt:** This is the unit of electrical potential difference or electromotive force.
- **d. VAR (Volt-Ampere Reactive):** This is the unit specifically for **Reactive Power (Q)**, which is the power that oscillates between the source and the load without being consumed.
In nickel cadmium cells, the electrolyte is a mixture of -
A)
potassium chloride and deionised water.
B)
potassium hydroxide and deionised water.
C)
sulphuric acid and water.
D)
potassium peroxide and deionised water.
A Nickel-Cadmium (Ni-Cd) battery is a type of rechargeable alkaline battery. The electrolyte used in these cells is an aqueous solution of potassium hydroxide (KOH), which is a strong base. Deionised water is used as the solvent to ensure there are no impurities that could interfere with the electrochemical reactions.
Breakdown of options:
- Option (a): Potassium chloride is a salt and not the standard electrolyte for Ni-Cd cells.
- Option (b): Correct. Potassium hydroxide (KOH) provides the necessary alkalinity for the cell to function.
- Option (c): Sulphuric acid is the electrolyte used in Lead-Acid batteries, not Ni-Cd batteries.
- Option (d): Potassium peroxide is not used as an electrolyte in these cells.
The primary and secondary of a transformer are-
A)
electrically coupled.
B)
magnetically coupled.
C)
electrically and magnetically coupled.
D)
directly coupled.
A transformer is a static electrical device that transfers electrical energy from one circuit to another through the process of electromagnetic induction.
1. **Mechanism of Operation**: When an alternating current (AC) flows through the primary winding, it creates a varying magnetic flux in the core. This flux links with the secondary winding, inducing an electromotive force (EMF) according to Faraday's Law of Induction.
2. **Magnetic Coupling**: Because the energy transfer happens via a magnetic field (flux) rather than a physical wire connection, the windings are said to be 'magnetically coupled'.
3. **Electrical Isolation**: In a standard transformer, the primary and secondary windings are physically and electrically separated (insulated from each other). This provides 'galvanic isolation', meaning there is no direct electrical path (current flow) between the two circuits.
Analysis of options:
- (a) Incorrect: They are not electrically coupled; if they were, it would be a short circuit or a direct connection.
- (b) Correct: The energy transfer is achieved solely through the magnetic field.
- (c) Incorrect: Since they are not electrically coupled, this combined option is wrong.
- (d) Incorrect: 'Directly coupled' implies a physical electrical connection, which contradicts the fundamental design of a transformer.
Which type of rotor is used in ceiling fan?
A)
Two phase induction motor
B)
Single phase induction motor
C)
Series motors
D)
Parallel motors
Step-by-step reasoning:
1. **Application Analysis**: A ceiling fan is a domestic appliance powered by a standard single-phase AC supply (typically 230V, 50Hz). Therefore, the motor must be compatible with a single-phase power source.
2. **Motor Type**: Ceiling fans use a specific type of single-phase induction motor known as a 'Permanent Split Capacitor (PSC)' motor. This motor uses a capacitor to create a phase shift between the main winding and the auxiliary winding to produce a rotating magnetic field, which is necessary to start the rotor.
3. **Rotor Construction**: The rotor used in these motors is a 'Squirrel Cage' rotor, which is the standard rotor for induction motors due to its robustness and low maintenance.
4. **Evaluating Options**:
- **Option (a)**: Two-phase induction motors are rare and not used in standard domestic ceiling fans.
- **Option (b)**: Correct. Ceiling fans operate on single-phase AC and utilize a single-phase induction motor.
- **Option (c)**: Series motors (like Universal motors) are used in vacuum cleaners or mixers where high starting torque and variable speed are needed; they are too noisy and unsuitable for ceiling fans.
- **Option (d)**: 'Parallel motors' is not a standard classification for the type of motor used in this application.
Majority of the induction motor have-
A)
totally enclosed frame
B)
drip proof type frame
C)
totally enclosed fan cooled frame
D)
good sound proof frame
In industrial applications, induction motors are exposed to dust, moisture, and varying environmental conditions. To ensure longevity and reliability, the most common construction used is the Totally Enclosed Fan Cooled (TEFC) frame.
1. **Totally Enclosed**: This means the internal windings and rotor are sealed off from the external atmosphere, preventing the entry of contaminants like dust and moisture, which would otherwise cause electrical failure or mechanical wear.
2. **Fan Cooled**: Since the motor is enclosed, it cannot draw in outside air for cooling. Therefore, an external cooling fan is mounted on the shaft, which blows air over the external ribs (fins) of the motor frame to dissipate heat generated by I²R losses.
Comparing other options:
- **Totally enclosed frame (a)**: While correct in part, it is incomplete because it doesn't specify the cooling mechanism, which is essential for the motor's operation.
- **Drip proof type frame (b)**: These are open frames that protect against falling liquids but allow dust and air to enter. They are used in clean environments but are not the 'majority' choice for general industrial use.
- **Good sound proof frame (d)**: Soundproofing is a specialized requirement and not a standard feature of the majority of general-purpose induction motors.
The World Radiation Centre (WRC) has adopted a value of solar constant as -
A)
1947 W/m²
B)
1467 W/m²
C)
1227 W/m²
D)
1367 W/m²
The solar constant is defined as the amount of solar energy received per unit time on a unit area perpendicular to the rays of the sun at the average distance between the Earth and the Sun. While various values have been proposed over time due to measurement refinements, the World Radiation Centre (WRC) adopted the standard value of 1367 W/m².
- Option (a) 1947 W/m² is incorrect as it is significantly higher than the measured solar irradiance.
- Option (b) 1467 W/m² is incorrect as it exceeds the standard accepted value.
- Option (c) 1227 W/m² is incorrect as it is lower than the standard accepted value.
- Option (d) 1367 W/m² is the internationally recognized standard value adopted by the WRC for solar radiation calculations.
Two things which are same for primary and secondary of transformer are-
A)
ampere-turns and voltage per turn
B)
resistance and leakage reactances
C)
currents
D)
voltages
In an ideal transformer, two fundamental parameters remain identical for both the primary and secondary windings:
1. **Voltage per Turn**: The voltage induced in a transformer is proportional to the number of turns. The formula is E = 4.44 f N Φm. Therefore, the voltage per turn (E/N) is equal to 4.44 f Φm. Since the frequency (f) and the maximum magnetic flux (Φm) are the same for both primary and secondary windings (as they share the same core), the voltage per turn is identical for both.
2. **Ampere-Turns (MMF)**: According to Ampere's Law, the Magnetomotive Force (MMF) required to establish a certain flux in the core is the same regardless of whether it is produced by the primary or secondary winding. Thus, N1I1 ≈ N2I2 (neglecting the small magnetizing current). This means the ampere-turns are the same for both sides.
Why other options are incorrect:
- **Option b**: Resistance and leakage reactances depend on the physical construction (wire gauge, length, and winding geometry), which differ between primary and secondary in almost all transformers.
- **Option c**: Currents differ based on the turns ratio (I1/I2 = N2/N1).
- **Option d**: Voltages differ based on the turns ratio (V1/V2 = N1/N2).
The metal used for making solar cells is -
A)
Gold
B)
Iron
C)
Aluminium
D)
Silicon
Solar cells (photovoltaic cells) are semiconductor devices that convert sunlight directly into electricity. Silicon (Si) is the most widely used material for this purpose because it is an abundant semiconductor with properties that allow it to efficiently absorb photons and release electrons. While the question uses the term 'metal', in the context of electrical engineering and solar energy, it refers to the primary elemental material used for the cell's construction.
Why other options are incorrect:
- Gold (a): While gold may be used in very small quantities for electrical contacts or thin-film coatings due to its high conductivity and corrosion resistance, it is not the primary material used to make the cell itself.
- Iron (b): Iron is a conductor and does not possess the semiconductor properties (band gap) required to create a p-n junction for the photovoltaic effect.
- Aluminium (c): Aluminium is used for the back-contact layer or frame of the solar panel, but it cannot function as the light-absorbing semiconductor material.
What should be the band gap of the semiconductors to be used as solar materials?
A)
0.5eV
B)
1.0eV
C)
1.5eV
D)
1.9eV
The efficiency of a solar cell depends heavily on the band gap (Eg) of the semiconductor material used. The goal is to maximize the absorption of the solar spectrum while minimizing energy loss due to thermalization. According to the Shockley-Queisser limit, the ideal band gap for a single-junction solar cell to achieve maximum efficiency under the AM1.5 solar spectrum is approximately 1.34 eV to 1.5 eV.
1. If the band gap is too small (e.g., 0.5 eV or 1.0 eV), the cell will absorb more photons, but a large portion of the photon energy will be lost as heat (thermalization), and the open-circuit voltage (Voc) will be very low.
2. If the band gap is too large (e.g., 1.9 eV), the cell will have a high Voc, but it will be unable to absorb a significant portion of the solar spectrum (low-energy photons will pass through), leading to a low short-circuit current (Isc).
Silicon (Si), the most common solar material, has a band gap of ~1.1 eV, but GaAs (Gallium Arsenide), which is closer to the ideal limit at ~1.42 eV, is significantly more efficient. Among the given options, 1.5 eV is the closest to the theoretical optimum for maximum efficiency.
Photovoltaic cells are materials,
A)
superconductor
B)
conductor
C)
semiconductor
D)
bad conductor
Photovoltaic (PV) cells are designed to convert light energy (photons) directly into electrical energy (electrons) through the photovoltaic effect. This process requires a material that has a specific 'band gap'—a gap between the valence band and the conduction band.
1. **Why Semiconductor (Correct):** Semiconductors (most commonly Silicon, Gallium Arsenide, or Cadmium Telluride) have electrical properties between those of a conductor and an insulator. When photons with energy greater than the band gap hit the semiconductor, they excite electrons from the valence band to the conduction band, creating electron-hole pairs. By creating a p-n junction within the semiconductor, these charges are separated, creating an electric current.
2. **Why not Superconductor (Incorrect):** Superconductors have zero electrical resistance at very low temperatures. They are used for high-efficiency power transmission or magnets, not for capturing light energy.
3. **Why not Conductor (Incorrect):** Conductors (like Copper or Aluminum) have overlapping energy bands. Electrons move freely, and they cannot maintain the internal electric field (depletion region) necessary to separate charges generated by light.
4. **Why not Bad Conductor (Incorrect):** Bad conductors (insulators) have a band gap that is too wide for visible light photons to excite electrons into the conduction band, meaning they cannot generate a current from sunlight.
In a 3-phase induction motor, iron loss mainly occurs in-
A)
stator core and stator teeth
B)
stator and rotor
C)
rotor core and rotor teeth
D)
stator winding
In a 3-phase induction motor, iron losses (also known as core losses) consist of hysteresis loss and eddy current loss. These losses occur due to the alternating magnetic flux passing through the ferromagnetic material of the motor.
1. Stator Core: The stator is connected to the AC supply, meaning the magnetic flux in the stator core is constantly alternating at the supply frequency. This leads to significant hysteresis and eddy current losses in the stator core and the stator teeth.
2. Rotor Core: In an induction motor, the rotor frequency is equal to the slip frequency (s * f). Since the slip (s) is typically very small (usually 2% to 5%) during normal operation, the frequency of the flux in the rotor is very low. Because iron losses are proportional to frequency (hysteresis) and the square of the frequency (eddy current), the iron losses in the rotor are negligible compared to those in the stator.
3. Stator Winding: Losses in the winding are 'copper losses' (I²R losses), not 'iron losses'.
Therefore, iron losses occur mainly in the stator core and stator teeth.
The gas used in sodium vapour lamp acts as a starter is -
A)
Neon
B)
Lithium
C)
Argon
D)
Hydrogen
In a sodium vapour lamp (specifically the High Pressure Sodium or Low Pressure Sodium lamps), sodium is a metal with a high boiling point, meaning it exists as a solid or liquid at room temperature and cannot be ionized directly to start the discharge process. To initiate the lamp, a small amount of a noble gas—typically Neon or Argon—is added to the discharge tube. This gas acts as a 'starter' because it has a much lower ionization potential than sodium. When the voltage is applied, the Argon/Neon gas ionizes first, creating a plasma discharge that generates heat. This heat vaporizes the sodium metal, which then takes over the discharge process and produces the characteristic yellow light. While Neon is sometimes used in specific lamp types, Argon is the standard starter gas used in the majority of industrial sodium vapour lamp configurations to facilitate the initial arc.
Battery cells are connected in series to -
A)
increase the current capacity
B)
increase the output voltage
C)
decrease the internal resistance
D)
decrease the ampere hour
In electrical circuits, when battery cells are connected in series (positive terminal of one cell connected to the negative terminal of the next), the total voltage of the battery bank is the sum of the individual voltages of each cell. For example, if you connect three 1.5V cells in series, the total output voltage becomes 4.5V.
Analysis of other options:
- Option (a) 'increase the current capacity': This is achieved by connecting cells in parallel, not series. In a series connection, the current capacity (Ah) remains the same as that of a single cell.
- Option (c) 'decrease the internal resistance': Connecting cells in series actually increases the total internal resistance, as the resistances of individual cells add up (R_total = R1 + R2 + ...).
- Option (d) 'decrease the ampere hour': Connecting cells in series does not decrease the ampere-hour (Ah) rating; it keeps it constant relative to a single cell's capacity.
A two blade turbine produces maximum power when the tip speed ratio is equal to -
A)
1
B)
2x
C)
3%
D)
0.593
For a two-blade wind turbine, the maximum power is obtained at a higher tip-speed ratio than for multi-blade rotors. The commonly accepted optimum tip-speed ratio for a two-blade turbine is about 2 (often written as 2x in such exam options). Hence option **b** is correct.
Why the others are incorrect:
- **a: 1** — too low for a two-blade high-speed turbine; tip-speed ratio 1 is more typical of very slow, high-solidity rotors.
- **d: 0.593** — this is the Betz limit (maximum theoretical power coefficient), not a tip-speed ratio.
- **c: 3%** — not a valid tip-speed ratio value; it is dimensionally and numerically incorrect in this context.
A 50 Hz, 4poles, a single phase induction motor is rotating in the clockwise direction at a speed of 1425 rpm. The slip of motor in the direction of rotation & opposite direction of the motor will be respectively.
A)
0.05, 0.95
B)
0.04, 1.96
C)
0.05, 1.95
D)
0.05, 0.02
Step-by-step calculation:
1. **Calculate Synchronous Speed ($N_s$):**
The formula for synchronous speed is $N_s = \frac{120f}{P}$
Given: Frequency ($f$) = 50 Hz, Poles ($P$) = 4
$N_s = \frac{120 \times 50}{4} = \frac{6000}{4} = 1500$ rpm.
2. **Calculate Slip in the direction of rotation ($s_{forward}$):**
The motor is rotating at $N = 1425$ rpm in the clockwise direction.
$s_{forward} = \frac{N_s - N}{N_s} = \frac{1500 - 1425}{1500} = \frac{75}{1500} = 0.05$.
3. **Calculate Slip in the opposite direction ($s_{backward}$):**
Slip is defined relative to the synchronous speed of the field. For a motor rotating in the opposite direction to the field's rotation (or calculating the slip relative to the reverse direction), the formula is:
$s_{backward} = \frac{N_s - (-N)}{N_s} = \frac{N_s + N}{N_s}$
$s_{backward} = \frac{1500 + 1425}{1500} = \frac{2925}{1500} = 1.95$.
Alternatively, using the property of single-phase induction motors (Double Revolving Field Theory), the forward slip is $s$ and the backward slip is $2 - s$.
$s_{backward} = 2 - 0.05 = 1.95$.
Therefore, the slips are 0.05 and 1.95 respectively.
Which of the following is the principal advantage of a solar cell?
A)
It shows oscillatory motion
B)
It has metal in the outer area
C)
it has moving parts
D)
It has no moving part
A solar cell (photovoltaic cell) is a solid-state semiconductor device that converts sunlight directly into electricity via the photovoltaic effect.
1. Why 'd' is correct: Because it is a static device with no mechanical moving parts, it requires virtually no maintenance, operates silently, and does not suffer from mechanical wear and tear. This is one of its primary advantages over traditional generators (like turbines or internal combustion engines) which rely on rotation and friction.
2. Why 'a' is incorrect: Solar cells do not exhibit oscillatory motion; they produce a Direct Current (DC) output based on light intensity.
3. Why 'b' is incorrect: While solar cells have metal contacts (grids) to collect current, having metal in the outer area is a design requirement for conductivity, not a 'principal advantage' of the technology itself.
4. Why 'c' is incorrect: This is the opposite of the truth. The absence of moving parts is the advantage; having moving parts would increase maintenance costs and failure rates.
What is the purpose of using bypass diodes in the series connected solar panels?
A)
Increasing the current through shaded cells
B)
To increase the resistance of shadow cells
C)
Protection of shaded cells
D)
To redirect light for making solar cells
In a series-connected string of solar cells, the current is limited by the cell with the lowest current production (the 'bottleneck'). When a cell is shaded, its resistance increases significantly. Because the other unshaded cells continue to push current through the string, the shaded cell begins to act as a load rather than a generator. This causes the shaded cell to dissipate power as heat, which can lead to 'hot-spot' formation, permanent cell damage, or even the melting of the module encapsulation.
Bypass diodes are connected in parallel with a group of cells. When a cell is shaded and its voltage drops below the diode's forward voltage, the diode becomes forward-biased and provides an alternative low-resistance path for the current to flow around the shaded section. This prevents the shaded cell from overheating, thereby protecting it from damage.
Analysis of other options:
- Option (a) is incorrect because the diode does not increase current 'through' the shaded cell; it diverts current 'around' it.
- Option (b) is incorrect because the diode decreases the overall resistance of the shaded path, it does not increase the resistance of the cell itself.
- Option (d) is incorrect because diodes are electronic components and have no physical capability to redirect light.
The light energy supplied to solar cell is in the form of -
A)
Electron
B)
Photons
C)
Neutron
D)
Isotope
A solar cell (photovoltaic cell) works on the principle of the photovoltaic effect. Light is an electromagnetic wave that consists of discrete packets of energy called 'photons'. When these photons strike the semiconductor material (usually silicon) of a solar cell, they transfer their energy to the electrons in the valence band. If the energy of the photon is greater than the bandgap energy of the semiconductor, an electron is excited to the conduction band, creating an electron-hole pair, which generates an electric current.
Why other options are incorrect:
- a. Electron: Electrons are the charge carriers that move within the cell to create current; they are not the form of light energy supplied to the cell.
- c. Neutron: Neutrons are subatomic particles found in the nucleus of an atom and are not components of light energy.
- d. Isotope: An isotope is a variant of a particular chemical element which differs in neutron number; it is a type of matter, not a form of energy.
The percentage of the incoming radiation reflected back to space by the earth is -
A)
10%
B)
20%
C)
30%
D)
40%
The percentage of incoming solar radiation reflected back into space is known as the 'Albedo' of the Earth. The Earth's average planetary albedo is approximately 0.30, meaning about 30% of the total solar radiation reaching the Earth's atmosphere is reflected back to space. This reflection occurs due to various factors: roughly 23% is reflected by clouds, 7% by the atmosphere (scattering), and 10% by the Earth's surface (such as ice, snow, and deserts). The remaining 70% is absorbed by the atmosphere and the Earth's surface, which then heats the planet. Therefore, option (c) 30% is the standard scientific value used in solar energy and climatology textbooks.
The element require for solar energy conversion is -
A)
Beryllium
B)
Silicon
C)
Tantalum
D)
Ultra pure carbon
Solar energy conversion, specifically in Photovoltaic (PV) cells, relies on the use of semiconductor materials that exhibit the photovoltaic effect. Silicon (Si) is the most widely used semiconductor material for this purpose because it is abundant, stable, and possesses an ideal bandgap to absorb a significant portion of the solar spectrum. When photons from sunlight hit the silicon wafer, they excite electrons, creating electron-hole pairs that generate an electric current.
Analysis of other options:
- Beryllium (a): An alkaline earth metal used primarily in aerospace and X-ray windows; it is not a semiconductor used for solar conversion.
- Tantalum (c): A transition metal used mainly in capacitors and surgical implants; it does not possess photovoltaic properties.
- Ultra pure carbon (d): While certain forms of carbon (like graphene or carbon nanotubes) are researched for next-generation solar cells, 'ultra pure carbon' (graphite/diamond) is not the standard element required for conventional solar energy conversion systems.
The winding of a transformer with greater number of turns will be-
A)
high-voltage winding.
B)
low-voltage winding.
C)
either high or low voltage winding.
D)
tertiary winding.
The fundamental principle of a transformer is based on Faraday's Law of Electromagnetic Induction. The relationship between the primary and secondary voltages and the number of turns in the windings is given by the transformer turns ratio formula: V1/V2 = N1/N2, where V represents voltage and N represents the number of turns. This indicates that the voltage is directly proportional to the number of turns (V ∝ N). Therefore, the winding that has a greater number of turns will produce or require a higher voltage. Conversely, the winding with fewer turns will be the low-voltage winding. Option (b) is incorrect because low-voltage windings have fewer turns. Option (c) is incorrect because the relationship is fixed by the turns ratio. Option (d) is incorrect as a tertiary winding is an additional third winding used for specific purposes (like harmonic suppression) and its turn count depends on its specific design requirement, not a general rule of 'greater turns'.
Which commutation technique is also called line commutation?
A)
Class F
B)
Class D
C)
Class E
D)
Class A
In power electronics, commutation refers to the process of turning off a thyristor (SCR). Commutation techniques are categorized into several classes based on the method used to achieve the turn-off condition (reducing the current to zero and applying a reverse voltage).
1. **Class A (Self-Commutation / Line Commutation):** In this technique, the thyristor is turned off by the natural commutation of the AC supply voltage. When the supply voltage reverses polarity, the current through the SCR naturally drops to zero, and the device is reverse-biased. Because this process relies on the AC line voltage, it is specifically called 'Line Commutation'. This is common in phase-controlled rectifiers.
2. **Class B (Self-Commutation):** Uses an LC resonant circuit to force the current to zero.
3. **Class C (Complementary Commutation):** Uses an auxiliary SCR to turn off the main SCR.
4. **Class D (Auxiliary Commutation):** Uses an external capacitor and an auxiliary SCR to force the main SCR to turn off.
5. **Class E (External Pulse Commutation):** Uses an external pulse source to achieve commutation.
Therefore, Class A is the correct answer as it is synonymous with line commutation.