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Past Questions
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.
A step down cyclo converter is a device which converts -
A)
DC to AC
B)
AC to AC only by changing higher input frequency into lower output frequency
C)
AC to AC only by changing lower input frequency into higher output frequency
D)
AC to DC
A cycloconverter is a power electronics device that converts AC power of one frequency to AC power of another frequency without an intermediate DC link. It is essentially an AC-to-AC frequency converter.
1. **Cycloconverter Definition**: It is a device that converts a fixed-frequency AC input to a variable-frequency AC output.
2. **Step-down Cycloconverter**: In a step-down (or descending) cycloconverter, the output frequency (f_out) is lower than the input frequency (f_in). For example, converting 50 Hz AC to 10 Hz AC. This is the most common type of cycloconverter used in industrial applications like cement mills or ship propulsion.
3. **Step-up Cycloconverter**: Conversely, a step-up (or ascending) cycloconverter converts a lower input frequency to a higher output frequency.
Evaluating the options:
- Option (a) describes an Inverter.
- Option (b) correctly describes a step-down cycloconverter (AC to AC, higher input to lower output).
- Option (c) describes a step-up cycloconverter.
- Option (d) describes a Rectifier.
The process of turning off a thyristor is called -
A)
Mounting
B)
Commutation
C)
Free-Wheeling
D)
Modulation
A thyristor (such as an SCR) is a semi-controlled device, meaning it can be turned ON by a gate pulse, but it cannot be turned OFF by the gate. To turn off a thyristor, the anode current must be reduced below the holding current level, or the polarity of the voltage across the device must be reversed. This specific process of turning off a thyristor is technically termed 'Commutation'.
Analysis of other options:
- Mounting: Refers to the physical installation of the device on a heat sink or PCB.
- Free-Wheeling: Refers to a circuit (usually involving a diode) that allows current to continue flowing in an inductive load after the main switch is turned off, preventing voltage spikes.
- Modulation: Refers to the process of varying a property of a periodic waveform (like in PWM - Pulse Width Modulation) to control the average power delivered to a load.
The capacitor used in single phase induction motor have no -
A)
voltage rating
B)
dielectric medium
C)
polarity marking
D)
definite value
In a single-phase induction motor, the capacitor used is an AC (Alternating Current) capacitor. Capacitors used in AC circuits are non-polarized, meaning they can be connected in either direction across the circuit without damage. Therefore, they have no polarity marking.
Analysis of other options:
- (a) Voltage rating: Every capacitor must have a voltage rating to prevent dielectric breakdown; otherwise, it would fail under the operating voltage of the motor.
- (b) Dielectric medium: A capacitor consists of two conducting plates separated by an insulating material known as the dielectric medium (e.g., polypropylene, polyester, or oil). Without this, the capacitor cannot function.
- (d) Definite value: Capacitors are manufactured with a specific capacitance value (measured in microfarads, μF) to provide the necessary phase shift for the starting torque of the motor.
If the capacity of a battery is 4000Ah, it can supply 4000 Ampere current for-
A)
120min
B)
240 min
C)
60min
D)
30min
The capacity of a battery is defined as the total charge it can deliver, typically measured in Ampere-hours (Ah). The formula relating capacity, current, and time is: Capacity (Ah) = Current (A) × Time (h). Given: Capacity = 4000 Ah and Current = 4000 A. Substituting these values into the formula: 4000 Ah = 4000 A × Time (h). Solving for Time: Time = 4000 / 4000 = 1 hour. Since the options are provided in minutes, we convert 1 hour to minutes: 1 hour × 60 minutes/hour = 60 minutes. Therefore, the battery can supply 4000 A for 60 minutes.
The specific gravity of a fully charged lead-cell is-
A)
1.0
B)
1.1
C)
1.22
D)
2.12
In a lead-acid battery, the state of charge is directly related to the specific gravity of the electrolyte (a mixture of sulfuric acid and water).
1. **Chemical Process**: During charging, sulfuric acid is produced and dissolved into the water, increasing the density (specific gravity) of the electrolyte. During discharging, the acid reacts with the lead plates to form lead sulfate, which decreases the density of the electrolyte.
2. **Standard Values**:
- A fully charged cell typically has a specific gravity ranging from **1.25 to 1.28** (depending on the grade of the battery), but in many technical textbooks and exam contexts, values around **1.22 to 1.28** are considered indicative of a charged state.
- A fully discharged cell typically drops to a specific gravity of approximately **1.10 to 1.15**.
3. **Evaluating Options**:
- Option (a) 1.0: This is the specific gravity of pure water.
- Option (b) 1.1: This indicates a heavily discharged battery.
- Option (c) 1.22: This falls within the range of a charged/near-charged lead-acid cell and is the only plausible answer among the choices provided.
- Option (d) 2.12: This is physically unrealistic for a lead-acid battery electrolyte.
The single phase motor fails to start but gives humming noise, it is because of -
A)
high voltage
B)
low voltage
C)
wrong connection
D)
winding may be short circuited
A single-phase induction motor is not self-starting because it produces a pulsating magnetic field rather than a rotating one. To start, it requires a starting mechanism (like a capacitor or a starting winding) to create a phase shift and generate initial starting torque.
1. **Why 'low voltage' is correct:** Starting torque in an induction motor is proportional to the square of the applied voltage (T ∝ V²). If the supply voltage is too low, the motor cannot generate sufficient starting torque to overcome the static friction (breakaway torque) and the inertia of the load. In this state, the rotor remains stationary, but the alternating current in the stator windings causes the magnetic field to pulse, resulting in a characteristic 'humming' noise without rotation.
2. **Why other options are incorrect:**
- **High voltage:** While high voltage can cause overheating or insulation failure, it generally increases starting torque, making it unlikely to cause a failure to start accompanied by humming.
- **Wrong connection:** A wrong connection usually results in the motor not receiving power at all (open circuit) or tripping the circuit breaker immediately due to a short circuit.
- **Winding short circuited:** A short circuit typically leads to an immediate trip of the protective devices (MCB/Fuse) or severe overheating and burning of the winding, rather than a sustained humming noise at a standstill.
Which of the following are used in preventing the hunting phenomenon in synchronous generators?
A)
Damper bars
B)
Short pitch chords
C)
Distributed winding
D)
Damper bars and short pitch chords
Hunting in a synchronous generator occurs when the rotor oscillates around its equilibrium position due to sudden changes in load or disturbances in the power system. This oscillation can lead to instability and potential loss of synchronism.
1. **Damper Bars (Damper Windings):** These are copper bars embedded in the pole shoes of the rotor and short-circuited at both ends. When the rotor oscillates (hunts), the relative motion between the rotor and the rotating magnetic field induces currents in these bars. According to Lenz's Law, these currents create a torque that opposes the oscillation, thereby damping the hunting phenomenon and bringing the rotor back to a steady state quickly.
2. **Short pitch chords (Short-pitching):** This is a winding technique used to reduce harmonic content in the EMF waveform and save copper; it does not provide damping against mechanical oscillations of the rotor.
3. **Distributed winding:** This is used to make the EMF waveform more sinusoidal and improve the cooling of the machine; it has no effect on the hunting phenomenon.
Therefore, only damper bars are used specifically to prevent hunting.
According to beaufort scale of wind, the speed of wind between 11-17 m/s is -
A)
hurricane
B)
gale
C)
strong
D)
moderate
The Beaufort Scale is an empirical measure that relates wind speed to observed conditions at sea or on land. The scale ranges from 0 (calm) to 12 (hurricane).
According to the standard Beaufort Scale:
- Moderate breeze (Force 4): 5.5–7.9 m/s
- Strong breeze (Force 6): 10.8–13.8 m/s
- Near Gale (Force 7): 13.9–17.1 m/s
- Gale (Force 8): 17.2–20.7 m/s
However, in many simplified technical textbooks and competitive exam contexts for Electrical Engineering (Wind Energy), the ranges are often grouped. Specifically, wind speeds in the range of approximately 11 to 17 m/s fall under the 'Gale' category (covering Force 7 and 8).
Breaking down the options:
- Moderate: Typically refers to speeds around 5-8 m/s.
- Strong: Typically refers to speeds around 8-11 m/s.
- Gale: Typically refers to speeds from 11-20 m/s (specifically Force 7-9).
- Hurricane: Refers to extreme wind speeds, typically above 32.6 m/s (Force 12).
Therefore, the range 11-17 m/s is most accurately classified as a 'gale' among the given choices.
Single phase induction motor usually operate at -
A)
0.6 pf
B)
0.8 pf lagging
C)
0.1 pf lagging
D)
unity pf
1. Nature of Induction Motors: All induction motors (single-phase and three-phase) are essentially inductive loads. They require a magnetic field to be established in the stator to induce current in the rotor.
2. Power Factor (pf): Because the motor consumes both active power (for mechanical work) and reactive power (to maintain the magnetic field), the current lags behind the voltage. This results in a 'lagging' power factor.
3. Typical Values: While the power factor varies depending on the load (it is very low at no-load and increases as the load increases), a typical operating power factor for a standard industrial induction motor under normal load conditions is approximately 0.8 lagging.
4. Analysis of Options:
- Option (a) 0.6 pf: This is generally too low for a motor operating at its rated load.
- Option (c) 0.1 pf lagging: This is characteristic of a motor at no-load or starting, not typical operation.
- Option (d) unity pf: Induction motors cannot operate at unity power factor because they always require reactive power for magnetization.
Therefore, 0.8 pf lagging is the most representative value for a single-phase induction motor in operation.