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Past Questions
In a single phase to single phase cyclo converter, the magnitude of harmonic components are quite large. How can they be reduced?
A)
By using a chopper circuit
B)
By using an RC oscillator
C)
By using a three phase input supply
D)
By adding alternator to the input
A cycloconverter is a power electronics device that converts AC power of one frequency to AC power of another frequency. In a single-phase to single-phase cycloconverter, the output waveform is composed of segments of the input sine wave, which results in significant harmonic distortion and high ripple content. To reduce these harmonic components and improve the quality of the output waveform (making it more sinusoidal), a three-phase input supply is used. In a three-phase to single-phase (or three-phase to three-phase) cycloconverter, the overlapping of phases and the interleaving of switching pulses significantly reduce the magnitude of the lower-order harmonics and the ripple, leading to a smoother output voltage and current.
Analysis of other options:
- (a) A chopper circuit is a DC-to-DC converter and cannot be used to reduce harmonics in an AC-to-AC cycloconverter.
- (b) An RC oscillator is used for signal generation at low power levels, not for power conversion or harmonic filtering in industrial power electronics.
- (d) Adding an alternator to the input simply provides the AC source; it does not inherently change the switching topology or reduce the harmonics generated by the cycloconverter's switching process.
When a fluorescent lamp shows a dense blocking at each end, it could possibly mean that the -
A)
gas filled in lamp was wrong
B)
lamp was started infrequently
C)
starter is working improperly
D)
lamp is new
In a new fluorescent lamp, the phosphor coating on the inner surface of the glass tube is uniform and pristine. When the lamp is first operated, the electrodes (filaments) at each end are heated, and the discharge begins. In brand new lamps, there is often a visible 'dark ring' or a dense appearance at the ends where the electrodes are located, as the plasma discharge is establishing itself and the phosphor has not yet undergone the initial 'burn-in' or aging process. As the lamp ages, the phosphor near the ends tends to degrade or 'burn off' due to the higher intensity of sputtering from the electrodes, making the ends appear lighter or more transparent over time. Therefore, a dense appearance or distinct blocking at the ends is a characteristic typically associated with a new lamp.
Analysis of other options:
- Option (a): If the gas mixture were wrong, the lamp would likely fail to strike an arc or would operate with an incorrect color/flicker, rather than showing specific 'blocking' at the ends.
- Option (b): Infrequent starting does not cause dense blocking; rather, excessive starting/stopping can shorten the life of the electrodes.
- Option (c): An improper starter would result in the lamp failing to start or flickering continuously, not a visual change in the phosphor density at the ends.
An ideal transformer is one which-
A)
has no losses and leakage reactance.
B)
does not work.
C)
have same primary and secondary turns.
D)
have good cooling system.
An ideal transformer is a theoretical model used in electrical engineering to simplify the analysis of transformer circuits. It is defined by the following characteristics:
1. No Losses: It has zero winding resistance (no copper losses/I²R losses) and zero core losses (no hysteresis or eddy current losses).
2. No Leakage Reactance: It is assumed that 100% of the magnetic flux produced by the primary winding links with the secondary winding (perfect magnetic coupling), meaning there is no leakage flux.
3. Infinite Permeability: The core is assumed to have infinite permeability, meaning no magnetizing current is required to establish the flux.
Evaluating the options:
- Option (a) is correct as it describes the fundamental assumptions of an ideal transformer.
- Option (b) is incorrect because an ideal transformer is a model of a perfectly efficient device, not a non-functional one.
- Option (c) describes a '1:1 transformer' or an isolation transformer, but not necessarily an 'ideal' one (an ideal transformer can be step-up or step-down).
- Option (d) refers to thermal management in real-world transformers; an ideal transformer produces no heat (no losses), so it doesn't require a cooling system.
Cores of large transformers are built upto nearly circular cross-section in order to reduce-
A)
leakage reactance.
B)
iron losses.
C)
eddy current loss.
D)
copper losses.
In large transformers, the core is designed with a nearly circular cross-section (often achieved through a stepped-core construction) to optimize the distribution of magnetic flux.
1. **Leakage Reactance**: Leakage reactance is caused by the magnetic flux that does not link both the primary and secondary windings (leakage flux). A circular or stepped-core design ensures that the magnetic path is as uniform as possible and minimizes the distance between the windings and the core. This reduces the amount of leakage flux, thereby reducing the leakage reactance of the transformer.
2. **Iron Losses (Option b)**: Iron losses (hysteresis and eddy current losses) depend primarily on the grade of the silicon steel used, the thickness of the laminations, and the flux density, not the overall geometric shape of the core cross-section.
3. **Eddy Current Loss (Option c)**: This is reduced by using thin, insulated laminations of the core, not by changing the cross-sectional shape to a circle.
4. **Copper Losses (Option d)**: Copper losses ($I^2R$) depend on the resistance of the windings, which is a function of the conductor material, length, and cross-sectional area of the wire, not the shape of the magnetic core.
Therefore, the primary objective of adopting a nearly circular cross-section in large transformers is to minimize leakage reactance.
Input ripple of SMPS is -
A)
high
B)
nil
C)
very low
D)
low
In a Switched-Mode Power Supply (SMPS), the input stage typically consists of a bridge rectifier followed by a large bulk filter capacitor. This capacitor acts as a reservoir, smoothing out the rectified AC voltage to provide a steady DC bus for the high-frequency switching converter. Because the switching frequency of an SMPS is very high (typically in the kHz or MHz range), the input filter can be highly effective, and the energy storage capacity of the input capacitor ensures that the voltage ripple seen by the switching stage is kept to a minimum. Compared to linear power supplies, the high-frequency nature of the conversion process allows for more efficient filtering, resulting in 'very low' input ripple relative to the operational requirements of the switching transistor. While 'low' is also a characteristic, in the context of technical examinations for SMPS, 'very low' is the preferred term to describe the effectiveness of the input DC bus stabilization.
For which of the following is the frequency is constant from input to output?
A)
AC voltage controller
B)
Chopper
C)
Inverter
D)
Cyclo converter
To determine which device maintains a constant frequency from input to output, let's analyze the function of each power electronic converter:
1. **AC Voltage Controller (a):** This device converts a fixed-frequency AC voltage to a variable-voltage AC voltage of the same frequency. It uses phase control (via thyristors or TRIACs) to change the RMS value of the output voltage, but the fundamental frequency of the input source remains unchanged at the output. Therefore, frequency is constant.
2. **Chopper (b):** A chopper is a DC-to-DC converter. It takes a constant DC input and produces a variable DC output. Since the input and output are DC, the concept of 'frequency' in the sense of a periodic AC waveform does not apply as a constant transfer from input to output.
3. **Inverter (c):** An inverter converts DC to AC. The input has no frequency (DC), while the output is a synthesized AC waveform with a specific frequency. Thus, the frequency changes from zero (DC) to a non-zero value.
4. **Cycloconverter (d):** A cycloconverter converts AC power at one frequency to AC power at a different (usually lower) frequency without an intermediate DC link. By definition, it changes the frequency from input to output.
Conclusion: Only the AC voltage controller maintains the input frequency at the output.
The hospital requires fresh air and that is why air condition system is require.
A)
50%
B)
70%
C)
90%
D)
100%
Hospitals require a very high percentage of fresh air in air-conditioning systems to maintain hygiene, control infection, and ensure proper ventilation. In standard HVAC practice for hospitals, the fresh air requirement is typically taken as 100% in such exam questions, meaning the system should supply only outdoor air rather than recirculated air. Therefore, option d (100%) is correct. The other options are incorrect because 50%, 70%, and 90% do not meet the full fresh-air requirement expected for hospital environments in this context.
The range of wind speed suitable for wind power generation is -
A)
0 to 5 m/s
B)
5 to 25 m/s
C)
25 to 50 m/s
D)
50 to 75 m/s
In wind energy conversion systems, there are two critical wind speed thresholds: the 'cut-in speed' and the 'cut-out speed'.
1. Cut-in Speed: This is the minimum wind speed at which the wind turbine blades begin to rotate and generate usable electricity. For most commercial wind turbines, this typically ranges from 3 to 5 m/s. Below this speed, the torque generated by the wind is insufficient to overcome the mechanical friction and inertia of the system.
2. Cut-out Speed: This is the maximum wind speed at which the turbine can safely operate. To prevent structural damage or mechanical failure due to extreme centrifugal forces and wind loads, the turbine is automatically braked or feathered (blades turned away from the wind) when speeds reach this limit. This typically occurs between 20 and 25 m/s.
Therefore, the operational range for power generation is generally between 5 m/s and 25 m/s.
Analysis of other options:
- Option (a) 0 to 5 m/s: This is below or at the cut-in speed; insufficient power is generated.
- Option (c) 25 to 50 m/s: This exceeds the cut-out speed; turbines are shut down to prevent destruction.
- Option (d) 50 to 75 m/s: These are storm-level speeds that would cause catastrophic failure to standard wind turbine structures.
A grid interactive solar system -
A)
always supplies power to the grid.
B)
supplies power to as well as receives power from the grid as required.
C)
always receives power from the grid.
D)
works only when the grid fails.
A grid-interactive solar system (also known as a grid-tied system) is designed to operate in parallel with the utility power grid.
1. **Supplying Power:** When the solar panels generate more electricity than the local load requires, the excess energy is exported (supplied) to the utility grid, often resulting in credits via 'Net Metering'.
2. **Receiving Power:** During periods of low sunlight (nighttime or cloudy weather) or when the local demand exceeds the solar production, the system automatically draws (receives) power from the grid to ensure a continuous power supply to the loads.
Analysis of other options:
- Option (a) is incorrect because the system only supplies power when there is a surplus; it cannot supply power at night.
- Option (c) is incorrect because the primary purpose of the solar installation is to generate power and reduce reliance on the grid.
- Option (d) describes a 'Stand-alone' or 'Backup' system with an inverter/battery setup; standard grid-interactive systems actually shut down during a grid failure (anti-islanding) for safety reasons unless they have specific backup capabilities.
A thyristor (SCR) is a -
A)
P-N-P device
B)
N-P-N device
C)
P-N-P-N device
D)
P-N device
A thyristor, specifically a Silicon Controlled Rectifier (SCR), is a four-layer semiconductor switching device. It consists of four alternating layers of P-type and N-type semiconductor materials, forming a P-N-P-N structure. This structure creates three P-N junctions. The device acts as a bistable switch, meaning it can be in either an 'off' state (blocking) or an 'on' state (conducting).
Analysis of options:
- Option (a) P-N-P: This describes a Bipolar Junction Transistor (BJT) with a P-type emitter and collector.
- Option (b) N-P-N: This describes a Bipolar Junction Transistor (BJT) with an N-type emitter and collector.
- Option (c) P-N-P-N: This is the correct structure for a thyristor/SCR.
- Option (d) P-N: This describes a simple diode.
For a practical synchronous motor, the pull-out torque will occur when the torque angle is nearly equal to -
A)
0°
B)
30°
C)
45°
D)
75°
1. Definition of Pull-out Torque: Pull-out torque (also known as maximum torque) is the maximum torque that a synchronous motor can develop before it loses synchronism and begins to slip.
2. Torque Equation: The torque (T) developed by a synchronous motor is given by the formula: T = (3 * V * E * Xs / ωs) * sin(δ), where V is the terminal voltage, E is the excitation voltage, Xs is the synchronous reactance, ωs is the synchronous speed, and δ is the torque angle (load angle).
3. Condition for Maximum Torque: Mathematically, the torque T is a function of sin(δ). The maximum value of sin(δ) occurs when δ = 90°.
4. Practical Considerations: In an ideal (cylindrical rotor) machine without armature reaction or saturation, the theoretical pull-out angle is 90°. However, for a 'practical' synchronous motor, factors such as armature reaction, magnetic saturation, and the fact that the motor is often over-excited or under-excited shift this peak. In practical operation and standard electrical machine textbooks (such as B.L. Theraja or Kothari & Nagrath), it is observed that the stability limit and the pull-out torque typically occur at angles significantly less than 90°, often falling in the range of 45° to 60° depending on the machine's design and loading conditions.
5. Evaluating Options: Among the given options (0°, 30°, 45°, 75°), 45° is the most representative value for the practical pull-out torque angle of a standard synchronous motor, as 0° is no-load, 30° is well within the stable operating region, and 75° is too close to the theoretical limit for most practical industrial machines.
If no load is connected to solar PV system -
A)
it will stop absorbing light.
B)
it will dissipate energy in the panel and increase its temperature.
C)
its voltage will go on increasing till its breakdown.
D)
it will start reflecting the light.
Step-by-step reasoning:
1. **Understanding Open Circuit Condition**: When no load is connected to a solar PV system, it is in an 'Open Circuit' state. In this state, the current (I) is zero.
2. **Energy Conversion**: A solar cell continues to absorb photons from sunlight regardless of whether a load is connected. This absorption creates electron-hole pairs.
3. **Recombination**: Since there is no external path (load) for the generated electrons to flow through, the charge carriers cannot be extracted as electrical current. Instead, they undergo 'recombination' within the semiconductor material of the cell.
4. **Thermal Dissipation**: The process of recombination releases the energy of the absorbed photons as heat (phonons) rather than electrical energy.
5. **Temperature Increase**: Because the absorbed solar energy is converted into heat instead of being exported as electricity, the temperature of the PV panel increases.
Why other options are incorrect:
- **Option a**: The absorption of light is a physical property of the semiconductor material (bandgap) and does not depend on the electrical load.
- **Option c**: The voltage of a solar cell is limited by its built-in potential (the p-n junction barrier). It will reach its Open Circuit Voltage (Voc) and stabilize; it will not increase indefinitely to a breakdown point.
- **Option d**: Reflection is determined by the surface properties (like anti-reflective coatings) and the angle of incidence, not by the electrical load connected to the circuit.
The no-load input power to a transformer is practically equal to-
A)
the iron loss in the transformer
B)
the copper loss in the transformer
C)
the eddy current in the transformer
D)
the hysteresis loss in the transformer
Step-by-step reasoning:
1. **Understanding No-Load Condition**: In a transformer, 'no-load' means the secondary winding is open-circuited. In this state, no current flows through the secondary winding, and only a small no-load current (excitation current) flows through the primary winding.
2. **Analyzing Power Consumption**: The power input at no-load is used to maintain the magnetic flux in the core and to compensate for the losses occurring within the core.
3. **Copper Loss**: Copper loss is caused by the resistance of the windings ($I^2R$). Since the no-load current is very small (typically 2-5% of full-load current), the $I^2R$ loss in the primary is negligible. Therefore, copper loss is practically zero at no-load.
4. **Iron Loss (Core Loss)**: Iron loss consists of Hysteresis loss and Eddy current loss. These losses depend on the supply voltage, frequency, and core material, not on the load. Since the transformer is connected to the rated supply voltage, the core is magnetized, and iron losses occur continuously.
5. **Conclusion**: Because copper losses are negligible at no-load, the total input power is almost entirely consumed by the core losses. Thus, no-load input power $\approx$ Iron Loss (Hysteresis Loss + Eddy Current Loss).
Why other options are incorrect:
- Option (b): Copper loss is load-dependent and negligible at no-load.
- Option (c) and (d): While eddy current and hysteresis losses are components of iron loss, neither one alone represents the total no-load power; their sum (the total iron loss) does.
Solar energy can be utilised directly by how many technologies?
A)
1
B)
2
C)
3
D)
4
Solar energy is utilized directly through two primary technological pathways:
1. Photovoltaic (PV) Technology: This converts sunlight directly into electricity using semiconductor materials (the photoelectric effect).
2. Solar Thermal Technology: This converts sunlight directly into heat (thermal energy), which can be used for water heating, space heating, or to drive a steam turbine for power generation.
While there are many variations of these (e.g., Concentrated Solar Power vs. Flat Plate Collectors), they all fall under these two fundamental direct utilization categories: conversion to electricity (PV) or conversion to heat (Thermal). Therefore, the number of primary direct utilization technologies is 2.
Windmill works on the principle of -
A)
rotation
B)
momentum
C)
gravitation
D)
collision
A windmill (or wind turbine) operates based on the principle of momentum. Specifically, it utilizes the change in momentum of the air flowing past the blades. According to Newton's Second Law of Motion, the force exerted by the wind on the blades is equal to the rate of change of momentum of the air. As the wind hits the blades, its velocity changes (direction and/or magnitude), creating a force (lift or drag) that causes the rotor to rotate. This conversion of the kinetic energy of the wind into mechanical energy is fundamentally a momentum transfer process.
Why other options are incorrect:
- Rotation (a): Rotation is the *result* of the wind's force, not the underlying physical principle that causes the movement.
- Gravitation (c): Gravitation is the force of attraction between masses and does not drive the rotation of a windmill.
- Collision (d): While air molecules do collide with the blades, the overall operation of a turbine (especially modern aerodynamic blades) is described by fluid dynamics and the conservation of momentum rather than simple discrete collisions.
A 4-pole, 3-phase, 60 Hz induction motor is operating at a speed of 1500 rpm. The frequency of the rotor current of the motor in Hz is
A)
5
B)
10
C)
2
D)
7
For a 4-pole, 60 Hz induction motor, the synchronous speed is Ns = 120f/P = 120×60/4 = 1800 rpm. The rotor speed is 1500 rpm, so slip s = (Ns − Nr)/Ns = (1800 − 1500)/1800 = 300/1800 = 1/6. Rotor current frequency is fr = s×f = (1/6)×60 = 10 Hz. Therefore, the correct value is 10 Hz. Among the options, 10 Hz corresponds to option b. The other options are incorrect because 7 Hz, 2 Hz, and 5 Hz do not match the rotor frequency obtained from the slip relation fr = sf.
How many layer does a solar cell has -
A)
2
B)
3
C)
4
D)
5
A basic solar cell (photovoltaic cell) is fundamentally a semiconductor diode. It is constructed by joining two different types of semiconductor materials: a p-type semiconductor and an n-type semiconductor. These two layers create a p-n junction. When sunlight hits the cell, it creates electron-hole pairs; the internal electric field at the p-n junction separates these charges, allowing current to flow. While a commercial solar panel includes additional components like an anti-reflective coating, a glass cover, and metal contacts for protection and efficiency, the core functional 'solar cell' itself consists of these 2 primary semiconductor layers.
What is the name of the device which controls temperature in window AC?
A)
OLP
B)
Relay
C)
Thermostat
D)
Selector switch
Step-by-step reasoning:
1. **Function Analysis**: The question asks for the device responsible for controlling the temperature in a window Air Conditioner (AC). Temperature control requires a sensing element that can trigger a switch based on a set thermal threshold.
2. **Evaluating Option (c) Thermostat**: A thermostat is a temperature-sensing switch. In a window AC, it monitors the room temperature; once the temperature drops to the user-defined set point, the thermostat opens the circuit to the compressor to stop cooling. When the temperature rises again, it closes the circuit to restart the compressor. This is the primary mechanism for temperature regulation.
3. **Evaluating Option (a) OLP**: OLP stands for Overload Protector. Its purpose is to protect the compressor motor from overheating or drawing excessive current by tripping the circuit. It is a safety device, not a temperature control device for the room.
4. **Evaluating Option (b) Relay**: A relay is an electromagnetic switch used to control a high-power circuit with a low-power signal. While a thermostat may use a relay to switch the compressor on/off, the relay itself does not 'sense' or 'control' the temperature; it merely executes the command.
5. **Evaluating Option (d) Selector switch**: A selector switch is used by the user to choose between different modes (e.g., Cool, Fan only, Dry). It does not automatically regulate the temperature based on the environment.
Conclusion: The thermostat is the only device listed specifically designed to maintain a set temperature.
In what form solar energy is radiated from the sun?
A)
Ultraviolet Radiation
B)
Infrared radiation
C)
Electromagnetic waves
D)
Transverse waves
Step-by-step reasoning:
1. Solar energy is produced in the sun's core through nuclear fusion, which releases energy in the form of photons.
2. These photons travel through space as electromagnetic radiation. Electromagnetic waves consist of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of propagation.
3. The solar spectrum is broad, meaning it includes a wide range of wavelengths: Ultraviolet (UV), Visible Light, and Infrared (IR).
4. Evaluating the options:
- Option (a) Ultraviolet Radiation: This is only a small portion of the solar spectrum.
- Option (b) Infrared radiation: This is also only a portion of the solar spectrum (thermal radiation).
- Option (c) Electromagnetic waves: This is the comprehensive term that encompasses UV, Visible, and Infrared radiation. Since solar energy is radiated across this entire spectrum, this is the most accurate and complete answer.
- Option (d) Transverse waves: While electromagnetic waves are indeed transverse waves, 'Electromagnetic waves' is the specific physical nature of the energy radiated from the sun, whereas 'transverse' is a general category of wave motion (which also includes certain mechanical waves).
Therefore, the most precise and authoritative answer is electromagnetic waves.
During the charging of a lead acid cell.
A)
Its voltage increase
B)
It gives out energy
C)
Its cathode becomes dark chocolate brown in colour
D)
Specific gravity of acid decreases
During the charging process of a lead-acid battery, electrical energy is converted back into chemical energy. Here is the detailed analysis of the options:
1. **Voltage (Option a):** As a battery charges, the state of charge (SoC) increases. The open-circuit voltage of a lead-acid cell is directly proportional to its state of charge. Therefore, as the cell charges, its voltage increases. This is the correct statement.
2. **Energy (Option b):** Charging is the process of storing energy. During charging, the cell *absorbs* electrical energy from an external source; it does not 'give out' energy. Giving out energy occurs during the discharging phase.
3. **Cathode Color (Option c):** In a lead-acid battery, the positive plate (cathode during discharge, anode during charge) consists of Lead Dioxide (PbO2), which is dark brown. During charging, the PbO2 is restored. However, the statement is misleading because the 'dark chocolate brown' color is the characteristic of the fully charged positive plate. The option implies a change occurring *during* charging, but the most definitive and universal electrical characteristic is the voltage increase.
4. **Specific Gravity (Option d):** During charging, sulfuric acid (H2SO4) is regenerated as sulfate ions move from the plates back into the electrolyte. This increases the concentration of the acid, thereby *increasing* the specific gravity. A decrease in specific gravity occurs during discharging.
A transformer transfers electrical energy from primary to secondary usually with a change in-
A)
frequency
B)
power
C)
voltage
D)
time period
A transformer is a static electrical device that operates on the principle of electromagnetic induction. Its primary purpose is to change the voltage level of an alternating current (AC) signal while maintaining the same frequency.
1. Why 'voltage' is correct: By varying the turns ratio (the ratio of the number of turns in the primary winding to the number of turns in the secondary winding), a transformer can either increase the voltage (Step-up transformer) or decrease the voltage (Step-down transformer).
2. Why 'frequency' and 'time period' are incorrect: A transformer does not change the frequency of the AC signal. The frequency of the secondary voltage is identical to the frequency of the primary voltage. Since the time period (T) is the reciprocal of frequency (T = 1/f), the time period also remains constant.
3. Why 'power' is incorrect: In an ideal transformer, the input power equals the output power (Power In = Power Out). In real transformers, there is a slight decrease in power due to losses (copper losses and iron losses), but the primary intended function of a transformer is not to change power, but to change voltage and current levels.
A three-phase slip ring induction motor is fed from the rotor side with the stator winding short-circuited. The frequency of the current flowing in the short-circuited stator is -
A)
slip frequency
B)
supply frequency
C)
the frequency corresponding to rotor speed
D)
zero
In a standard induction motor operation, the stator is fed with supply frequency (f), and the rotor carries slip frequency (sf). However, in this specific scenario, the roles are reversed: the motor is fed from the rotor side and the stator is short-circuited.
1. When a three-phase supply of frequency 'f_rotor' is applied to the rotor, the rotor becomes the 'source' of the rotating magnetic field.
2. The stator, being short-circuited, acts as the 'rotor' in this inverted configuration.
3. The frequency of the current induced in the stator depends on the relative motion between the rotating magnetic field (created by the rotor) and the stationary stator windings.
4. Since the stator is stationary, the frequency of the induced current in the stator is exactly the frequency of the rotating magnetic field relative to the stator.
5. In the context of induction motor theory, when the rotor is driven or fed, the frequency seen by the stationary part (stator) is the slip frequency (sf = s * f), where 's' is the slip. Specifically, if the rotor is fed at a frequency f_r, the stator will experience that same frequency relative to its stationary position. In standard terminology for these types of problems, this is referred to as the slip frequency because it is the frequency associated with the relative motion between the field and the conductors.
The frame of an induction motor is made of -
A)
Aluminium
B)
Silicon steel
C)
Cast iron
D)
Stainless steel
The frame of an induction motor serves as the outer protective enclosure that supports the stator core and protects the internal components from environmental damage and mechanical stress.
1. Why Cast Iron: Cast iron is the most common material for motor frames because it provides high mechanical strength, rigidity, and excellent vibration-damping properties. It is also cost-effective and durable for industrial applications.
2. Why not Silicon Steel: Silicon steel is used for the stator and rotor cores (the magnetic circuit) to reduce hysteresis and eddy current losses, but it is not used for the outer structural frame.
3. Why not Aluminium: While some lightweight or specialized motors use aluminium frames for heat dissipation or weight reduction, cast iron remains the standard authoritative answer for general-purpose induction motor frames in technical examinations.
4. Why not Stainless Steel: Stainless steel is used only in specialized environments (like food processing or chemical plants) due to its high cost; it is not the standard material for general induction motor frames.
In an auto transformer, power from the primary is transferred to the secondary
A)
inductively only
B)
conductively only
C)
inductively and conductively
D)
resistively
An auto transformer differs from a conventional two-winding transformer because it has only one continuous winding that serves as both the primary and the secondary. Because the primary and secondary circuits are electrically connected (sharing a common section of the winding), power is transferred in two ways: 1. Conductively: Power flows directly through the electrical connection between the primary and secondary windings. 2. Inductively: Power is transferred via the magnetic flux in the core, similar to a standard transformer. The total power transferred is the sum of the conductive power and the inductive power. In contrast, a standard transformer transfers power 'inductively only' because there is galvanic isolation between the primary and secondary.
Thyristors have controlled turn-on by a -
A)
Gate signal
B)
Voltage Controller
C)
Rectified signal
D)
Surge Current
A thyristor (specifically the SCR - Silicon Controlled Rectifier) is a four-layer p-n-p-n semiconductor device that acts as a bistable switch. It has three terminals: Anode, Cathode, and Gate.
1. **Mechanism of Turn-on**: A thyristor is forward-biased when the anode is positive relative to the cathode. However, it will not conduct (remain in the 'off' state) until a small positive current/voltage is applied to the **Gate** terminal. This gate signal triggers the device into conduction mode (turn-on).
2. **Latching**: Once the thyristor is turned on by the gate signal, it remains on even if the gate signal is removed, provided the anode current stays above the 'latching current' level.
Why other options are incorrect:
- **Voltage Controller**: While a voltage controller might generate the gate signal, the controller itself is the system, not the direct trigger mechanism of the device.
- **Rectified signal**: A rectified signal is a type of waveform; while it can be used to trigger a gate, the device is controlled by the signal applied to the gate, not by the fact that a signal is rectified.
- **Surge Current**: A very high surge current (exceeding the breakover voltage) can force a thyristor to turn on, but this is generally considered an uncontrolled or accidental trigger (breakover) rather than the intended 'controlled turn-on' method.