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Electrical Engineering interview questions and answers
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1.Why star delta starter is preferred with induction motor?2. State the difference between generator and alternator3. Why AC systems are preferred over DC systems?4. How can you relate power engineering with electrical engineering?5. What are the various kind of cables used for transmission?6. Why back emf used for a dc motor? highlight its significance.7. What is slip in an induction motor?8. Explain the application of storage batteries.
1. Why star delta starter is preferred with induction motor?
Star delta starter is preferred with induction motor due to following reasons:• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.• In addition, starting torque is increased and it prevents the damage of motor winding.
2. State the difference between generator and alternator
Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.
3. Why AC systems are preferred over DC systems?
Due to following reasons, AC systems are preferred over DC systems:a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmissionc. From power stations, AC is produced so it is better to use AC then DC instead of converting it.d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
4. How can you relate power engineering with electrical engineering?
Power engineering is a sub division of electrical engineering. It deals with generation, transmission and distribution of energy in electrical form. Design of all power equipments also comes under power engineering. Power engineers may work on the design and maintenance of the power grid i.e. called on grid systems and they might work on off grid systems that are not connected to the system.
5. What are the various kind of cables used for transmission?
Cables, which are used for transmitting power, can be categorized in three forms:• Low-tension cables, which can transmit voltage upto 1000 volts.• High-tension cables can transmit voltage upto 23000 volts.
• Super tension cables can transmit voltage 66 kV to 132 kV.
6. Why back emf used for a dc motor? Highlight its significance.
The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature rotates, is called back emf. Its value depends upon the speed of rotation of the armature conductors. In starting, the value of back emf is zero.
7. What is slip in an induction motor?
Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘S’.
8. Explain the application of storage batteries.
Storage batteries are used for various purposes, some of the applications are mentioned below:
• For the operation of protective devices and for emergency lighting at generating stations and substations.• For starting, ignition and lighting of automobiles, aircrafts etc.• For lighting on steam and diesel railways trains.• As a supply power source in telephone exchange, laboratories and broad casting stations.• For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible.
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9. Explain advantages of storage batteries
Few advantages of storage batteries are mentioned below:• Most efficient form of storing energy portably.• Stored energy is available immediately because there is no lag of time for delivering the stored energy.• Reliable source for supply of energy.• The energy can be drawn at a fairly constant rate.
10. What are the different methods for the starting of a synchronous motor.
Starting methods: Synchronous motor can be started by the following two methods:• By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained.• By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field.
11. Name the types of motors used in vacuum cleaners, phonographic appliances, vending machines, refrigerators, rolling mills, lathes, power factor improvement and cranes.
Following motors are used: -• Vacuum cleaners- Universal motor.
• Phonographic appliances – Hysteresis motor.• Vending machines – Shaded pole motor.• Refrigerators – Capacitor split phase motors.• Rolling mills – Cumulative motors.• Lathes – DC shunt motors.• Power factor improvement – Synchronous motors.
12. State Thevenin’s Theorem:
According to Thevenin’s theorem, the current flowing through a load resistance Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e. the voltage across the two terminals when RL is removed) and sum of load resistance and internal resistance of the network. It is given by Voc / (Ri + RL).
13. State Norton’s Theorem
The Norton’s theorem explains the fact that there are two terminals and they are as follows:• One is terminal active network containing voltage sources • Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to the constant source of current and it allows giving the parallel resistance. The Norton’s theorem also explains about the constant current that is equal to the current of the short circuit placed across the terminals. The parallel resistance of the network can be viewed from the open circuit terminals when all the voltage and current sources are removed and replaced by the internal resistance.
14. State Maximum power transfer theorem
The Maximum power transfer theorem explains about the load that a resistance will extract from the network. This includes the maximum power from the network and in this case the load resistance is being is equal to the resistance of the network and it also allows the resistance to be equal to the resistance of the network. This resistance can be viewed by the output terminals and the energy sources can be removed by leaving the internal resistance behind.
15. Explain different losses in a transformer.
There are two types of losses occurring in transformer:• Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses. Two types of iron losses are:o eddy current loss o Hysteresis loss. These losses depend upon the supply voltage, frequency, core material and its construction. As long as supply voltage and frequency is constant, these losses remain the same whether the transformer is loaded or not. These are also known as constant losses.• Variable losses or copper losses: when the transformer is loaded, current flows in primary and secondary windings, there is loss of electrical energy due to the resistance of the primary winding, and secondary winding and they are called variable losses. These losses depend upon the loading conditions of the transformers. Therefore, these losses are also called as variable losses.
16. Explain different types of D.C motors? Give their applications
Different type of DC motors and their applications are as follows:-• Shunt motors: It has a constant speed though its starting torque is not very high. Therefore, it is suitable for constant speed drive, where high starting torque is not required such as pumps, blowers, fan, lathe machines, tools, belt or chain conveyor etc.• Service motors: It has high starting torque & its speed is inversely proportional to the loading
conditions i.e. when lightly loaded, the speed is high and when heavily loaded, it is low. Therefore, motor is used in lifts, cranes, traction work, coal loader and coal cutter in coalmines etc.• Compound motors: It also has high starting torque and variable speed. Its advantage is, it can run at NIL loads without any danger. This motor will therefore find its application in loads having high inertia load or requiring high intermittent torque such as elevators, conveyor, rolling mill, planes, presses, shears and punches, coal cutter and winding machines etc.
17. Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are required in a dc machine.
Commutation: It is phenomenon when an armature coil moves under the influence of one pole- pair; it carries constant current in one direction. As the coil moves into the influence of the next pole- pair, the current in it must reverse. This reversal of current in a coil is called commutation. Several coils undergo commutation simultaneously. The reversal of current is opposed by the static coil emf and therefore must be aided in some fashion for smooth current reversal, which otherwise would result in sparking at the brushes. The aiding emf is dynamically induced into the coils undergoing commutation by means of compoles or interpoles, which are series excited by the armature current. These are located in the interpolar region of the main poles and therefore influence the armature coils only when these undergo commutation.
18. Comment on the working principle of operation of a single-phase transformer.
Working principle of operation of a single-phase transformer can be explained asAn AC supply passes through the primary winding, a current will start flowing in the primary winding. As a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced in both the windings. Now, when the load is connected to the secondary side, the current will start flowing in the load in the secondary winding, resulting in the flow of additional current in the secondary winding. Hence, according to Faraday’s laws of electromagnetic induction, emf will be induced in both the windings. The voltage induced in the primary winding is due to its self inductance and known as self induced emf and according to Lenze’s law it will oppose the cause i.e. supply voltage hence called as back emf. The voltage induced in secondary coil is known as mutually induced voltage. Hence, transformer works on the principle of electromagnetic induction.
19. Define the following terms:-
• Reliability,• Maximum demand,• Reserve-generating capacity, • Availability (operational).
Reliability: It is the capacity of the power system to serve all power demands without failure over long periods.Maximum Demand: It is maximum load demand required in a power station during a given period.Reserve generating capacity: Extra generation capacity installed to meet the need of scheduled downtimes for preventive maintenance is called reserve-generating capacity.Availability: As the percentage of the time a unit is available to produce power whether needed by the system or not.
20. Mention the disadvantages of low power factor? How can it be improved?
Disadvantages of low power factor:• Line losses are 1.57 times unity power factor.• Larger generators and transformers are required.• Low lagging power factor causes a large voltage drop, hence extra regulation equipment is required to keep voltage drop within prescribed limits.• Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the conductors will have to carry more current at low power factor. This requires a large conductor size.
21. State the methods of improving power factor?
Methods of improving power factor:• By connecting static capacitors in parallel with the load operating at lagging power factor. • A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.• By using phase advancers to improve the power factor of induction motors. It provides exciting ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor can be made to operate on leading power factor like an overexcited synchronous motor.
22. State the factors, for the choice of electrical system for an aero turbine.
The choice of electrical system for an aero turbine is guided by three factors:• Type of electrical output: dc, variable- frequency ac, and constant- frequency ac.• Aero turbine rotational speed: constant speed with variable blade pitch, nearly constant speed with simpler pitch- changing mechanism or variable speed with fixed pitch blades.• Utilization of electrical energy output: in conjunction with battery or other form of storage, or interconnection with power grid.
23. What are the advantages of VSCF wind electrical system?
Advantages of VSCF wind electrical system are:• No complex pitch changing mechanism is needed.• Aero turbine always operates at maximum efficiency point.• Extra energy in the high wind speed region of the speed – duration curve can be extracted• Significant reduction in aerodynamic stresses, which are associated with constant – speed operation.
24. Explain the terms real power, apparent power and reactive power for ac circuits and also the units used.
• Real Power: It is the product of voltage, current and power factor i.e. P = V I cos j and basic unit of real power is watt. i.e. Expressed as W or kW.• Apparent power: It is the product of voltage and current. Apparent power = V I and basic unit of apparent power is volt- ampere. Expressed as VA or KVA.• Reactive Power: It is the product of voltage, current and sine of angle between the voltage and current i.e. Reactive power = voltage X current X sinj or Reactive power = V I sin j and has no other unit but expressed in VAR or KVAR.
25. Define the following: Average demand, Maximum demand, Demand factor, Load factor.
• Average Demand: the average power requirement during some specified period of time of considerable duration is called the average demand of installation.• Maximum Demand: The maximum demand of an installation is defined as the greatest of all the demand, which have occurred during a given period. It is measured accordingly to specifications, over a prescribed time interval during a certain period.• Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the rating of the connected load.• Load Factor: It is defined as the ratio of the average power to the maximum demand.
26. Explain forward resistance, static resistance and dynamic resistance of a pn junction diode.
• Forward Resistance: Resistance offered in a diode circuit, when it is forward biased, is called forward-resistance. • DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across the diode to the direct current flowing through it. • AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward characteristic of the diode. It is the resistance offered by a diode to the changing forward current.
27. How does Zener phenomenon differ from Avalanche breakdown?
The phenomenon when the depletion region expands and the potential barrier increases leading to a very high electric field across the junction, due to which suddenly the reverse current increases under a very high reverse voltage is called Zener effect. Zener-breakdown or Avalanche breakdown may occur independently or both of these may occur simultaneously. Diode junctions that breakdown below 5v are caused by Zener Effect. Junctions that experience breakdown above 5v are caused by avalanche-effect. The Zener-breakdown occurs in heavily doped junctions, which produce narrow depletion layers. The avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers.
28. Compare JFET’s and MOSFET’s.
Comparison of JFET’s and MOSFET’s:• JFET’s can only be operated in the depletion mode whereas MOSFET’s can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier injunction occurs and the gate-current is substantial.• MOSFET’s have input impedance much higher than that of JFET’s. Thus is due to negligible small leakage current.• JFET’s have characteristic curves more flat than that of MOSFET is indicating a higher drain resistance.• When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it would be in a comparable MOSFET.
29. Explain thin film resistors and wire-wound resistors
a. Thin film resistors- It is constructed as a thin film of resistive material is deposited on an insulating substrate. Desired results are obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. During this process, the value of the resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of resistance is obtained.b. Wire wound resistors – length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel
30. What is a differential amplifier? Also, explain CMRR.
Differential Amplifier: The amplifier, which is used to amplify the voltage difference between two input-lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise injected into the amplifier, because any noise appearing simultaneously on both the input-terminals as the amplifying circuitry rejects it being a common mode signal.CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a differential amplifier is perfect, CMRR would be infinite because in that case common mode voltage gain would be zero.
1. How grounding is different than earthing
2. What is diversity factor in electric installations?3. Explain Marx circuit.4. Why human body feel electric shock?5. What is the principal of motor?6. What is power factor?7. Should power factor be low or high?8. Difference between field rheostat and armature rheostat9. Why field rheostat is kept in minimum position? 10. Why armature rheostat is kept in maximum position?11. What is meant by derating factor?12. What is stiffness factor?13. What is the dependency of stiffness on load angle??14. What is 100% protection of generator? Why 100% is not used but 95% is generally used?15. What is the difference between a Verilog task and Verilog function?16. What is the unit of magnetic flux density?17. Why increase in current leads to increase in conductor temperature?18. How can a equal potential zone be carried out in conductors?19. What is essential to prove safe isolation of electrical circuit?20. What is the ratio of true power to apparent power in an AC circuit?21. What is power relay?22. Differentiate between power relay and reverse power relay.23. What is the suitable transmission voltage?24. What is the maximum operating temperature for a thermoplastic insulated cable?25. Explain the working of variable frequency transformer?26. Which oil can be used in transformer?27. What is excitation in case of DC motor?28. Why is the starting current high in DC motor?29. What is rotary phase converter?30. Differentiate between digital phase converter and ordinary phase converter.31. What is knee point voltage?32. List the advantages of star-delta starter with induction motor?33. Which type of transformer is used for lighting loads?34. What is star-delta transformer?
1. What is a System?
When a number of elements or components are connected in a sequence to perform a specific function, the group of elements that all constitute a System
2. What is Control System?
In a System the output and inputs are interrelated in such a manner that the output quantity or variable is controlled by input quantity, and then such a system is called Control System.The output quantity is called controlled variable or response and the input quantity is called command signal or excitation.
3. What are different types of Control Systems?
Two major types of Control Systems are 1) Open loop Control System 2) Closed Loop Control SystemsOpen loop Control Systems: The Open loop Control System is one in which the Output Quantity has no effect on the Input Quantity. No feedback is present from the output quantity to the input quantity for correction.Closed Loop Control System: The Closed loop Control System is one in which the feedback is provided from the Output quantity to the input quantity for the correction so as to maintain the desired output of the system.
4. What is a feedback in Control System?
The Feedback in Control System in one in which the output is sampled and proportional signal is fed back to the input for automatic correction of the error (any change in desired output) for further processing to get back the desired output.
5. Why Negative Feedback is preferred in the Control System?
The role of Feedback in control system is to take the sampled output back to the input and compare output signal with input signal for error (deviation from the desired result). Negative Feedback results in the better stability of the system and rejects any disturbance signals and is less sensitive to the parameter variations. Hence in control systems negative feedback is considered.
6. What is the effect of positive feedback on stability of the system?
Positive feedback is not used generally in the control system because it increases the error signal and drives the system to instability. But positive feedbacks are used in minor loop control systems to amplify certain internal signals and parameters
7. What is latching current?
Gate signal is to be applied to the thyristor to trigger the thyristor ON in safe mode. When the thyristor starts conducting the forward current above the minimum value, called Latching current, the gate signal which is applied to trigger the device in no longer require to keep the scr in ON position.
8. What is holding current?
When scr is conducting current in forward conduction state, scr will return to forward blocking state when the anode current or forward current falls below a low level called Holding currentNote: Latching current and Holding current are not same. Latching current is associated with the turn on process of the scr whereas holding current is associated with the turn off process. In general holding current will be slightly lesser than the latching current.
9. Why thyristor is considered as Charge controlled device?
During the triggering process of the thyristor from forward blocking state to forward conduction state through the gate signal, by applying the gate signal (voltage between gate and cathode) increases the minority carrier density in the p-layer and thereby facilitate the reverse break over of the junction J2 and thyristor starts conducting. Higher the magnitude of the gate current pulse, lesser is the time required to inject the charge and turning on the scr. By controlling the amount of charge we can control the turning on time of the scr.
10. What are the different losses that occur in thyristor while operating?
Different losses that occur area) Forward conduction losses during conduction of the thyristorb) Loss due to leakage current during forward and reverse blocking.c) Power loss at gate or Gate triggering loss. d) Switching losses at turn-on and turn-off.
11. What is meant by knee point voltage?
Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated. (CT- current transformer)
12. What is reverse power relay?
Reverse Power flow relay are used in generating station’s protection. A generating station is supposed to fed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.
13. What will happen if DC supply is given on the primary of a transformer?
Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance , only resistance will act in the electrical circuit. So high electrical current will flow through primary side of the transformer So for this reason coil and insulation will burn out.
14. What is the difference between isolators and electrical circuit breakers? What is bus-bar?
Isolators are mainly for switching purpose under normal conditions but they cannot operate in fault conditions .Actually they used for isolating the CBs for maintenance. Whereas CB gets activated under fault conditions according to the fault detected. Bus bar is nothing but a junction where the power is getting distributed for independent loads.
15. What are the advantages of freewheeling diode in a Full Wave rectifier?
It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load.
16. Mention the methods for starting an induction motor?
The different methods of starting an induction motor:a) DOL: direct online starterb) Star delta starterc) Auto transformer starterd) Resistance startere) Series reactor starter
17. What is the power factor of an alternator at no load?
At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.
18. What is the function of anti-pumping in circuit breaker?
When breaker is close at one time by close push button, the anti pumping contactor prevent re close the breaker by close push button after if it already close.
19. What is stepper motor what is its uses?
Stepper motor is the electrical machine which acts upon input pulse applied to it. it is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle. So, in automation parts it is used.
20. There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum? And why?
The motor has max load current compare to that of transformer because the motor consumes real power.. And the transformer is only producing the working flux and it’s not consuming.. Hence the load current in the transformer is because of core loss so it is minimum.
21. What is SF6 Circuit Breaker?
SF6 is Sulphur hexa Flouride gas. If this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.
22. What is ferrantic effect?
Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.
23. What is meant by insulation voltage in cables? explain it?
It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is known as insulation level of the cable.
24. What is the difference between MCB & MCCB, Where it can be used?
MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A.
25. Where the lighting arrestor should be placed in distribution lines?
Near distribution transformers and outgoing feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations
26. Define IDMT relay?
It is an inverse definite minimum time relay. In IDMT relay its operating is inversely proportional and also a characteristic of minimum time after which this relay operates. It is inverse in the sense, the tripping time will decrease as the magnitude of fault current increase.
27. What are the transformer losses?
TRANSFORMER LOSSES - Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance of the wire (I2R). Magnetic losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and therefore a measurable loss. Hysteresis loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer.
28. what is the full form of KVAR?
We know there are three types of power in Electricals as Active, apparent & reactive. So KVAR is stand for ``Kilo Volt Amps with Reactive component.
29. Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?
Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb (P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.
30. Why temperature rise is conducted in bus bars and isolators?
Bus bars and isolators are rated for continuous power flow that means they carry heavy currents which rises their temperature. so it is necessary to test this devices for temperature rise.
31. What is the difference between synchronous generator & asynchronous generator?
In simple, synchronous generator supply’s both active and reactive power but asynchronous generator (induction generator) supply’s only active power and observes reactive power for magnetizing. This type of generators is used in windmills.
32. What is Automatic Voltage regulator (AVR)?
AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators, it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.
33. Difference between a four point starter and three point starters?
The shunt connection in four point stater is provided separately form the line where as in three point stater it is connected with line which is the drawback in three point stater
34. Why the capacitors works on ac only?Generally capacitor gives infinite resistance to dc components(i.e., block the dc components). it allows the ac components to pass through.
35. How many types of colling system it transformers?
1. ONAN (oil natural, air natural)2. ONAF (oil natural, air forced)3. OFAF (oil forced, air forced)4. ODWF (oil direct, water forced)5. OFAN (oil forced, air forced)
36. Operation carried out in Thermal power stations?
The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit the turbine , the turbine which is coupled with the generator generates the electricity.
37. What is 2 phase motor?
A two phase motor is a motor with the the starting winding and the running winding have a phase split. e.g;ac servo motor.where the auxiliary winding and the control winding have a phase split of 90 degree.
38. What is the principle of motor?Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.
39. What is meant by armature reaction?
The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.
40. What is the difference between synchronous generator & asynchronous generator?
In simple, synchronous generator supply's both active and reactive power but asynchronous generator(induction generator) supply's only active power and observe reactive power for magnetizing.This type of generators are used in windmills.
41. Whats is MARX CIRCUIT?
It is used with generators for charging a number of capacitor in parallel and discharging them in series.It is used when voltage required for testing is higher than the available.
42. What are the advantages of speed control using thyristor?
Advantages :1. Fast Switching Characterstics than Mosfet, BJT, IGBT 2. Low cost 3. Higher Accurate.
43. What is ACSR cable and where we use it?
ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.
44. Whats the one main difference between UPS & inverter ? And electrical engineering & electronics engineering ?
Uninterrupt power supply is mainly use for short time . means according to ups VA it gives backup. ups is also two types : on line and offline . online ups having high volt and amp for long time backup with with high dc voltage.but ups start with 12v dc with 7 amp. but inverter is startwith 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup.
45. What will happen when power factor is leading in distribution of power?
If their is high power factor, i.e if the power factor is close to one:a)Losses in form of heat will be reduced,
b)Cable becomes less bulky and easy to carry, and very cheap to afford, &c)It also reduces over heating of tranformers.
46. What are the advantages of star-delta starter with induction motor?
(1). The main advantage of using the star delta starter is reduction of current during the starting of the motor.Starting current is reduced to 3-4 times Of current of Direct online starting.(2). Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.
47. Why Delta Star Transformers are used for Lighting Loads?
For lighting loads, neutral conductor is must and hence the secondary must be star winding. and this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads.
48. Why computer humming sound occurred in HT transmission line?
This computer humming sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss.
49. What is rated speed?
At the time of motor taking normal current (rated current)the speed of the motor is called rated speed. It is a speed at which any system take small current and give maximum efficiency.
50. If one lamp connects between two phases it will glow or not?
If the voltage between the two phase is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.
Reference:
http://placement.freshersworld.com/power-preparation/technical-interview-preparation/electronics-interview-questions-25073
1. What is Electronic?
The study and use of electrical devices that operate by controlling the flow of electrons or other electrically charged particles
2. What is communication?
Communication means transferring a signal from the transmitter which passes through a medium then the output is obtained at the receiver. (or)communication says as transferring of message from one place to another place called communication.
3. Different types of communications? Explain.
Analog and digital communication as a technology, analog is the process of taking an audio or video signal (the human voice) and translating it into electronic pulses. Digital on the other hand is breaking the signal into a binary format where the audio or video data is represented by a series of "1"s and "0"s.Digital signals are immune to noise, quality of transmission and reception is good, components used in digital communication can be produced with high precision and power consumption is also very less when compared with analog signals.
4. What is sampling?
The process of obtaining a set of samples from a continuous function of time x(t) is referred to as sampling.
5. State sampling theorem.
It states that, while taking the samples of a continuous signal, it has to be taken care that the sampling rate is equal to or greater than twice the cut off frequency and the minimum sampling rate is known as the Nyquist rate.
6. What is cut-off frequency?
The frequency at which the response is -3dB with respect to the maximum response.
7. What is pass band?
Passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.
8. What is stop band?
A stopband is a band of frequencies, between specified limits, in which a circuit, such as a filter or telephone circuit, does not let signals through, or the attenuation is above the required stopband attenuation level.
9. Explain RF?
Radio frequency (RF) is a frequency or rate of oscillation within the range of about 3 Hz to 300 GHz. This range corresponds to frequency of alternating current electrical signals used to produce and detect radio waves. Since most of this range is beyond the vibration rate that most mechanical systems can respond to, RF usually refers to oscillations in electrical circuits or electromagnetic radiation.
10. What is modulation? And where it is utilized?
Modulation is the process of varying some characteristic of a periodic wave with an external signals.Radio communication superimposes this information bearing signal onto a carrier signal.These high frequency carrier signals can be transmitted over the air easily and are capable of travelling long distances. The characteristics (amplitude, frequency, or phase) of the carrier signal are varied in accordance with the information bearing signal. Modulation is utilized to send an information bearing signal over long distances.
11. What is demodulation?
Demodulation is the act of removing the modulation from an analog signal to get the original baseband signal back. Demodulating is necessary because the receiver system receives a modulated signal with specific characteristics and it needs to turn it to base-band.
12. Name the modulation techniques.
For Analog modulation--AM, SSB, FM, PM and SM Digital modulation--OOK, FSK, ASK, Psk, QAM, MSK, CPM, PPM, TCM, OFDM
13. Explain AM and FM.
AM-Amplitude modulation is a type of modulation where the amplitude of the carrier signal is varied in accordance with the information bearing signal. FM-Frequency modulation is a type of modulation where the frequency of the carrier signal is varied in accordance with the information bearing signal.
14. Where do we use AM and FM?
AM is used for video signals for example TV. Ranges from 535 to 1705 kHz.FM is used for audio signals for example Radio. Ranges from 88 to 108 MHz.
15. What is a base station?
Base station is a radio receiver/transmitter that serves as the hub of the local wireless network, and may also be the gateway between a wired network and the wireless network.
16. How many satellites are required to cover the earth?
3 satellites are required to cover the entire earth, which is placed at 120 degree to each other. The life span of the satellite is about 15 years.
17. What is a repeater?
A repeater is an electronic device that receives a signal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances without degradation.
18. What is an Amplifier?
An electronic device or electrical circuit that is used to boost (amplify) the power, voltage or current of an applied signal
19. Example for negative feedback and positive feedback?
Example for –ve feedback is ---Amplifiers And for +ve feedback is – Oscillators
20. What is Oscillator?
An oscillator is a circuit that creates a waveform output from a direct current input. The two main types of oscillator are harmonic and relaxation. The harmonic oscillators have smooth curved waveforms, while relaxation oscillators have waveforms with sharp changes.
21. What is an Integrated Circuit?
An integrated circuit (IC), also called a microchip, is an electronic circuit etched onto a silicon chip. Their main advantages are low cost, low power, high performance, and very small size.
22. What is crosstalk?
Crosstalk is a form of interference caused by signals in nearby conductors. The most common example is hearing an unwanted conversation on the telephone. Crosstalk can also occur in radios, televisions, networking equipment, and even electric guitars.
23. What is resistor?
A resistor is a two-terminal electronic component that opposes an electric current by producing a voltage drop between its terminals in proportion to the current, that is, in accordance with Ohm's law:V = IR.
25. What is inductor?
An inductor is a passive electrical device employed in electrical circuits for its property of inductance. An inductor can take many forms.
26. What is conductor?
A substance, body, or device that readily conducts heat, electricity, sound, etc. Copper is a good conductor of electricity.
27. What is a semi conductor?
A semiconductor is a solid material that has electrical conductivity in between that of a conductor and that of an insulator (An Insulator is a material that resists the flow of electric current. It is an object intended to support or separate electrical conductors without passing current through itself); it can vary over that wide range either permanently or dynamically.
28. What is diode?
In electronics, a diode is a two-terminal device. Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional current property.
29. What is transistor?
In electronics, a transistor is a semiconductor device commonly used to amplify or switch electronic signals. The transistor is the fundamental building block of computers, and all other modern electronic devices. Some transistors are packaged individually but most are found in integrated circuits
30. What is op-amp?
An operational amplifier, often called an op-amp , is a DC-coupled high-gain electronic voltage amplifier with differential inputs[1] and, usually, a single output. Typically the output of the op-amp is controlled either by negative feedback, which largely determines the magnitude of its output voltage gain, or by positive feedback, which facilitates regenerative gain and oscillation.
31. What is a feedback?
Feedback is a process whereby some proportion of the output signal of a system is passed (fed back) to the input. This is often used to control the dynamic behaviour of the system.
32. Advantages of negative feedback over positive feedback.
Much attention has been given by researchers to negative feedback processes, because negative feedback processes lead systems towards equilibrium states. Positive feedback reinforces a given tendency of a system and can lead a system away from equilibrium states, possibly causing quite unexpected results.
33. What is Barkhausen criteria?
Barkhausen criteria, without which you will not know which conditions, are to be satisfied for oscillations.“Oscillations will not be sustained if, at the oscillator frequency, the magnitude of the product of thetransfer gain of the amplifier and the magnitude of the feedback factor of the feedback network ( the magnitude of the loop gain ) are less than unity”. The condition of unity loop gain -A = 1 is called theβ Barkhausen criterion. This condition implies that A = 1and that the phase of - A is zero.β β
34. What is CDMA, TDMA, FDMA?
Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency.An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different directions (spatial division). In CDMA, they would speak different languages. People speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.
35. explain different types of feedback
Types of feedback: Negative feedback: This tends to reduce output (but in amplifiers, stabilizes and linearizes operation). Negative feedback feeds part of a system's output, inverted, into the system's input; generally with the result that fluctuations are attenuated. Positive feedback: This tends to increase output. Positive feedback, sometimes referred to as "cumulative causation", is a feedback loop system in which the system responds to perturbation (A perturbation means a system, is an alteration of function, induced by external or internal mechanisms) in the same direction as the perturbation. In contrast, a system that responds to the perturbation in the opposite direction is called a negative feedback system.Bipolar feedback: which can either increase or decrease output.
36. What are the main divisions of power system?
The generating system, transmission system, and distribution system
37. What is Instrumentation Amplifier (IA) and what are all the advantages?
An instrumentation amplifier is a differential op-amp circuit providing high input impedances with ease of gain adjustment by varying a single resistor.
38. What is meant by impedance diagram?
The equivalent circuit of all the components of the power system are drawn and they are interconnected is called impedance diagram.
39. What is the need for load flow study?
The load flow study of a power system is essential to decide the best operation existing system and for planning the future expansion of the system. It is also essential for designing the power system.
40. What is the need for base values?
The components of power system may operate at different voltage and power levels. It will be convenient for analysis of power system if the voltage, power, current ratings of the components of the power system is expressed with referance to a common value called base value.
