Electrical interview Questions contains interview question asked in interview
1. What is electric current?(units coulomb per second or Ampere)
Electric current
The controlled movement or drift of electrons through a substance is called the electric current.
Current may also be defined as the rate of net motion of an electric charge across a cross sectional boundary(as shown in fig).
A random motion of electrons in a metal doesn't constitute a unless there is a net transfer of charge with time i.e. electric current, i= Rate of transfer of electric charge
= dQ/dt
Coulomb is the practical as well as SI unit for measurement of electric charge charge. One coulomb is approximately equal to 624 x 10^16 electrons.
Since current is the rate of flow of electron through the conductor and coulomb is the unit of electric charge, the current may be specified in coulombs per second.
= dQ/dt
Coulomb is the practical as well as SI unit for measurement of electric charge charge. One coulomb is approximately equal to 624 x 10^16 electrons.
Since current is the rate of flow of electron through the conductor and coulomb is the unit of electric charge, the current may be specified in coulombs per second.
In practice the term coulomb per second is seldom used, a shorter term, ampere is used instead.
See the Below video tutorial for More details:
See the Below video tutorial for More details:
2. What is Resistance ? (units of resistance) (ohm definition)
Resistance(R)
Resistance may be defined as that property of a substance which opposes (or restricts) the flow of an electric current (or electrons) through it.
Units and ohm definition
The practical as well as mks (or SI) unit of resistance is ohm (Ω), which is defined as that resistance between two points of a conductor when a potential difference of one volt, applied between these points, produces in this conductor a current of one ampere, the conductor not being a source of any emf.
For insulators having high resistance , much bigger units kilo ohm or kΩ (10^3 ohm) are used. in case of very small resistances smaller units like milli ohm (10^-3) are employed.
Relation Between electric conductivity and resistance :
Electric Conductivity and Resistance are inversely proportional to each other. the reciprocal of resistance is called conductance.
Conductance = 1/R
For More Detailed Explanation see the below video.
Resistance may be defined as that property of a substance which opposes (or restricts) the flow of an electric current (or electrons) through it.
Units and ohm definition
The practical as well as mks (or SI) unit of resistance is ohm (Ω), which is defined as that resistance between two points of a conductor when a potential difference of one volt, applied between these points, produces in this conductor a current of one ampere, the conductor not being a source of any emf.
For insulators having high resistance , much bigger units kilo ohm or kΩ (10^3 ohm) are used. in case of very small resistances smaller units like milli ohm (10^-3) are employed.
Relation Between electric conductivity and resistance :
Electric Conductivity and Resistance are inversely proportional to each other. the reciprocal of resistance is called conductance.
Conductance = 1/R
For More Detailed Explanation see the below video.
Ohm's Law and It's limitations
Ohm's Law
The current flowing through the conductor is directly proportional to the voltage difference across the ends of the conductor and inversely proportional to the resistance.
The current flowing through the conductor is directly proportional to the voltage difference across the ends of the conductor and inversely proportional to the resistance.
This relation was discovered by Georg Simon Ohm and so it is known as Ohm's Law.
If 'I'current flowing through the conductor of resistance 'R' across which potential difference 'V' is applied then according to Ohm's law
Limitations
Ohm's Law cannot be applied to circuits consisting of electronic tubes or transistors because such elements are not bilateral i.e they behave in different way when the direction of flow of current reversed as in case of diode.
If 'I'current flowing through the conductor of resistance 'R' across which potential difference 'V' is applied then according to Ohm's law
Ohm's Law cannot be applied to circuits consisting of electronic tubes or transistors because such elements are not bilateral i.e they behave in different way when the direction of flow of current reversed as in case of diode.
Ohm's law also cannot be applied to circuits consisting of nonlinear elements like powdered carbon, thyrite, electric arc etc.
For More Explanation see the below video:
3. Laws of resistance (Resistivity ,specific resistance)
Laws of resistance
The resistance of a wire depends upon its length, area of cross section, type of material,purity and hardness of material of which it is made of and the operating temperature.
Resistance of a wire is
Directly proportional to its length, l i.e R α l
Inversely proportional to its area of cross section, a i.e Rα 1/a
Combining above two facts we have R α l/a
where ρ is a constant depending upon the nature of the material and is known as the specific resistance or resistivity of teh amterial of the wire.
Resistivity or specific resistance of amaterial may also be defined as the resistance of the material of unit length and unit cross section area.
The resistance of a wire depends upon its length, area of cross section, type of material,purity and hardness of material of which it is made of and the operating temperature.
Resistance of a wire is
Directly proportional to its length, l i.e R α l
Inversely proportional to its area of cross section, a i.e Rα 1/a
Combining above two facts we have R α l/a
where ρ is a constant depending upon the nature of the material and is known as the specific resistance or resistivity of teh amterial of the wire.
Resistivity or specific resistance of amaterial may also be defined as the resistance of the material of unit length and unit cross section area.
Specific resistance or resistivity of a material may also defined as the resistance of the material may also defined as the resistance between opposite faces of a unit cube of that material.
Resistivity is measured in ohm-metres Ω-m or ohms per meter cube
For More Details about Ohm's Law See the Video
4. What electrical value can be measured across the copper and zinc rods?
Voltage.
5. What is alternating current (AC)?
Alternating current changes direction. (50 Hz Supply)
6. What is Direct Current (DC)?
Does not change direction.
7. What does a fuse protect against in a circuit?
High current.
8. What unit is used to measure an electrical resistance?
Ohms.
9. What are the major parts of an electrical motor?
1. Motor enclosure
2. Stator
3. Rotor
4. Shaft
5. Bearings
6. End fan cover.
10. What is diffrent Electrical and Electronics Element?
Electrical elements deal with electricity while electronic elements deal with controlling the flow of electricity.
Electrical elements are passive components that do not control the flow of electricity.
Electronic elements are active components that control the flow of electricity.
Examples of electrical elements include resistors, capacitors, and inductors.
Examples of electronic elements include transistors, diodes, and integrated circuits.
11. What you do if motor stator core is short, what maintenance you do in HT panel, transformer maintenance, innovative modifications?
If motor stator core is short, maintenance in HT panel and transformer is necessary.
Isolate the affected equipment and de-energize it
Check the insulation resistance of the equipment
Identify the cause of the short circuit and repair or replace the affected parts
Perform regular maintenance on the HT panel and transformer to prevent future issues
Consider innovative modifications to improve the efficiency and reliability of the equipment
12. What is different SCR and thrisitor?
SCR stands for Silicon Controlled Rectifier.
SCR is a type of thyristor used for high power applications.
It is a type of thyristor used for high power applications.
SCR can handle high voltage and current levels.
It is commonly used in power control circuits, motor control circuits, and lighting dimmer circuits.
Thyristors are a family of semiconductor devices that include SCR, TRIAC, and DIAC.
13. What is transformers working star delta stars?
Transformers work by transferring electrical energy from one circuit to another through electromagnetic induction.
Transformers consist of two coils of wire, a primary coil and a secondary coil, wrapped around a magnetic core.
When an alternating current flows through the primary coil, it creates a magnetic field that induces a voltage in the secondary coil.
The voltage in the secondary coil can be either higher or lower than the voltage in the primary coil, depending on the number of turns in each coil.
Transformers are used in power distribution systems to step up or step down voltage levels, and in electronic devices to isolate circuits and match impedances.
14. 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.
15. 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.
16. 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 emfinto 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.
17. 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 transmission
C. 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.
18. 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.
19. 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 upto1000 volts.
• High-tension cables can transmit voltage upto23000 volts.
• Super tension cables can transmit voltage 66 kV to 132 kV.
20. Why back emfused 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.
21. 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.
22. 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.
23. 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.
24. 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.
25. 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).
26. 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.
27. 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.
28. 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:
-eddy current loss
-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.
29. 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.
30. 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 compolesor interpoles, which are series excited by the armature current. These are located in the inter polar region of the main poles and therefore influence the armature coils only when these undergo commutation.
31. Comment on the working principle of operation of a single-phase transformer.
Working principle of operation of a single-phase transformer can be explained as An 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.
32. 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.
33. 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.
34. 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.
35. 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.
36. 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.
37. 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 cosj 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.
38. 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.