Interview Questions

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 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.

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.

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:

  • 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.

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 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.

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.

30. 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

31. 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.

Electrical Circuit Theorems:

To solve problems in electrical networks, there are number circuit theorems have been developed like
(1) Kirchhoff’s laws
(2) Superposition theorem
(3) Thevenin theorem
(4) Norton Theorem
(5) The maximum power transfer theorem

In this post we have listed all these theorems. We believe that this post will serve as a ready reference for you.

(1)Kirchhoff’s laws:
(a) Current Law:
At any junction in an electric circuit the total current flowing towards that junction is equal to the total current flowing away from the junction, i.e. I =0

(b) Voltage Law:
In any closed loop in a network, the algebraic sum of the voltage drops (i.e.products of current and resistance) taken around the loop is equal to the resultant e.m.f. acting in that loop.

(2)Superposition Theorem:
In any network made up of linear resistances and containing more than one source of EMF, the resultant current flowing in any branch is the algebraic sum of the currents that would flow in that branch if each source was considered separately, all other sources being replaced at that time by their respective internal resistances.

(3)Thevenin Theorem:
The current in any branch of a network is that which would result if an EMF. equal to the potential differential across a break made in the branch, were introduced into the branch, all other EMFs being removed and represented by the internal resistances of the sources.

(4)Norton Theorem:
The current that flows in any branch of a network is the same as that which would flow in the branch if it were connected across a source of electrical energy, the short-circuit current of which is equal to the current that would flow in a short-circuit across the branch, and the internal resistance of which is equal to the resistance which appears across the open-circuited branch terminals.

(5)The Maximum Power Transfer Theorem:
The power transferred from a supply source to a load is at its maximum when the resistance of the load is equal to the internal resistance of the source.

What is the Scope of Electrical Engineering?

[This question is mostly asked to check your communication. Speak casually. Tell the importance of electrical and electronics engineering course, what are the topics to be covered in the course, what are the job prospects etc.]

Answer: Electrical Engineering has a very broad scope with the science and technology which involves electricity, electronics, and electromagnetism in order to design, constructs, and maintains products, services, and information systems.

Electrical Engineering is a branch that deals with the field associated with power transmission and motor control.

Electrical engineering is one of the core fields of engineering, so it is always evergreen in terms of job prospects.

Electrical Engineering is the heart of whole engineering discipline.

Nowadays, we see Electronics playing an important role in the implementation and control of Electrical Machines. Keeping this fact file in mind the course have transformed from core electrical Engineering into a Fusion of Electronics and Electrical and has been renamed as Electronics and Electrical engineering.

Electricity has become the basic need for survival, globally. From household to industrial plants, communication and satellite navigation system, electronic equipment, computers etc., all require electricity.

Electrical engineering thus deals with study and application of electrical systems for use in these different environments. It equips you with the knowledge of transmission and generation of electrical power, electrical circuit design, electronics, instrumentation, control system, understanding electrical and electronic networks etc.

The course also covers the study of electronic devices and circuits involved in measurement, instrumentation, control and protection of electrical equipments and conversion systems. Concept of computer and recent applications of computer based systems in design, analysis and efficient operation of power system, maintaining quality and security, also included in the course.

What does ‘230V AC Supply’ really mean? Is it the RMS or peak voltage?

If the peak value is meant it should be clearly stated, otherwise assume it is the RMS value. The supply is 230V AC means 230V RMS, so the peak voltage of the mains is about 320V.

What does the multimeter in volt meter mode show? Is it the RMS or peak voltage?

If the multimeter is in voltmeter mode and when it connects to AC circuits it shows the RMS value of the voltage or current. When it connects to varying DC signals it will show RMS value only.

How does Induction Motor work?

  • Induction motors are also known as rotating transformer. ( Primary winding is stator and secondary winding is rotor).
  • When the 3 phase stator windings are fed by 3 phase supply a magnetic flux of constant magnitude, rotating at synchronous speed is generated. These flux passes through the air gap, sweeps past the rotor surface, cuts stationary rotor conductors.
  • According to Faraday’s laws of electro-magnetic induction, an emf is induced in the rotor. This emf frequency is same as supply frequency.
  • As rotor conductors form a closed circuit, the induced emf produces a current. It’s direction will oppose the cause producing it (according to lenz’s law).

In this case, the cause is relative velocity between the rotating flux of the stator and stationary rotor conductors. So to reduce the relative speed, the rotor starts to run in the same direction of the stator flux.

[Q] Define Power and Energy?
The rate at which work is done in an electric circuit is known as Electric power.
Power = Voltage X Current
The basic unit for electric power is Watts
The instrument used to measure the power is known as watt meter.

The total amount of work done in an electric circuit is called as electric energy.
Energy = Power X Time
the basic unit is Joule or watt-sec
The practical unit is Kilo watt-hour
The instrument used to measure the energy is known as Energy meter.

[ Like this you have to explain elaborately in the interviews. The interviewer will expect detailed answer from the candidate.
If he feels that you are deviating too much, then he will stop you and ask another question. Please don’t answer in one line. Add related points
where ever possible]

[Q] In our house are we monitoring/measuring power or energy?
Energy. It is done by energy meter.

[Q] what is meant by cycle in ac waveform? what is period? Define frequency?
One complete set of change in value is known as waveform.
If you any basic question on AC waveform, then first draw an ac waveform and explain with reference to the waveform.

[Q] Explain about various AC Values?
Instantaneous value:
Instantaneous values are the values of the alternating quantities at any instant of time. They are represented by small letters like i, e etc.
Peak value:
The largest value reached in a half cycle is called the peak value or the maximum value or the amplitude of the waveform. Such values are represented by Vm, Im etc.
Peak-to-peak value is the difference between the maximum and minimum values in a cycle.
Average value:
The average or mean value of a symmetrical alternating quantity (such as a sine wave), is the average value measured over a half cycle (since over a complete cycle the average value is zero). For a sine wave, average value = 0.637 times maximum value
RMS Value:
The effective value of an alternating current is that current which will produce the same heating effect as an equivalent direct current. The effective value is called the root mean square (rms) value and whenever an alternating quantity is given, it is assumed to be the rms value.

[Q] What is form factor and peak factor?
The ratio of rms value to average value of an ac quantity is called form factor.
Form Factor = RMS value/ Average value
The ratio of maximum value to the RMS value of an ac quantity is called as peak factor.
Peak Factor = Maximum Value/RMS value

Power Electronics Interview Questions with Answers:

Here, we had collected Various Power Electronics and Drives related Interview questions, Viva Questions. We are appending the new Q & A whenever we are getting from readers.
Some of the questions are asked to Experienced candidates based on their previous experience. If it is not related to your field, just ignore it.
[Q] What is AC voltage controller?
It is defined as the power electronics converter which converts the fixed AC voltage into a variable AC voltage without change in the frequency.

[Q] What is inverter? What are the types of Inverter?
A device which converts dc power into ac power at desired output voltage and frequency is called as Inverter.
Inverters are broadly classified into
Voltage Source Inverter (VSI)
Current Source Inverter (CSI)

[Q] What is chopper?
It is equivalent to a dc transformer in ac circuit. The Chopper is a static switch which is used to get the variable dc output voltage from a constant dc input voltage.
Chopper basics, types and principle of operation is discussed in a separate page. Click here to know more about Choppers

Step down chopper:
In this circuit, the average output voltage [VO = D . VS] is less than the input supply voltage. It is also known as Buck converter.

Step up chopper:
It is also known as Boost converter. Here the average output voltage [VO = VS / 1 – D]  is more than the input supply voltage.

[Q] What is load commutation?
In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR.

[Q] What is meant by pulse triggered devices?
As the name indicates, to trigger these devices just a pulse is sufficient. Continuous gate voltage of entire on time is not required. The advantage of pulse triggering is, it will avoid the hard triggering.
Ex. Thyristor, GTO

[Q] What is level-sensitive devices? give some examples
Some of the level sensitive devices are

  • MOSFET
  • IGBT
  • MCT
  • IGCT

In order to keep these devices in the ON state condition, we have to apply  gate current/voltage continuously to these devices.

[Q] What is meant by GTO?
It is a three terminal, four layer PNPN  Power Semiconductor device that can be turned on by a positive gate current and can be turned off by a reverse gate current.

[Q] What are the types of GTO?

  • Asymmetrical GTO
  • Symmetrical GTO

[Q] What does 10V AC mean? Is it the RMS voltage or Peak voltage or Average voltage?
In general,  AC voltages and currents are mentioned in RMS values only. It is sensible to compare with steady DC voltages and currents.
For example 230V AC supply means, 230Vrms AC Supply.

[Q] When we have to connect the SCRs in a parallel manner?
To meet the high current demand we will connect the SCRs in a parallel manner.
Similarly to meet the high voltage demand we have to connect the SCRs in series.

[Q] Define Gate Charge in MOSFET?
The gate charge is defined as the amount of charge required for the device during turn-on and turn-off time. It is the most important parameter we have to consider while selecting the MOSFET. The switching speed of the MOSFET depends on the speed at which the gate driver can charge or discharge the input gate charge.

Flip-Flop:

A flip-flop continuously checks its inputs and correspondingly changes its output only at times determined by clocking signal.

A latch is a device which continuously checks all its input and correspondingly changes its output, independent of the determined by clocking signal.

Types of flip-flop:

There are basically four main types of flip-flops: SR, D, JK, and T.

[1] Explain the amplifier operation of OP-AMP?
The golden rules of OP-AMP are:

  1. Vout = A. Delta Vin  [ A = Gain, Delta Vin = V+  –  V-]
  2. V–  < Vout  < V++
  3. Zout = 0    ,      Zin+ = Zin- = Infinity

The opamp will work as inverting amplifier and non- inverting amplifier based on the supply applied to its terminals.

[2] Explain how transistor acts as a switch?

  • The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region.
  • This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its “fully-OFF” (cut-off) and “fully-ON” (saturation) regions as shown below.

[3] Draw and Explain the SCR V-I characteristic?

Vak = +ve & Vg = 0

  1. When a positive voltage is applied to anode with respect to cathode, the junctions J1 and J3 are forward biased, but the junction J2 is reverse biased.
  2. The SCR is in its forward blocking state. At this time the Gate signal is not applied.
  3. As shown in figure a small amount of forward leakage current is flows through the device.

Vak=+&Vg=+ve

  1. When the small amount of positive voltage is applied, while positive voltage is applied to anode with respect to cathode, the junction J3 becomes forward biased.
  2. Thus the SCR conducts a large value of forward current with small voltage drop. With the application of gate signal the SCR changed from forward blocking state to forward conducting state. It is called as latching.Without gate signal it happens at forward breakdown voltage (Vfbd).
  3. When the gate signal value is increased, the latching happens for low Vak voltages as mentioned in the figure. In the presence of forward current (i.e. after the thyristor is turned on by a suitable gate voltage) it will not turn off even after the gate voltage has been removed.
  4. The thyristor will only turn off when the forward current drops below holding current. The holding current is defined as the minimum current required to hold the SCR in the forward conduction state.

Vak = -ve

  1. When a negative voltage is applied to anode with respect to cathode, the junctions J1 and J3 are reversed biased, but the junction J2 is forward biased. The SCR is in its reverse blocking state.
  2. As shown in figure a small amount of reverse leakage current flows through the device.

If the applied voltage is more than reverse breakdown voltage, the device will collapse and the large amount of current flows through it.

[3] Explain Buck topology

  1. When the switch is ON condition (ie, TON), the input provides energy to inductor and output.
  2. The difference between the input and output voltage is applied to inductor as shown in the below waveform.
  3. When the switch is turned off, the inductor current will flow in the same direction to load. It is inductor property to maintain the current flow in the same direction.
  4. The freewheeling diode D1 is forward biased at this time and completes the inductor current path.

The output of the buck converter with continuous output current is

  • Vout=D*VIN

D = Duty cycle = TON / [TON + TOFF]

[4] Compare MOSFET and BJT

  • BJT                                                                                   MOSFET
  • More Power handling Capability                             Less Power handling capability
  • Low switching speed                                                 Fast switching speed
  • High On state resistance                                          Low on state resistance
  • Has second breakdown voltage problem              No second breakdown voltage problem

 

Power System Interview Question and Answers:

[Q] What is meant by synchronous condenser?

An over excited synchronous motor running on no load is known as synchronous condenser. It is used to improve the power factor of the system.

[Q] Define Power Factor.

The cosine of angle between voltage and the current in an AC circuit is known as power factor.

  • The value of power factor will always be less than 1.
  • It is a usual practice to attach a word lagging or leading with the numerical value of power factor to signify whether the current lags or leads the voltage.

[Q] What is the difference between fuse and circuit breaker?

  1. Fuse performs both detection and interruption  functions. Circuit breaker performs interruption function only. The detection of fault is made by relay system.
  2. The breaking capacity of fuse is very small compare to that of circuit breaker.
  3. The operating time of the fuse is smaller than operating time of CB.
  4. After every operation, replacement is required in the fuse. In case of circuit breaker, replacement is not required.

[Q] What is meant by Sag?

The difference in level between points of supports and the lowest point on the conductor is called sag.

[Q] What is corona?

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona.

[Q] How can we reduce the effect of corona?

  1. By increasing conductor size:

By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced.

  1. By increasing conductor spacing:

By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated.

[Q] What is tariff?

The rate at which electrical energy is supplied to a consumer is known as tariff.

The tariff should include the following items:

  1. Recovery of cost of producing electrical energy at the power station
  2. Recovery of cost on the capital investment in transmission and distribution systems.
  3. Recovery of cost of operation and maintenance of supply
  4. A suitable profit on the capital investment.

[Q] What are the various types of tariff?

  1. Simple tariff
  2. Flat rate tariff
  3. Block rate tariff
  4. Two-part tariff
  5. Maximum demand tariff
  6. Power factor tariff
  7. Three-part tariff

[Q] What are the advantages and disadvantages of nuclear power plant?

Advantages:

  1. The amount of fuel required is quite small.
  2. It requires less space so it can be located near to the load centre.
  3. Huge deposits of nuclear fuels available all over the world. So supply of electricity can be ensured.
  4. It ensures reliability of operation.

Disadvantages:

  1. The fuel used is expensive and is difficult to recover.
  2. The capital cost on a nuclear plant is very high as compared to other types of plants
  3. The disposal of the by-products is a big problem. They are radioactive in nature.
  4. The maintenance charges are high.

[Q] Why transformers rated in KVA instead of KW?
The copper loss (I2R) in the transformer depends on the current I through the winding while the iron or core loss depends on the voltage V as frequency of operation is constant. None of these losses depend on the power factor (cosφ) of the load. Hence losses decide the temperature rise and hence the rating of the transformer. As losses depends on V and I only, the rating of the transformer is specified as a product of these two parameters VxI. Thus the transformer rating is in kVA and not in kW.

[Q] Why is the efficiency of transformers more than that of other rotating machines?

There are no moving parts in transformer hence the friction and mechanical losses are absent in transformer. Hence efficiency of the transformer is more than of other rotating machines.

[Q] Define leakage inductance

Inductance offered by the winding due to the leakage flux associated with it is called leakage inductance. It is the ratio of leakage flux linkages with the winding to the current passing through the winding.

[Q] What material is used in the transformer core?
High grade silicon steel laminations are used for the construction of the core.

[Q] What is meant by leakage flux?
Part of the primary flux as well as the secondary flux completes the path through air and links with the respecting winding only. Such a flux is called leakage flux. Leakage fluxes link with the respective windings only and not to both the windings.

[Q] Define voltage regulation of a transformer?
The regulation is defined as change in the magnitude of the secondary terminal voltage, when full load ie, rating load of specified power factor supplied at rated voltage is reduced to no load, with primary voltage maintained constant expressed as the percentage of the rated terminal voltage.

[Q] What is an ideal transformer?

A transformer is said to be ideal if it satisfies following properties:

  • It has no losses
  • Its windings have zero resistance
  • Leakage flux is zero ie, 100% flux produced by primary links with the secondary
  • Permeability of core is so high that negligible current is required to establish the flux in it.

 

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