Electrical Interview questions Part 1

Electrical Engineering interview questions and answers

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.

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.

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.

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.

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.

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.

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.

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 Thevenins Theorem:

According to thevenins theorem, the current flowing through a load resistanceConnected 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 Nortons Theorem

The Nortons 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 Nortons 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 losso 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 Faradays 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 Lenzes 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 JFETs and MOSFETs.

Comparison of JFETs and MOSFETs: JFETs can only be operated in the depletion mode whereas MOSFETs 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. MOSFETs have input impedance much higher than that of JFETs. Thus is due to negligible small leakage current. JFETs 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.

Mechanical Engineering interview questions and answers

What are the different types of fits? Explain?

On the basis of Indian standards fits can mainly be categorized into three groups:> Clearance Fit: These types of fits are characterized by the occurrence of a clearance between the two mating parts. The difference between the minimum size of the hole and the maximum size of the shaft is called the minimum clearance, the difference between the maximum size of the hole and the minimum size of the shaft is known as maximum clearance.> Interference Fit: In these types of fits the size of the mating parts are predefined so that interference between them always occurs. The tolerance zone of the hole is completely below the tolerance zone of the shaft.> Transition Fit: As the name suggests these type of fit has its mating parts sized limited to allow either clearance or interference. The tolerance zone of the hole and the shaft overlaps in case of such fits.

For a shaft designated as 40 H8/f7, calculate the tolerances.

Given: Shaft designation = 40 H8/f7The shaft designation 40 H8/f 7 means that the basic size is 40 mm and the tolerance grade forthe hole is 8 ( i. e. I T 8) and for the shaft is 7 ( i. e. I T 7).Since 40 mm lies in the diameter steps of 30 to 50 mm, therefore the geometric mean diameter,D = Square root of (30 x 50) = 38.73 mmWe know that standard tolerance unit,i = 0.45 x Cube root of (D) + 0.001 Di = 0.45 3.38 + 0.03873 = 1.559 73 or 1.56 micronsi = 1.56 0.001 = 0.001 56 mm ...(1 micron = 0.001 mm)The standard tolerance for the hole of grade 8 (IT8)= 25 i = 25 0.001 56 = 0.039 mmThe standard tolerance for the shaft of grade 7 (IT7)= 16 i = 16 0.001 56 = 0.025 mm

What are the factors that can affect the Factor of safety selection?

The factor of safety is used in designing a machine component. Prior to selecting the correct factor of safety certain points must be taken into consideration such as:> The properties of the material used for the machine and the changes in its intrinsic properties over the time period of service.> The accuracy and authenticity of test results to the actual machine parts.> The applied load reliability.> The limit of stresses (localized).> The loss of property and life in case of failures.> The limit of initial stresses at the time period of manufacture.> The extent to which the assumptions can be simplified.

The factor of safety also depends on numerous other considerations such as the material, the method of manufacturing , the various types of stress, the part shapes etc.

What is heat treatment and why is it done?

Heat treatment can be defined as a combination of processes or operations in which the heating and cooling of a metal or alloy is done in order to obtain desirable characteristics without changing the compositions. Some of the motives or purpose of heat treatment are as follows:> In order to improve the hardness of metals.> For the softening of the metal.> In order to improve the machinability of the metal.> To change the grain size.> To provide better resistance to heat, corrosion, wear etc.

Heat treatment is generally performed in the following ways:> Normalizing> Annealing> Spheroidising> Hardening> Tempering> Surface or case hardening

What are the rules that must be kept in mind while designing castings?

Some of the points that must be kept in mind during the process of cast designing are as follows:> To avoid the concentration of stresses sharp corners and frequent use of fillets should be avoided.> Section thicknesses should be uniform as much as possible. For variations it must be done gradually.> Abrupt changes in the thickness should be avoided at all costs.> Simplicity is the key, the casting should be designed as simple as possible.> It is difficult to create true large spaces and henceforth large flat surfaces must be avoided.> Webs and ribs used for stiffening in castings should as minimal as possible.> Curved shapes can be used in order to improve the stress handling of the cast.

What are the points that should be kept in mind during forging design?

Some of the points that should be followed while forging design are:> A radial flow of grains or fibers must be achieved in the forged components.> The forged items such as drop and press forgings should have a parting line that should divide the forging into two equal halves.> The ribs in a forging should not be high or thin.> In order to avoid increased die wear the pockets and recesses in forgings should be minimum.> In forgings the parting line of it should lie as far as possible in a single plane.> For ease of forging and easy removal of forgings the surfaces of the metal should contain sufficient drafts.

Describe briefly the different cold drawing processes.

Some of the important cold drawing processes are as follows:> Bar and Rod Drawing: In the case of bar drawing the hot drawn bars are at first pickled, washed and coated to prevent oxidation. Once this is done a draw bench is used for the process of cold drawing. In order to make an end possible to enter a drawing die the diameter of the rod is reduced by the swaging operation. This end is fastened by chains to the draw bench and the end is gripped by the jaws of the carriage. In this method a high surface finish and accuracy dimensionally is obtained. The products of this process can be used directly without any further machining.> Wire Drawing: Similar to the above process the bars are first pickled, washed and coated to prevent any oxidation. After this the rods are passed through several dies of decreasing diameter to provide a desired reduction in the size ( diameter ). The dies used for the reduction process is generally made up of carbide materials.>Tube Drawing: This type of drawing is very similar to the bar drawing process and in majority of cases it is accomplished by the use of a draw bench.

What are the different theories of failure under static load, explain briefly?

The main theories of failure of a member subjected to bi-axial stress are as follows:> Maximum principal stress theory ( Rankines theory): This theory states that failure occurs at a point in member where the maximum principal or normal stress in a bi-axial system reaches the maximum strength in a simple tension test.> Maximum shear stress theory ( Guests or Trescas theory): This theory states that failure occurs when the biaxial stress reaches a value equal to the shear stress at yield point in a simple tension test.> Maximum principal strain theory ( Saint Venant theory): This theory states that failure occurs when bi-axial stress reaches the limiting value of strain.> Maximum strain energy theory ( Haighs theory): This theory states that failure occurs when strain energy per unit volume of the stress system reaches the limiting strain energy point.> Maximum distortion energy theory ( Hencky and Von Mises theory): This theory states that failure occurs when strain energy per unit volume reaches the limiting distortion energy.

8. What are the assumptions made in simple theory of bending?

The assumptions made in the theory of simple bending are:> The material of the beam is homogeneous this implies that it is uniform in density, strength and have isotropic properties meaning possessing same elastic property in all directions.> Even after bending the cross section of the beam remains constant.> During the initial stages the beam is straight and unstressed.> All the stresses in the beam are within the elastic limit of its material.> The layers of the beam are free to contract and expand longitudinally and laterally> On any cross section the perpendicular resultant force of the beam is zero.> Compared to the cross-sectional dimension of the beam the radius of curvature is very large.

9. Why is stress considered important in a shaft?

The following types of stresses are prevalent in shafts:> At the outermost surface of the shaft the max shear stress occurs on the cross-section of the shaft.> At the surface of the shaft on the longitudinal planes through the axis of the shaft the maximum longitudinal shear stress occurs.> At 45 degrees to the maximum shearing stress planes at the surface of the shafts the major principal stress occurs. It equals the max shear stress on the cross section of the shaft.> For certain materials where the tensile and compressive strengths are lower in measure as compared to the shear strength, then the shaft designing should be carried out for the lowest strengths.> All these stresses are of significance as they play a role in governing the failure of the shaft. All theses stresses get generated simultaneously and hence should be considered for designing purposes

10. What do you understand by the Hooke`s Coupling what are its purposes?

The Hooke`s coupling is used to connect two shafts whose axes intersect at a small angle. The two shafts are inclined at an angle and is constant. During motion it varies as the movement is transferred from one shaft to another. One of the major areas of application of this coupling is in gear boxes where the coupling is used to drive the rear wheels of trucks and other vehicles. In such usage scenarios two couplings are used each at the two ends of the coupling shaft. they are also used to transfer power for multiple drilling machines. The Hooke`s coupling is also known as the Universal coupling. The torque transmitted by the shafts is given by :T= (pie/16) x t x (d) cubeWhere T = torque, t = shear stress for the shaft material and d the diameter of the shaft.

11. What kind of materials should be used for shafts manufacturing?

Some of the qualities that should be present in materials for shafts are as follows:> The material should have a high index of strength.> Also it should have a high level of machinability.> The material should possess a low notch sensitivity factor.> The material must also have wear resistant properties.> Good heat treatment properties should also be presentThe common material used to creates shafts of high strengths an alloy of steel like nickel is used. The shafts are manufactured by hot rolling processes and then the shaft is finished using drawing or grinding processes.

12. Why should a chain drive be used over a belt or rope driven drive? State pro`s and con`s?

The advantages of using a chain drives are:> In a chain drive no slip occurrence takes place.> The chains take less space as compared to rope or belts as they are made of metal and offer much strength.> The chain drives can be used at both short and long ranges and they offer a high level of transmission efficiency.> Chain drives can transmit more load and power as compared to belts.> A very high speed ratio can be maintained in one step of chain drives.Some of the cons of using a chain drive are:> The cost of producing chain drives is higher as compared to that of belts.> The chain drives must be serviced and maintained at regular intervals and henceforth their cost of ownership is high comparatively.

13. What are the different types of springs and explain them briefly?

Springs can be broadly classified into the following types:> Helical Springs: These springs as their name suggests are in coil form and are in the shape of helix. The primary purpose of such springs are to handle compressive and tensile loads. They can be further classified into two types: compression helical spring and tension helical spring each having their own unique areas of application./> Conical and volute springs: Both these spring types have specialized areas of usage where springs with adaptable rate according to the load is required. In case of conical springs they are wound so as to have a uniform pitch while on the other hand volute springs are wound in a slight manner of a parabloid.> Torsion Springs: The characteristics of such springs is that they tend to wind up by the load. They can be either helical or spiral in shape. These types of springs are used in circuit breaker mechanisms.> Leaf springs: These types of springs are comprised of metal plates of different lengths held together with the help of bolts and clamps. Commonly seen being used as suspensions for vehicles.> Disc Springs: As the name suggests such types of springs are comprised of conical discs held together by a bolt or tube.> Special Purpose Springs: These springs are all together made of different materials such as air and water.

14. During the design of a friction clutch what are the considerations that should be made?

In order to design a friction clutch the following points must be kept in mind:> The material for the contact surfaces must be carefully selected.> For high speed devices to minimize the inertia load of the clutch, low weight moving parts must be selected.> The contact of the friction surfaces must be maintained at all the times without the application of any external forces.> Provisions for the facilitation of repairs must be there.> In order to increase safety the projecting parts of a clutch must be covered.> A provision to take up the wearing of the contact surfaces must be present.> Heat dissipaters to take away the heat from the point of contacting surfaces must be there.

15. What are the different types of brakes and explain them briefly?

Brakes can be classified on the basis of their medium used to brake, they are as follows:> Hydraulic Brakes: These brakes as their name suggest use a fluid medium to push or repel the brake pads for braking.> Electric Brakes: These brakes use electrical energy to deplete or create a braking force.Both the above types of breaks are used primarily for applications where a large amount of energy is to be transformed.> Mechanical Brakes: They can be further classified on the basis of the direction of their acting force: Radial Brakes: As their names suggests the force that acts on the brakes is of radial direction. They can further be classified into internal and external blades. Axial Brakes: In these types of brakes the braking force is acting in an axial direction as compared to radial brakes.

16. On what basis can sliding contact bearings be classified? Explain?

Sliding contact bearings can be classified on the basis of the thickness of the lubricating agent layer between the bearing and the journal. They can be classified as follows:> Thick film bearings: These type of bearings have their working surface separated by a layer of the lubricant. They are also known as hydrodynamic lubricated bearings.> Thin film bearings: In this type of bearings the surfaces are partially in direct contact with each other even after the presence of a lubricant. The other name for such type of bearings is boundary lubricated bearings.> Zero Film Bearings: These type of bearings as their name suggests have no lubricant present between the contact layers.> Externally or hydrostatically pressurized lubricated bearings: These bearings are able to without any relative motion support steady loads.

17. What are the basis on which the best material for Sliding Contact Bearings manufacturing?

Some of the important properties to lookout for in the material for sliding contact bearings are as follows:> Compressive Strength: In order to prevent the permanent deformation and intrusion of the bearing the material selected should be possess a high compressive strength to bear the max bearing pressure.> Fatigue Strength: the material selected for the bearing should be able to withstand loads without any surface fatigue cracks getting created. This is only possible if the material has a high level of fatigue strength.> Comfortability: The material should be able to adjust or accommodate bearing inaccuracies and deflections without much wear and heating.> Embeddability: The material should allow the embedding of small particles without effecting the material of the journal.> Bondability: The bearings may be created by bringing together ( bonding ) multiple layers of the material. Due to the above reason the bondability of the material should be sufficiently high.> Thermal conductivity and corrosion resistance: Thermal conductivity is an essential property for bearing materials as it can help in quickly dissipating the generated heat. Also the material should have a level of corrosion resistance against the lubricant.

18. Briefly explain the advantages of Cycloidal and Involute gears?

The advantages of the Cycloidal gears are as follows:> Having a wider flank as compared to Involute gears they are considered to have more strength and hence can withstand further load and stress.> The contact in case of cycloidal gears is between the concave surface and the convex flank. This results in less wear and tear.> No interference occurs in these types of gears.The advantages of Involute gears are as follows:> The primary advantage of involute gears is that it allows the changing of the centre distance of a pair without changing the velocity ratio.> The pressure angle remains constant from start to end teeth, this results in less wear and smooth running of the gears.> The involute gears are easier to manufacture as they can be generated in a single curve ( the face and flank ).

19. How can the reaction of support of a frame be evaluated?

Generally roller or hinged support are used to support the frames. The conditions of equilibrium are used to determine the reaction support of a frame. The condition of equilibrium takes place when the sum of the horizontal and vertical forces sum equal to zero. The system must form a state of equilibrium even after considering the external loads and the reactions at the supports. For equilibrium to be prevalent in the system the following conditions are required to be in occurrence:> Summation of V = 0. This implies that the summation of all the forces in the vertical direction results to zero.> Summation of H = 0 . This implies that the total of all the forces acting in horizontal direction is also zero.> Summation of M = 0. The sum of all the moment of forces around a point must be zero.

20. Explain in an orderly manner how the force in the member of a truss be detected using the method of joint.

The steps required to calculate the force are as follows:> The reaction at the support has to be first calculated.> Once the reaction is calculated the direction of force of the member is made to make it tensile. On getting the result to be negative the direction assumed is wrong and this implies the force being compressive in nature.> A joint needs to be selected whose 2 members are not known. The lami`s theorem is used on the joint on which less than three forces are acting.> After the above process is complete the free body diagrams of the joint needs to be made. Since the system is in equilibrium the condition of Summation of V and H must result in zero.> After the above step the resolution of forces method needs to be used on the joint on which more than 4 forces are acting.

21. In order to derive the torsional formulas what are the assumptions taken?

The torsion equation is derived on the basis of following assumptions:> The shaft material is uniform, throughout the shaft.> Even after loading the shaft circular remains circular.> After the application of torques the plain section of a shaft remains plain.> Any twist that occurs in the shaft remains uniform and constant.> After the application of torque the distance between any two cross-sectional references remains constant.> The elastic limit value of a shaft is never exceeded even after the shear stress induced because of torque application.

22. What are Bevel Gears and what are its types?

Bevel gears are the type of gears in which the two shafts happen to intersect. The gear faces which are tooth bearing are conical in shape. They are generally mounted on shafts which are 90 degrees apart but they can be made to work at other angles as well. The bevel gears are classified into the following types on the basis of pitch surfaces and shaft angles:> Mitre Gears: These types of gears are similar to each other ie. they have the same pitch angles and contain the same number of teeth. The shaft axes intersect at 90 degrees angle.> Angular bevel gears: When two bevel gears connect at any angle apart from 90 degrees.> Crown bevel gears: When the two shaft axes intersect at an angle greater than 90 and one of the bevel gears have a pitch angle of 90 degrees they are known as crown bevel gears.> Internal bevel gears: In these type of gears the teeth on the gears is cut on the inside area of the pitch cone.

23. What are the different values that need to be determined in order to design a cylinder for an ICE?

The following values are needed to be determined:> Thickness of the cylinder wall: The cylinder walls in an engine is made witness to gas pressure and the side thrust of a piston. This results in two types of stresses: longitudinal and circumferential stress. Both the types of stresses are perpendicular to each other and hence it is aimed to reduce the resulting stress as much as possible.> Length and bore of the cylinder: The length of the cylinder and the length of the stroke is calculated on the basis of the formula: length of cylinder L = 1.15 times the length of the stroke (l). L = 1.15(l)> Cylinder flange and studs: The cylinders are always cast integral as a part of the upper crankcase or in some cases attached to it by means of nuts and bolts. The flange is integral to a cylinder and henceforth its thickness should be greater than that of the cylinder wall. The thickness of flange should generally be between 1.2t-1.4t where t is the cylinder thickness.The stud diameter is calculated by equating gas load ( due to max pressure ) to the grand total of all the resisting forces of the studs.

24. What are considerations taken into account while creating a piston head?

The piston head is designed on the basis of the following considerations:> The crown should have enough strength to absorb the explosion pressure inside the engine cylinder.> The head must always dissipate the heat of the explosion as quickly as possible to the engine walls. The thickness of the head is calculated on the basis of another formula which takes into consideration the heat flowing through the head, the conductivity factor of the material. The temperature at the center and edges of the head.> The thickness of the piston head is calculated on the basis of the Grashoff`s formula which takes into consideration the maximum gas pressure of an explosion , the permissible bending and the outside diameter of the piston.

What is mechanism?

A mechanism is an assembly of different parts which perform a complete motion and is often part of a machine.

State Newton's three laws of Motion.

- The law of inertia: Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.- Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object). Force=Mass times acceleration.- For every action there is an equal and opposite reaction.

State the laws of thermodynamics and its importance of in Mechanical Engineering.

Thermodynamics is a physical science which studies the interrelation between heat, work and the internal energy of any system. Thermodynamics helps study all the systems of mechanical engineering. There are three laws of thermodynamics.

First Law:Energy can be neither created nor destroyed. It can only change forms. In any process in an isolated system, the total energy remains the same.

Eg: turning on a light would seem to produce energy; however, it is electrical energy that is converted.

Second Law:The second law of thermodynamics states that the entropy of any isolated system not in thermal equilibrium almost always increases.

Eg: A car that has run out of gas will not run again until you walk 10 miles to a gas station and refuel the car.

Third Law:As temperature approaches absolute zero, the entropy of a system approaches a minimum.

Eg: Water in gas form has molecules that can move around very freely. Water vapor has very high entropy (randomness). As the gas cools, it becomes liquid which can still move around but not as freely. At this point they have lost some entropy. On cooling further it becomes solid ice where molecules can no longer move freely but can only vibrate within the ice crystals. The entropy is now very low. As the water nears absolute zero, the vibration of the molecules diminishes. If the solid water reaches absolute zero, all molecular motion would stop completely. And at this point the water would have no entropy at all.

What is Hess law?

According to the Hess law the energy transfer is simply independent of the way being followed. If the reactant and the product of the whole process are the same then same amount of energy will be dissipated or absorbed.

What is PS?

Personal Statement. It is something that gives an informative background about an individual.

What is a bearing? What are the different types of bearings?

Bearing is a device that helps smoother movement with minimal friction which in turn helps enhances efficiency and speed. Considering two types of loading, radial and thrust, there are different types of bearings which help handle these loads. The basic difference in the types of loads is essentially due to their ability to handle weight and different kinds of loads for various applications. Different types of bearings are:

-Ball bearing-Roller bearing-Ball Thrust bearing-Roller thrust bearing-Tapered roller bearing-Magnetic bearings-Giant Roller Bearing

What is a process flow diagram?

A Process Flow Diagram is a user friendly, simplified sketch which illustrates general plant streams, major equipments and key central loops and shows the relationship between major components in the system. By using symbols to identify instruments and vessels it describes the primary flow course. They also provide e detail of mass/energy balance data along with stream composition and physical properties, however do not show minor components.

What is a time and motion theory?

Frederick Taylor was pioneer of the time and motion theory. This technique monitors the amount of time required to complete a task along with observing the steps taken by a worker to complete the given task.

Which is the hardest material on earth?

Diamond is currently the hardest material, made up of carbon atoms which cannot move. Carbon is the only atom that can have four electrons in the second shell surrounding the carbon nucleus, precisely why making a diamond the hardest material. However, there also are claims by a few to a new rare material called Wurtzite Boron Nitride which has a structure similar to a diamond but has some other atoms in place of carbon.

One unit of BTU is how many Joules?

1 BTU=1055.06 Joules

What does a pump develop? Give reason to support your answer.

Pump is a device that is used to transfer fluid from one place to another place which means it develops flow not pressure.

Explain the difference between pipe and a tube.

A pipe is measured based on its inner diameter (ID) whereas a tube is measured based on the outer diameter (OD). Other than the dimensions there is no major difference between the two.

Explain the formula of heat loss in a pipe.

In order to find total heat loss through the pipes it is imperative to know the thermal conductivity and the differing thicknesses of each layer. The ground that surrounds the pipe also acts as a layer of insulation. Considering this, the thermal properties of the stoneless sand is included in the heat loss calculation.

What kind of pipes are used for steam lines?

Pressure and temperature are two of the most important factors to be considered before selecting the type of material to be used. Steam is a compressible gas due to which the capacity of the pipe line depends on the size of the pipes and pressure of the steam. Since steam at even low pressure can be dangerous, extra care is to be taken. Galvanized pipes are usually not used for steam. Pipes made of mild steel with welded fittings are instead largely used.

Who invented the four stroke engine? According to you which one is more efficient, four stroke engine or a two stroke engine and why?

Nikolaus Otto invented the four stroke engine. Both, two stroke and four stroke have their own advantages and disadvantages. 4 stroke is more fuel efficient. However, 2 stroke produces more power.

What do you understand by the concept of a 6 stroke engine? Explain how it works.

The six stroke engine is based on the concept of four stroke engine but built with an intention of improvising the efficiency in reducing emission. Every cycle of a four stroke engine involves the upward and downward movement of the piston which happens twice in the chamber, resulting in four total strokes and one of which is the power stroke which provides the torque to move the vehicle. A six stroke engine works similarly except that there are two power strokes.

Explain what is torque

Torque is the force that causes rotation. It is a measure of how much force is acting on an object making it rotate.

What is the difference between torque and power?

While power determines the speed of a vehicle, torque determines the time in which that speed can be reached. The greater the torque figure, the faster the acceleration. The more torque in the engine, the faster you accelerate. Power is the rate at which work is done, so it is basically the potential of the engine.

Explain why diesel engine is known as high torque and petrol engine as high speed engine?

Each power stroke in a petrol engine releases more heat and is converted into mechanical energy due to a higher rate of burning. This is the reason petrol engine has higher power and acceleration.

Diesel engine is a compression ignition engine with higher compression ratio, therefore extreme pressure is high. Since the piston of a diesel engine is larger, more torque is delivered in produced.

Why do heavy vehicles use diesel engine?

The compression ratio of diesel engine is higher than the petrol engine, the reason for producing higher torque. The efficiency of diesel engine increases with load. Diesel produces slow energy on burning of fuel and the efficiency of the engine increases with load which is why diesel engine is suitable and largely used for heavy vehicles.

Explain the difference between projectile motion and rocket motion.

The major difference is that a projectile has no motor or a rocket on it, due to which the momentum is given to it as it is launched. A pen thrown across a room is a classic example of a projectile motion. On the other hand, a rocket or missile has a motor on it which helps in accelerating while moving. This helps in resisting other forces such as gravity. A projectile does not have any specific shape, it is a point mass, whereas rocket has a particular shape having its center of gravity situated at a particular point on its body. Thus rocket motion comes under kinetics and projectile comes under kinematics.

Between steel, copper and brass, which conduct faster heat.

Copper conducts heat faster than steel or brass. In most cases, material that is good for conducting heat is also good for electricity.

Explain the types of sensors.

(i) Temperature Sensor-This device collects information about the temperature from a source and converts it to a form which is understandable by another device or person. Glass thermometer is the best example where mercury acts as the temperature sensor.

(ii) IR Sensors-This device detects and/or emits infrared radiation to sense a particular phase in the environment. Mostly thermal radiation is emitted by all the objects in the infrared spectrum. This type of radiation is not visible to the human eye but the infrared sensor detects it.

(iii) UV Sensors-These sensors measure the intensity of the ultraviolet radiation. This form of electromagnetic radiation has wavelengths which are longer than x-rays yet shorter than visible radiation. UV sensors can discover the exposure of environment to ultraviolet radiation.

(iV) Touch Sensor-A touch sensor acts as a variable resistor based on the location where it is sensed. Proximity Sensor- A proximity sensor detects the presence of objects that are placed in close proximity without any point of contact.

What minerals are used in the manufacture of cars?

Cars are made from many various minerals. The steel body is made from the iron-rich minerals like magnetite and hematite. Door handles and badges are often coated in chromium which comes from chromite. Some of the other minerals used are aluminum, quartz, copper, magnesium, zinc, tin etc.

Which instrument measures speed in a car?

A speedometer helps measure the speed in the car.

What are the advantage and disadvantage of using LPG in a car?

Advantages:

The high octane rate enables it to blend better with air and to burn completely, generating less carbon. With less carbon buildup, spark plugs often last longer and oil changes are needed less frequently.

Since it burns in the gaseous phase, it results in less corrosion and engine wear.

In case of a spill, LPG evaporates quickly.

Disadvantages

The LPG requires servicing at approximately once a year.

Since complete combustion occurs, more heat is liberated which is not advised for a long journey as the engine will get over heated.

Installation of LPG is rather difficult.

Why gas containers are mostly in a cylindrical shape?

The ideal shape would be a sphere. The container must have the capacity to withstand the extremely high pressure of liquefied gas. A spherical shape helps in distributing these forces uniformly.

Explain why re-heater is used in gas turbine

The advantage of reheater is that it significantly increases the thrust; which is a prime reason for its use in gas turbines.

How many types of suspensions are used in automobiles?

McPherson strutsLeaf springCoil springTorsion beamWishboneAir Suspension

What is DTSI? Why it is used in motor bikes?

Digital Twin Spark Ignition. This is used for a better fuel combustion in the cylinder head which helps provide better efficiency and optimum use of fuel.

What are the advantages of DTSI over normal engines?

The cylinder head has two spark plugs, instead of the usual one. When two sparks are generated at either ends of the combustion chamber, the air-fuel mixture is ignited in a way that creates two flame fronts. A higher rate of combustion is achieved leading to higher rise in pressure. The outcome of this is more torque, better fuel efficiency and lower emissions.

What is meant by gear ratio?

A gear ratio is a direct measure of ratio of the rotational speeds of two or more interlocking gears.

What is the ratio of specific heat of air?

The ratio of specific heat =CP/CV is a factor in adiabatic engine processes and in determining the speed of sound in gas. This ratio =1.66 for an ideal monoatomic gas and =1.4 for air, which is predominantly a diatomic gas.

What is the percentage of carbon present in pig iron?

Carbon content is generally 3.5-4.5%.

Instrumentation engineering interview questions and answers

What are different types of orifice plates? State their uses.

Different orifice plates are: 1. Concentric 2. Segmental 3. Eccentric- Concentric: These plates are used for ideal liquid as well as gases and steam service. Concentric holes are present in these plates, thats why it is known as concentric orifice.- Segmental: This plate has hole in the form of segment of the circle. This plate is used for colloidal and sherry flow measurement.- Eccentric: This plate has the eccentric holes. This plate is used in viscous and sherry flow measurement.

How do you identify an orifice in the pipeline?

An orifice tab is welded on the orifice plate which extends out of the line giving an indication of the orifice plate.

Why is the orifice tab provided?

Following reasons justify for providing orifice tab:1. Indication of orifice plate in a line2. The orifice diameter is marked on it.3. The material of the orifice plate.4. The tag number of the orifice plate.5. To mark the inlet of an orifice.

Explain Bernoullis theorem. State its application.

Bernoullis theorem states that the total energy of a liquid flowing from one point to another remains constant. It is applicable for non-compressible liquids. For different types of liquid flow Bernoullis equation changes. There is direct proportion between speed of fluid and its dynamic pressure and its kinetic energy. It can be used in various real life situations like measuring pressure on aircraft wing and calibrating the airspeed indicator. It can also be used to low pressure in the venturi tubes present in carburetor.

How can a D.P. transmitter be calibrated?

D.P. transmitter can be calibrated using following steps:1. Adjust zero of Xmtrs.2. Perform static pressure test: Give equal pressure on both sides of transmitter. Zero should not shift either side. If the zero shifts then carry out static alignment.3. Perform vacuum test: Apply equal vacuum to both the sides. Zero should not shift.4. Calibration procedure: Give 20 psi air supply to the transmitter and vent L.P. side to atmosphere. Connect output of the instrument to the standard test gauge. Adjust zero. Apply required pressure to the high pressure side and adjust the span. Adjust zero gain if necessary.

How is flow measured in square root?

Flow varies directly as the square root of pressure. Thus, F=K of square root of applied pressure. Since this flow varies as the square root of differential pressure. The pressure pen does not directly indicate flow. Thus flow can be determined by taking the square root of the pen. Assume the pen reads 50% of the chart. So, flow can be calculated using the pen measure in the chart.

Name different parts of a pressure gauge. Explain the use of hair spring in the pressure gauge.

Pressure gauge includes following components:a. C type bourdon tube.b. Connecting linkc. Sector geard. Pinion Geare. Hair springf. Pointerg. Dial

Use of hair spring: Hair spring is responsible for controlling torque. It is also used to eliminate any play into linkages.

How D.P. transmitter can be applied to close tank?

In closed tank, bottom of the tank is connected to the high pressure side of the transmitter. Top of tank is connected to the lower pressure side of the transmitter. In this way vessel pressure can be measured.

How D.P. transmitter can be applied to open tank?

In open tank the lower pressure side is vented to the atmosphere. All pressure is applied to the high pressure side. This vessel pressure is measured through high pressure side.

Explain the working of an electronic level troll.

The variation in level of buoyancy resulting from a change in liquid level varies the net weight of the displacer increasing or decreasing the load on the torque arm. This change is directly proportional to change in level and specific gravity of the liquid. The resulting torque tube movement varies the angular motion of the rotor in RVDT providing a rotor change proportional to the rotor displacement, which is converted and amplified to a D.C. current.

Explain the working of an enraf level gauge.

Enraf level gauge is based on the ser powered null balance technique. A displacer serves as continuous level sensing element. A two phase ser motor controlled by a capacitive balance system winds unwinds the the measuring wire until the tension in the weight springs is in balance with the weight of the displaced part immersed in the liquid. The sensing system in balance measures the two capacitance formed by the moving central sensing rod provided by the two capacitor plates and the si plates.

What is the constant voltage unit?

The constant voltage circuit consists of a rectifier, CR and a filter capacitor. It is followed by two stages of zener regulation. Abridge configuration is provided to lamp line voltage zener regulation. Regulation 1 and regulation 2 provides relatively provide constant current. Resistors form a bridge that may remoment line voltage effects.

Explain the burnout feature.

Burnout provides the warnsug feature of driving indicator at the end of scale, if the input circuit is open. A burnout resistor is provided which develops a voltage drop between the measuring circuit and amplifier. The polarity of the signal determines the direction of the servo drive upon an open circuit in the input.Upscale burnout: R value 10 MDownscale burnout: R value 22 M

Why thermo wells are used? What materials are used in thermo wells?

In numerous applications, it is neither desirable nor practical to expose a temperature sensor directly to a material. Wells are therefore used to protect against damage corresion, arosion, aborsion and high pressure processes. A thermo well is also useful in protecting a sensor from physical damage during handling and normal operations.Materials used in thermo wells: Stainless steel, Inconel, Monel, Alloy Steel, Hastelloy

How is automatic reference junction compensation carried out in temperature recorders?

In automatic reference junction compensation, variable nickel resistor is used. As the temperature changes, so does its resistance. This reference junction compensator is located, so that it will be at the temperature of the reference junction. The reference junction is at the poset where the dissimilar wire of the thermocouple is rejoined. This joint is invariably at the terminal strip of the instrument.

What are de-saturators?

When, in some processes, e.g. batch processes, long transient responses are expected during which a sustained deviation is present the controller integral action continuously drives the output to a minimum or maximum value. This phenomenon is called integral saturation of the control unit. When this condition is met, then this unit is de-saturated.

Explain the working of Rota meter.

Variable area meters are special form of head meters. Where in the area of flow restrictor is varied. So as to hold the differential pressure constant. The rota meter consists of a vertical tapered tube through which the metered fluid flows in upward direction. As the flow varies the float rises or falls to vary the area of the passages that the differential across it balances the gravitational force on the float. The differential pressure is maintained constant. The position of the float is the measure of the rate of flow.

What is the working principle of the magnetic meter?

An electric potential is developed when a conductor is moved across the magnetic field. In most electrical machinery the conductor is a wire. The principle is equally applicable to a moving, electrically conductive liquid. The primary device of commercial magnetic meters consist of straight cylindrical electrically insulated tube with a pair of electrodes nearly flush with the tube walls and located at opposite end of a tube diameter. This device is limited to electrically conducting liquids. The magnetic meter is particularly suited to measurement of slurries and dirty fluids.

Explain the mechanism behind the turbine meter.

Turbine meters consist of straight flow tube within which a turbine or fan is free to rotate about it s axis which is fixed along g the centre line of the tube. Mostly, a magnetic pick up system senses the rotation of the rotor through the tube walls. The turbine meter is a flow rate device, since the rotor speed is directly proportional to the flow rate. The output is usually in the form of electric pulses from the magnetic pick up with a frequency proportional to the flow rate.

How would you choose differential range?

The most common range for differential range for liquid measurement is 0-100. This range is high enough to minimize the errors caused by unequal heads in the seal chambers. It is also dependent on the differences in the temperature of the load lines. The 100 range permits an increased in capacity up to 400. While decrease down up to 20 by merely changing the range tubes or range adjustments.

What is the use of single seated valve?

The single seated valve is used on smaller sizes where an absolute shut off is required. The use of single seated valve is limited by pressure drop across the valve in the closed or almost closed position.

What is the use of double seated valve?

In double seated valves the upward and downward forces on the plug due to reduction of fluid pressure are nearly equalized. It is generally used on bigger size valves and high pressure systems. Actuator forces required are less.

What is the use of valve positioner?

Valve positioner can be used for following reasons:a. Quick actionb. Valve hysterisisc. Viscous liquidsd. Split range.e. Line pressure changes on valvef. Bench set not standardg. Reverse valve operations

What are primary elements of measuring pressure? Which type of pressure can be measured by these elements?

Primary elements of measuring pressure are:a. Bourdon Tubeb. Diaphragmc. Capsuled. Bellowse. Pressure springsThese elements are known as elastic deformation pressure elements.

Name different types of bourdon tubes.

Types of bourdon tubes:1. C type2. Spiral3. Helix

What are different types of control valves?

The commonly used control valves can be defined as follows:a. Depending on Action:Depending on action there are two types of control valves 1. Air to close 2. Air to closeb. Depending on body:Depending on body there are 4 types of control valves1. Globe valves single or double seated2. Angle valves3. Butterfly valves4. Three way valves

What is furnace draft control?

Balanced draft boilers are generally used negative furnace pressure. When both forced draft and induced draft are used together, at some point in the system the pressure will be same as that of atmosphere. Therefore the furnace pressure must be negative to prevent hot gas leakage. Excessive vacuum in the furnace however produces heat losses through air infiltration. The most desirable condition is that the one have a very slight negative pressure of the top of furnace.

What is intrinsically safe system?

Intrinsic safety is a technique for designing electrical equipment for safe use in locations made hazardous by the presence of flammable gas or vapours in the air. Intrinsically safe circuit is one in which any spark or thermal effect produce either normally or under specified fault conditions is incapable of causing ignition of a specified gas or vapour in air mixture at the most ignited concentration.

What is zener diode? What is voltage regulator?

The breakdown region of a p-n diode can be made very sharp and almost vertical diodes with almost vertical breakdown region are known a s zener diodes. A zener diode operating in the breakdown region is equivalent to a battery. Because of this current through zener diode can change but the voltage remains constant. It is this constant voltage that has made the zener diode an important device in voltage regulation.Voltage regulator: The output remains constant despite changes in the input voltage due to zener effect.

What is force balance principle? State some of its advantages.

Force balance principle: A controller which generates an output signal by opposing torque. The input force is applied on the input bellows which moves the beam. This crackles nozzle back pressure. The nozzle back pressure is sensed by the balancing bellows which brings the beam to balance. The baffle movement is very less about 0.002 for full scale output.

Advantages:a. Moving parts are fewer.b. Baffle movement is negligiblec. Frictional losses are less

What is motion balance principle?

A controller which generates an output signal by motion of its parts. The increase in the baffle is to move towards the nozzle. The nozzle back pressure will increase. This increase in the back pressure acting on the balancing bellows, will expand the bellows. The nozzle is moved upward due to this. The nozzle will move until motion almost equals the input baffle motion.

How will you test a transistor with a multimeter?

1. Emitter +ve of meter and base -ve output =Low resistance2. Emitter -ve of meter and base +ve output =High resistance3. Collector +ve and base -ve output =Low4. Collector -ve and base +ve output =Low

Emitter: Collector = High resistancePNP: Opposite Results

Explain ratio control system.

A ratio control system is characterized by the fact that variations in the secondary variable dont reflect back on the primary variable. A ratio control system is the system where secondary flow is hold in some proportion to a primary uncontrollable flow.

If we assume that the output of a primary transmitter is A. and the output of the secondary transmitter is B, and that the multiplication factor of the ratio relay is K, then for equilibrium conditions which means set valve is equal to measured valve, we find the following relation:

KA-B=0 or B/A = K where K is the ratio setting off the relay.

What is solenoid valve? Where it is used?

A solenoid is electrically operated valve. It consists of solenoid coil in which magnetic plunger moves. This plunger is connected to the plug and tends to open or close the valve. There are two types of solenoid valves:

1. Normally Open2. Normally closed

Use: It is used for safety purpose in different electric work

Review a list of frequently asked Electrical interview questions for electrical graduates. Frequently asked interview questions with answers under the subjects like electrical machines,Transmission and distribution,Power electronics and some general basic questions.

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, 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 futher processing to get back the desired output.

5. Why Negative Feedback is preffered 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 stations's protection. A generating stations 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 advantage of free wheeling 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 act 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 its 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 Sulpher hexa Flouride gas.. if this gas is used as arc quenching medium in a Circuitbreaker 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 should the lighting arrestor be placed in distribution lines?Near distribution transformers and out going feeders of 11kv and incomming 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 observe reactive power for magnetizing.This type of generators are 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 theoutput 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 starter?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, IGBT2. Low cost3. 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.

Electrical Interview questions Part 1Q:What is electric traction?A:Electric traction means using the electric power for traction system (i.e. for railways,trams, trolleys etc). Electric traction means use of the electricity for all the above machines. Now a days, magnetic traction is also used for bullet trains.and basically dc motors are used for electric traction systems.Q:How can you start-up the 40w tube lite with 230v AC/DC without using any choke/Coil?A:It's possible by means of Electronic chokes,otherwise it's not possible to ionize the particles in tube light with normal voltage.Q:What is "pu" in electrical engineering?A:Pu stands for per unit and this will be used insingle line diagram of power distributionand it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis.Those values are called as pu values. (p.u=actual value/base value).Q:Operation carried out in Thermal power stations?A: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.Q:Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?A:Link is provided at a Neutral common point in the circuit from which various connection are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e load) only.So if any malfunction happen the fuse connected in the particular control circuit alone will blow off.Q: What is the difference between electronic regulator and ordinaryelectrical rheostatregulator for fans?A:The difference between the electronic and ordinaryelectricalregulator is that in electronic regulator power losses are less because as we decrease the speed the electronic regulator gives the power needed for that particular speed but in case of ordinary rheostat type regulator, the power wastage is same for every speed and no power is saved.In electronic regulator, triac is employed for speed control by varying the firing angle speed and it is controlled but in rheostatic ,control resistance is decreased by steps to achieve speed control.Q:How tube light circuit is connected and how it works?A:A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided ,the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000volts . This volt will capable of to break the electrons inside the tube to make electron flow. once the current passes through the tube the starter circuit will be out of part. now there is no change of supply causes choke voltage normalized and act as minimize the current.Q:whats is MARX CIRCUIT?A: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.Q:What is encoder, how it function?A:An encoder is a device used to change a signal (such as a bitstream) or data into a code. The code may serve any of a number of purposes such as compressing information for transmission or storage, encrypting or adding redundancies to the input code, or translating from one code to another. This is usually done by means of a programmed algorithm,especially if any part is digital, while most analog encoding is done with analog circuitry.Q:What are the advantages of speed control using thyristor?A:Advantages :1. Fast Switching Characterstics than Mosfet, BJT, IGBT 2. Low cost 3. Higher Accurate.Q:Why Human body feel Electric shock ?? n in an Electric train during running , We did nt feel any Shock ? why?A:Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.Q:what is the principle of motor?A:Whenever a current carrying conductor is placed in an magnetic field it produce turning