Electrical Interview questions Part 18

Query:Difference between megger test equipment and contact resistance meter test instruments?
Resolution: Megger test equipment used to measure cable electric resistance, conductor continuity, phase identification where as contact resistance meter test instruments used to measure low resistance like relays ,contactors.

Query:When we connect the large capacitor bank in series ?
Resolution :we connect large capacitor bank in series to improve the voltage power supply at the load end in balanced transmission line when there is considerable voltage drop along the balanced transmission line due to high impedance of the line.So in order to bring the voltage at the load terminals within its limits (i.e (+ or - %6 )of the rated high terminal voltage )the large capacitor bank is used in series.

Query:What is electrical diversity factor in electrical installations?
Resolution :Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Electrical diversity factor is usually more than one.

Query:Why field rheostat is kept in minimum position while armature rheostat at maximum position?
Resolution :In motors at the time of starting the armature resistance is introduced to reduce the high starting current and the field resistance is kept minimum to have high starting
torque.

Query:Why computer humming sound occurred in HT transmission line?
Resolution :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.

Query:What is rated speed?
Resolution :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.

Query:What is different between resistance grounding system and resistance earthing system?
Resolution :Resistance grounding system means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas resistance earthing system is done in an electric equipment in order to protect he equipment in occurrence of fault in the system.

Query:Why should be the frequency 50 Hz 60Hz only why not others like 45, 95 56 or anything , why should we maintain the frequency constant if so why it is only 50 Hz 60Hz?
Resolution :We can have the frequency at any frequency you like, but than you must also make your own motors,high voltage transformers or any other equipment you want to use.We maintain the frequency at 50hz or 60hz because the world maintains a standard at 50 /60hz and the equipments are are made to operate at these frequency.

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Electrical Interview questions Part 17

Query:Use of switch mode power converter in real-time basis?
Resolution:Switch mode power converter can be used in the following 5 different ways
1) step down an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Buck Converter or Step-Down SMPS,
2)step up an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Boost Converter or Step-Up SMPS,
3)step up or step down an unregulated dc input voltage to produce a regulated dc output voltage,
4)invert the input dc voltage using usually a circuit such as the Cuk converter, and
5)produce multiple dc outputs using a circuit such as the fly-back converter.

Query:Which type of oil is used as a transformer oil?

Resolution:Transformer oil, or insulating oil, is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.

Well into the 1970s, polychlorinated biphenyls (PCB)s were often used as a dielectric fluid since they are not flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan. Starting in the early 1970s, concerns about the toxicity of PCBs have led to their banning in many countries.

Today, non-toxic, stable silicon-based or fluoridated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Combustion-resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and higher flash points than mineral oil.

Query: If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV?
Resolution:As we know the Voltage & current relation for transformer-V1/V2 = I2/I1
We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps.
By putting these value on Relation-
540/11000= I2/2334
So,I2 = 114.5 Amps

Query:what are the points to be consider for MCB(miniature circuit breaker selection?
Resolution:I(L)*1.25=I(MAX) maximum current. Mcb specification are done on maximum current flow in circuit.

Query:what is the full form of KVAR? 

Resolution:We know there are three types of power in Electrical as Active, apparent & reactive. So KVAR is stand for ``Kilo Volt Amps with Reactive component.

Query:What is excitation?
Resolution:Excitation is applying an external voltage to DC shunt coil
in DC motors.

Query:In three pin plug 6 Amp. 220v AC rating. why earth pin diameter is higher than other two pin? what its purpose ? 

Resolution:Because Current flow in the conductor is inversely proportional to the conductor diameter. So if any short circuits occur in the system first high currents bypassed in the Earthling terminal.( R=Pl/a area of the conductor increases resistance
value decreases) 

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