Sunday, January 16, 2011

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)