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Q. Why the transformer & generator are rated in KVA instead of KW.
Ans. Here are the following reason's.
1-The power factor of generator & transformer depend on the power factor of load , as we don't know the power factor of load so it is rated in KVA.
2- The copper loss depend on the current & iron loss depend on the voltage so total loss depend on the volt-amp not on the phase angle of voltage and current i.e is independent of load power factor , that is why rating of transformer is in kva instead of kw.
Q. What is the difference between generator and alternator?
Ans. Here is the following difference are noted as.
1> Both works on the same principle of electromagnetic induction , generator generate D.C through commutator while the alternator generates A.C through slip rings.
2> In generator the field is fixed and the armature is rotating while in alternator armature is fixed and field is rotating.
3> Generator is used where small loads while alternator is used for large loads or large power plants.
4> Operation and flexibility of alternator is more than the generator.
Q. What are the types of winding used in electrical machine?
Ans. There are two types of winding is generally used as wave winding and lap winding . In wave winding the no of parallel path is two, where as in lap winding the no of parallel path is equal to no of poles.
Q. What is the power factor ?
Ans. It is the cosine angle between voltage and current in an ac circuit.There are generally three types of power factor as,
1.Active power factor = v*i*cosɸ.
Q. Causes of low power factor? Ans.Here are the reason of low power factor ,
1.Most of A.C motor are induction type which have low lagging power factor.
2.Arc lamps,industrial heating surface, electrical discharge lamps operate at low lagging power factor.
Q .What is the classification of transformer?
Ans. The transformer is classified as,
1. According to voltage- High voltage transformer ( H.T) and low voltage transformer(L.T).
2. According to winding - Core type winding and shell type winding.
3. According to cooling – Natural oil cooling, natural oil forced air cooling, forced air forced oil cooling.
Q. What is corona effect?
Ans. It is the phenomena of hissing sound, production of ozone gas, radio interference and power loss in overhead transmission line. At higher voltage corona loss will be found more.
Theory of corona :- Generally air contains some ionized particles due to ultraviolet radiation & cosmic rays . In normal condition air contains free electron and + ve ions when ever the voltage increase to a certain value in between the conductor , The potential gradient is setup in the air . Under the presence of potential gradient the ionized air occurred more velocity and air starts conducting in between the conductor. The air starts conducting when the potential gradient of the surface increase to 30 v/cm . More voltage more free electron available in between the conductor hence more corona effect. Due to this result either corona will formed of spark occurred in between the conductor.
Factors affecting corona :-
1.Atmosphere- Corona depends on the physical condition of atmosphere . In a rainy or stormy weather corona will be more at less voltage.2.
2.Space between the conductor- Spacing between the conductor is also depend on the corona more space in between the conductor less electrostatic effect and less corona.
3. Line voltage- Line voltage is also affect on corona, greater the line voltage more corona will be formed.
4.Surface of the conductor- Irregular and rough surface have more corona than the smooth surface.
Advantages of corona:-
1.Due to formation of corona surrounding conductors starts conducting, hence the diameter of the conductor is increased.
2.The effect of transient produced by surges is also reduced.
Disadvantages of corona:-
1.It increase line loss and decrease transmission efficiency.
2.Non sinusoidal voltage drop across the line as the current drawn by the corona is non sinusoidal .
3.Corrosion of conductor may cause due to production of ozone gas.
Methods of reducing corona effect:-
1.By increasing conductor spacing
2.By increasing conductor size.
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Ans. The Generator is an electrical machine which convert mechanical energy to electrical energy . There are two types of generator AC generator & DC generator.