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Electrical Interview questions Part 7

Q:What happens if i connect a capacitor to a generator load?
A:Connecting a capacitor across a generator always improves powerfactor,but it will help depends up on the engine capacity of the alternator,other wise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don't connect a capacitor across an alternator while it is picking up or without any other load.

Q:Why the capacitors works on ac only?
A:Generally capacitor gives infinite resistance to dc components(i.e., block the dc components). it allows the ac components to pass through.

Q:Explain the working principal of the circuit breaker?
A:Circuit Breaker is one which makes or breaks the circuit.It has two contacts namely fixed contact & moving contact.under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions(when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open circuitArc is extinguished by the Arc Quenching media like air, oil, vaccum etc.

Q:How many types of colling system it transformers?
A: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)

Q:What is the function of anti-pumping in circuit breaker?
A: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.

Q:what is stepper motor.what is its uses?
A: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.
 
Q:How to calculate capacitor bank value to maintain unity power factor with some suitable example?
A:KVAR= KW(TAN(COS(-1)#e)- TAN(COS(-1)#d) )
#e= EXISTING P.F.
#d= DESIRED P.F.

Q:Tell me in detail about c.t. and p.t. ?(Company:reliance)
A:The term C.T means current transformer,and the term P.T means potential transformer.In circuit where measurements of high voltage and high current is involved they are used there.Particularly when a measuring device like voltmeter or ammeter is not able to measure such high value of quantity because of large value of torque due to such high value it can damage the measuring device.so, CT and PT are introduced in the circuits. They work on the same principle of transformer, which is based on linkage of electromagneticflux produced by primary with secondary.They work on the ratio to they are designed.E.g if CTis of ratio 5000\5A and it has to measure secondary current of
8000A.then ANS=8000*5\5000=8Aand this result will be given to ammeter .and after measuring 8A we can calculate the primary current.same is
the operation of PT but measuring voltage.

Q: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?
A: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.

Q:what is power factor? whether it should be high or low? why?
A:Power factor should be high in order to get smooth operation of the system.Low power factor means losses will be more.it is the ratio of true power to apperent power. it has to be ideally 1. if it is too low then cable over heating & equipment overloading will occur. if it is greater than 1 then load will act as capacitor and starts feeding the source and will cause tripping.(if pf is poor ex: 0.17 to meet actual power load has to draw more current(V constant),result in more lossesif pf is good ex: 0.95 to meet actual power load has to draw less current(V constant),result in less losses).
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