Interview and short exam Question for Electrical engineer-PART-1

1. What happens if I connect a capacitor to a generator load?

Answer : Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, otherwise 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.

2. Why the capacitors works on ac only?

Answer: Generally capacitor gives infinite resistance to dc components (i.e., block the DC components). It allows the ac components to pass through.

3. Explain the working principal of the circuit breaker?

Answer: 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 circuit, Arc is extinguished by the Arc Quenching media like air, oil, vacuum etc.

4. How many types of colling system it transformers?

Answer: 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)

5. What is the function of anti-pumping in circuit breaker?

Answer :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.

6. What is stepper motor? What is its uses?

Answer: 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.

7. Tell me in detail about C.T and P.T?

Answer: 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 electromagnetic flux produced by primary with secondary.

They work on the ratio to they are designed. E.g. if CT is 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.

8. 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?

Answer: 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 it’s not consuming. Hence the load current in the transformer is because of core loss so it is minimum.

9. What is power factor? Whether it should be high or low? Why?

Answer: 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 apparent 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 losses if pf is good ex: 0.95 to meet actual power load has to draw less current (V constant), result in less losses)

10. What is the difference between Isolator and Circuit Breaker?

Answer: Isolator is an off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. It is manually operated and does not contain any solenoid unlike circuit breaker. It should not be operated while it is having load. First the load on it must be made zero and then it can safely operate. Its specification only rated current is given. But circuit breaker is on load automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current .

11. What is Boucholz relay and the significance of it in to the transformer?

Answer: Boucholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay. whenever any internal fault occurs in a transformer, the Boucholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own tripping mechanism.

12. What is SF6 Circuit Breaker?

Answer: SF6 is Sulphur hexa Fluoride gas. If this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.

13. What is ferrantic effect?

Answer: Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.

14. What is meant by insulation voltage in cables? Explain it?

Answer: 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.

15. Why we do 2 types of earthing on transformer (i.e. 🙂 body earthing & neutral earthing, what is function. i am going to install a 5oo kva transformer & 380 kva DG set what should the earthing value?

Answer: The two types of earthing are Familiar as Equipment earthing and system earthing. In Equipment earthing: body (non-conducting part) of the equipment should be earthed to safeguard the human beings. System Earthing: In this neutral of the supply source (Transformer or Generator) should be grounded. With this, in case of unbalanced loading neutral will not be shifted.so that unbalanced voltages will not arise. We can protect the equipment also. With size of the equipment (transformer or alternator) and selection of relying system earthing will be further classified into directly earthed, Impedance earthing, resistive (NGRs) earthing.

16. What is the difference between MCB & MCCB, Where it can be used?

Answer: 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 shortcircuit condition. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A

17. What is use of lockout relay in ht voltage?

Answer: A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

18. What is the difference between earth resistance and earth electrode resistance?

Answer: Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away. The resistance of the electrode has the following components 🙁 A) The resistance of the metal and that of the connection to it. (B) The contact resistance of the surrounding earth to the electrode.

19. Which power plant has high load factor?

Answer:All base load power plants have a high load factor. If we use high efficiency power plants to supply the base load, we can reduce the cost of generation. Hydro power plants have a higher efficiency than thermal & nuclear power plants.

20. Why an ac solenoid valve attract the plunger even though we interchanges the terminal? Will the poles changes?

Answer: Yes because the poles changes for every half-cycle of ac voltage so the polarity of AC voltage is continuously changing for every half cycle. So, interchanging of terminals in ac system does not show any difference. That’s why the ac solenoid attract the plunger even though it’s terminals are interchanged.

21. Define IDMT relay?

Answer: 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.

22. What are the transformer losses?

Answer: 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 is loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer.

23. What is meant by regenerative braking?

Answer: When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply. So that back emf voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as “Regenerative Breaking”.

24. Why is the starting current high in a DC motor?

Answer: In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage, Eb = Back emf in Motor, Ia = Armature current, Ra = Armature resistance).At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra ,where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.

25. What are the advantages of star-delta starter with induction motor?

Answer :(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.

26. Why Delta Star Transformers are used for Lighting Loads?

Answer: 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.

27. Why in a three pin plug the earth pin is thicker and longer than the other pins?

Answer: It depends upon R=rho l/a where area(a) is inversely proportional to resistance(R), so if (a) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the First to make the connection and last to disconnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

28. Why series motor cannot be started on no-load?

Answer: Series motor cannot be started without load because of high starting torque. Series motor are used in Trains, Crane etc.

29. Why ELCB can’t work if N input of ELCB do not connect to ground?

Answer: ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.

30. How electrical power is generated by an A.C Generator?

Answer: For the generation of elect power we need a prime mover which supplies mechanical power input to the alternator, can be steam turbines, or hydro turbines. When poles of the rotor moves under the armature conductors which are placed on the stator ,field flux cut the armature conductor ,therefore voltage is generated and is of sinusoidal in nature…due to polarity change of rotor poles(i,e) N-S-N-S.