Electrical Job Interview Questions and Answers - Part 08

Question No. 01: 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 ckt 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 ckts. 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\5 A and it has to measure secondary current of 8000 A, then ANS = (8000 ×5)/5000 = 8 A and this result will be given to ammeter   and after measuring 8 A we can calculate the primary current. Same is the operation of PT but measuring voltage.

Question No. 02: What is the difference between surge arrestor and lightning arrestor?
Answer: LA is installed outside and the effect of lightning is grounded, where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Question No. 03: What is vacuum circuit breaker? Define with cause and where be use this Device?
Answer: A breaker is normally used to break a circuit. While breaking the circuit, the contact terminals will be separated. At the time of separation an air gap is formed in between the terminals. Due to existing current flow the air in the gap is ionised and results in the arc. Various mediums are used to quench this arc in respective CB's. But in VCB the medium is vacuum gas. Since the air in the CB is having vacuum pressure, the arc formation is interrupted. VCB's can be used upto 11 kV.

Question No. 04: Why temperature rise is conducted in bus bars and isolators? 
Answer: Bus bars and isolators are rated for continuous power flow that means they carry heavy currents which rises their temperature. So it is necessary to test these devices for temperature rise.

Question No. 05: Explain the operation of variable frequency transformer?
Answer: A variable frequency transformer is used to transmit electricity between two asynchronous alternating current domains. A variable frequency transformer is a doubly-fed electric machine resembling a vertical shaft hydroelectric generator with a three-phase wound rotor, connected by slip rings to one external ac power circuit. A direct-current torque motor is mounted on the same shaft. Changing the direction of torque applied to the shaft changes the direction of power flow; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator. The variable frequency transformer behaves as a continuously adjustable phase-shifting transformer. It allows control of the power flow between two networks.

Question No. 06: Why should be the frequency 50 Hz 60 Hz 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 60 Hz? 
Answer: We can have the frequency at any frequency you like, but then you must also make your own motors, high voltage transformers or any other equipment you want to use. We maintain the frequency at 50 Hz or 60 Hz because the world maintains a standard at 50 /60 Hz and the equipments are made to operate at these frequency.

Question No. 07: What is slip in an induction motor?
Answer: Slip can be defined as the distinction between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘S’.

Question No. 08: Name power loses in rotating electrical machines.
Answer:
Power loses in rotating electrical machines are Copper losses, core losses, mechanical losses and stray losses.

Question No. 09: Explain the terms real power, apparent power and reactive power for ac circuits and also the units used.
Answer:
  • Real Power: It is the product of voltage, current and power factor i.e. P = V I cos j and basic unit of real power is watt. I.e. Expressed as W or kW.
  • Apparent power: It is the product of voltage and current. Apparent power = V I and basic unit of apparent power is volt- ampere. Expressed as VA or KVA.
  • Reactive Power: It is the product of voltage, current and sine of angle between the voltage and current i.e. Reactive power = voltage × current × sin j or Reactive power = V I sin j and has no other unit but expressed in VAR or KVAR.

Question No. 10: What is a differential amplifier? Also, explain CMRR.
Answer:
  • Differential Amplifier: The amplifier, which is used to amplify the voltage difference between two input-lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise injected into the amplifier, because any noise appearing simultaneously on both the input-terminals as the amplifying circuitry rejects it being a common mode signal.
  • CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a differential amplifier is perfect, CMRR would be infinite because in that case common mode voltage gain would be zero.

Question No. 11: Difference between a four point starter and three point starters?
Answer: The shunt connection in four point starter is provided separately from the line where as in three point states it is connected with line which is the drawback in three point starters

Question No. 12: How do you select a cable size (Cu & Al) for a particular load?
Answer: At first calculate the electrical current of the load, after that derate the electrical current considering derating factor(depending on site condition and laying of cable) after choose the cable size from cable catalogue considering derating electrical current. After that measure the length of cable required from supply point of load to load point. Calculate the voltage drop which will max 3% (resistance and reactance of cable found from cable catalogue of selecting cable) if voltage drop>3%
then choose next higher size of cable.

Question No. 13: Why computer humming sound occurred in HT transmission line?
Answer: 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.

Question No. 14: Capacitor is load free component but why ampere meter shows current when capacitor bank breaker closes?
Answer:
As we know that Electrical is having two type of load, Active and Reactive. Capacitor is a reactive load which is not considering as a load, & its factor is Isin@ .Meter is design based on Current RMS value because of it meter is showing the current RMS value.

Question No. 15: Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are required in a dc machine.
Answer: Commutation: It is phenomenon when an armature coil moves under the influence of one pole- pair; it carries constant current in one direction. As the coil moves into the influence of the next pole- pair, the current in it must reverse. This reversal of current in a coil is called commutation. Several coils undergo commutation simultaneously. The reversal of current is opposed by the static coil emf and therefore must be aided in some fashion for smooth current reversal, which otherwise would result in sparking at the brushes. The aiding emf is dynamically induced into the coils undergoing commutation by means of Compoles or Interpoles, which are series excited by the armature current. These are located in the interpolar region of the main poles and therefore influence the armature coils only when these undergo commutation.

Question No. 16: 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 is not consuming. Hence the load current in the transformer is because of core loss so it is minimum.

Question No. 17: 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 aground 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.

Question No. 18: 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 ckt. Arc is extinguished by the Arc quenching media like air, oil, vacuum etc.

Question No. 19: What is the function of anti-pumping in circuit breaker?
Answer: when breaker is close at one time by close push button he anti pumping contractor prevent re close the breaker by close push button after if it already close.

Question No. 20: What is Buchholz relay and the significance of it into the transformer?
Answer:  Buchholz 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 Buchholz relay at once gives a horn for sometime, if the transformer is isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own tripping mechanism.

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