Post by pandikumar on Nov 22, 2013 17:29:05 GMT 5.5
ULTRA College of Engineering & Technology for Women Madurai
Electrical Interview questions
Electrical interview question: What is electric traction?
Answer: Traction means using the electric power for traction system i.e. for railways, trams, trolleys etc. electric traction means use of the electricity for all these. Now a day, magnetic traction is also used for bullet trains. Basically dc motors are used for electric traction systems.
Electrical interview question: How can you start-up the 40w tube lite with 230v AC/DC without using any choke / Coil?
Answer: It's possible by means of Electronic choke. Otherwise it's not possible to ionize the particles in tube light, with normal voltage.
Electrical interview question: What is "pu" in electrical engineering?
Answer: Pu stands for per unit and this will be used in power system single line diagram there it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as pu values. (p.u=actual value/base value).
Electrical interview question: Operation carried out in Thermal power station?
Answer: The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit the turbine; the turbine which is coupled with the generator generates the electricity
Electrical interview question: Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?
Answer: Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e load) only. So if any malfunction happens the fuse connected in the particular control circuit alone will blow off.
Electrical interview question: what is the difference between electronic regulator and ordinary rheostat regulator for fans?
Answer: The difference between the electronic and ordinary regulator is that in electronic reg. power losses are less because as we decrease the speed the electronic reg. give the power needed for that particular speed but in case of ordinary rheostat type reg. the power wastage is same for every speed and no power is saved. In electronic regulator triac is employed for speed control.by varying the firing angle speed is controlled but in rheostatic control resistance is decreased by steps to achieve speed control.
Electrical interview question: How tube light circuit is connected and how it works?
Answer: A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000 volts. This volt will capable of to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part. Now there is no change of supply causes choke voltage normalized and act as minimize the current.
Electrical interview question: what is MARX CIRCUIT?
Answer: It is used with generators for charging a number of capacitor in parallel and discharging them in series. It is used when voltage required for testing is higher than the available.
Electrical interview question: What is encoder, how it function?
Answer: An encoder is a device used to change a signal (such as a bit stream) or data into a code. The code may serve any of a number of purposes such as compressing information for transmission or storage, encrypting or adding redundancies to the input code, or translating from one code to another. This is usually done by means of a programmed algorithm, especially if any part is digital, while most analog encoding is done with analog circuitry.
Electrical interview question: What are the advantages of speed control using thyristor?
Answer: Advantages :1. Fast Switching Characteristics than Mosfet, BJT, IGBT. 2. Low cost. 3. Higher Accurate.
Electrical interview question: Why Human body feels Electric shock? in an Electric train during running , We didn’t feel any Shock ? Why?
Answer: Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.
Electrical interview question: what is the principle of motor?
Answer: Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.
Electrical interview question: why, when birds sit on transmission lines or current wires doesn't get shock?
Answer: It’s true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds touch 2 lines than the circuit is closed and they get electrical shock. so if a human touch single one line (phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral)। and in the most of electric lines the neutral is grounded. So that means that human who touch the line closes the circuit between phase and neutral.
Electrical interview question: what is meant by armature reaction?
Answer: The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.
Electrical interview question: what happen if we give 220 volts dc supply to d bulb r tube light?
Answer: Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.
Electrical interview question: Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?
Answer: DC Series motor has high starting torque. We cannot start the Induction motor and Synchronous motors on load, but cannot start the DC series motor without load.
Electrical interview question: what is ACSR cable and where we use it?
Answer: ACSR means Aluminum conductor steel reinforced, this conductor is used in transmission & distribution.
Electrical interview question: What is vacuum circuit breaker? Define with cause and where be use it 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 ionized 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 11kv.
Electrical interview question: What will happen when power factor is leading in distribution of power?
Answer: If there is high power factor, i.e. if the power factor is close to one:
1. Losses in form of heat will be reduced,
2. Cable becomes less bulky and easy to carry, and very cheap to afford, &
3. It also reduces over heating of transformers.
Electrical interview question: what’s the one main difference between UPS & inverter? and electrical engineering & electronics engineering?
Answer: uninterrupted power supply is mainly use for short time. Means according to ups VA it gives backup. ups is also two types : on line and offline. Online ups having high volt and amp for long time backup with high dc voltage. But ups start with 12v dc with 7 amps. but inverter is start with 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup.
Electrical interview question: What is 2 phase motor?
Answer: A two phase motor is a motor with the starting winding and the running winding have a phase split. e.g; A.C servo motor, where the auxiliary winding and the control winding have a phase split of 90 degree
Electrical interview question: Advantages of vvvf drives over non vvvf drives for EOT cranes?
Answer: 1. smooth start and stop. 2. No jerking of load. 3. Exact positioning 4. Better protection for motor. 5. high/low speed selection. 6. Reliability of break shoe. 7. Programmable break control. 8. Easy circuitry 9.reduction in controls 10. Increases motor life
Electrical interview question: What is the significance of vector grouping in Power Transformers?
Answer: Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phase difference between the current and voltage. EG. DYN11 means Delta primary, Wye Secondary and the current is at 11 o clock referred to the voltage.
Electrical interview question: Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, pedestal fan, bracket fan etc) which are find in the houses?
Answer: Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.
Electrical interview question: Give two basic speed control scheme of DC shunt motor?
Answer: 1. By using flux control method: in this method a rheostat is connected across the field winding to control the field current.so by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled 2.armature control method: in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop(IaRa)can be varied, and since speed is directly proportional to Eb-IaRa the speed can be controlled.
Electrical interview question: what is the principle of motor?
Answer: Whenever a current carrying conductor is placed in a magnetic field it produce turning or twisting movement is called as torque.
Electrical interview question: what is meant by armature reaction?
Answer: The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.
Electrical interview question: Give two basic speed control scheme of DC shunt motor?
Answer: 1. By using flux control method: in this method a rheostat is connected across the field winding to control the field current.so by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled 2.armature control method: in this method a rheostat is connected across armature wdg.by varying the resistance the value of resistive drop(IaRa)can be varied, and since speed is directly proportional to Eb-IaRa the speed can be controlled.
Electrical interview question: what is the difference between synchronous generator & asynchronous generator?
Answer: In simple, synchronous generator supply's both active and reactive power but asynchronous generator (induction generator) supply's only active power and observe reactive power for magnetizing. This type of generators is used in windmills.
Electrical interview question: What is the Polarization index value? (pi value)and simple definition of polarization index ?
Answer: Its ratio between insulation resistance(IR) i.e meggar value for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5.
Electrical interview question: Why syn. generators r used for the production of electricity?
Answer: synchronous machines have capability to work on different power factor(or say Different imaginary power varying the field emf. Hence syn. generators r used for the production of electricity.
Electrical interview question:What is the difference between synchronous generator & asynchronous generator?
Answer: In simple, synchronous generator supply's both active and reactive power but asynchronous generator (induction generator) supply's only active power and observe reactive power for magnetizing. This type of generators is used in windmills.
Electrical interview question: 1 ton is equal to how many watts?
Answer: 1 ton = 12000 BTU/hr and to convert BTU/hr to horsepower,
12,000 * 0.0003929 = 4.715 hp therefore 1 ton = 4.715*.746 = 3.5 KW
Electrical interview question: why syn. generators r used for the production of electricity?
Answer: synchronous machines have capability to work on different power factor (or say different imaginary power varying the field emf. Hence syn. generators r used for the production of electricity.
Electrical interview question: Enlist types of dc generator?
Answer: D.C. Generators are classified into two types 1) separately excited d.c.generator 2) self-excited d.c. generator, which is further classified into;1) series 2)shunt and 3) compound(which is further classified into cumulative and differential).
Electrical interview question: What is Automatic Voltage regulator (AVR)?
Answer: AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.
Electrical interview question: What is an exciter and how does it work?
Answer: There are two types of exciters, static exciter and rotary exciter. Purpose of exciter is to supply the excitation dc voltage to the fixed poles of generator. Rotary exciter is an additional small generator mounted on the shaft of main generator. if it is dc generator, it will supply dc to the rotary poles through slip ring and brushes( conventional alternator). if it is an ac exciter, output of ac exciter is rectified by rotating diodes and supply dc to main fixed poles.ac exciter is the ac generator whose field winding are stationary and armature rotates. Initial voltage is built up by residual magnetism. It gives the starting torque to the generator.
Electrical interview question: Difference between a four point starter and three point starters?
Answer: The shunt connection in four point starter is provided separately form the line whereas in three point starter it is connected with line which is the drawback in three point starter
Electrical interview question: Why use the VCB at High Transmission System? Why can't use ACB?
Answer: Actually the thing is vacuum has high arc quenching property compare to air because in VCB, the die electric strengths equal to 8 times of air. That y always vacuum used as in HT breaker and air used as in LT.
Electrical interview question: What is the difference between surge arrestor and lightning arrestor?
Answer: LA is installed outside and the effect of lightning is grounded, whereas surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.
Electrical interview question: 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.
Electrical interview question: Why the capacitors work 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.
Electrical interview question: 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, vaccum etc.
Electrical interview question: How many types of cooling 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)
Electrical interview question: 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.
Electrical interview question: What is stepper motor? What are its uses?
Answer: Stepper motor is the electrical machine which acts 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.
Electrical interview question: How to calculate capacitor bank value to maintain unity power factor with some suitable example?
Answer: KVAR= KW(TAN(COS(-1)#e)- TAN(COS(-1)#d) )
#e = EXISTING P.F. #d= DESIRED P.F.
Electrical interview question: Tell me in detail about c.t. and p.t. ?(Company: reliance)
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.
Electrical interview question: 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.
Electrical interview question: 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).
Electrical interview question: 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.
Electrical interview question: 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.
Electrical interview question: What is SF6 Circuit Breaker?
Answer: SF6 is Sulpher hexa Flouride gas. If this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.
Electrical interview question: what is Ferranti effect?
Answer: Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.
The Ferranti effect is a rise in voltage occurring at the receiving end of a long transmission line, relative to the voltage at the sending end, which occurs when the line is energized but there is a very light load or the load is disconnected.
This effect is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. Therefore both capacitance and inductance are responsible for producing this phenomenon. [1] The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied. [2] The relative voltage rise is proportional to the square of the line length. [3] Due to high capacitance, the Ferranti effect is much more pronounced in underground cables, even in short lengths.
Electrical interview question: 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.
Electrical interview question: Why we do 2 types of earthing on transformer (ie:)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 safe guard 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.
Electrical interview question: 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 short circuit condition. under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A.
Electrical interview question: Where should the lighting arrestor be placed in distribution lines?
Answer: Near distribution transformers and outgoing feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations.
Electrical interview question: 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.
Electrical interview question: 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 loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer.
Electrical interview question: 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 (resitor) across the armature winding and the motor should have maintained continuous field supply. so that back e.m.f 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".
Electrical interview question: 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 = Aramture 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.
Electrical interview question: 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.
Electrical interview question: 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.
Electrical interview question: 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 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.
Electrical interview question: Why series motor cannot be started on no-load?
Answer: Series motor cannot be started without load because of high starting torque. Series motor is used in Trains, Crane etc.
Electrical interview question: 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 wil 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.
Electrical interview question: 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.
Electrical interview question: Why an ac solenoid valve attract the plunger even though we interchanges the terminal? Will the poles changes?
Answer: Yes because the poles change 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.
Electrical interview question: What is de-rating? Why it is necessary, it is same for all means for drives, motors, and cables.
Answer: The current currying of cables will change depending upon the site temperature (location of site), type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, and distance between cables. Considering this condition actual current currying capacity of cable reduce than current currying capacity (which given to cable Catalogue) this is called de-rating.
Electrical interview question: 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 this devices for temperature rise.
Electrical interview question: When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?
Answer: No. We can't sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in nature). So, the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing the system failure & risk of fire, So, for a better protection of the system. It should have both over voltage & over current relay.
Electrical interview question: If one lamp connects between two phases it will glow or not?
Answer: If the voltage between the two phase is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.
Electrical interview question: 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 de-rate the electrical current considering de-rating factor(depending on site condition and laying of cable) after choose the cable size from cable catalog considering de-rating 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 catalog of selecting cable) if voltage drop>3% then choose next higher size of cable.
Electrical interview question: What are HRC fuses and where it is used?
Answer: HRC stand for "high rupturing capacity" fuse and it is used in distribution system for electrical transformers.
Electrical interview question: 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. Hydel power plants have a higher efficiency than thermal & nuclear power plants.
Electrical interview question: Mention the methods for starting an induction motor?
Answer: The different methods of starting an induction motor DOL: direct online starter
Star delta starter
Auto transformer starter
Resistance starter
Series reactor starter
Electrical interview question: 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.
Electrical interview question: 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.
Electrical interview question: What is the power factor of an alternator at no load?
Answer: At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.
Electrical interview question: Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?
Answer:4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation .For example- a pressure instrument gives output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.
Electrical interview question: Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?
Answer: Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb (P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.
Electrical interview question: What is meant by knee point voltage?
Answer: Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).
Electrical interview question: What is reverse power relay?
Answer: Reverse Power flow relay are used in generating stations's protection. A generating stations is supposed to fed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.
Electrical interview question: What will happen if DC supply is given on the primary of a transformer?
Answer: Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance, only resistance will act in the electrical circuit. So high electrical current will flow through primary side of the transformer. So for this reason coil and insulation will burn out.
Electrical interview question: What is the difference between isolators and electrical circuit breakers? What is bus-bar?
Answer: Isolators are mainly for switching purpose under normal conditions but they cannot operate in fault conditions .Actually they used for isolating the CBs for maintenance. Whereas CB gets activated under fault conditions according to the fault detected. Bus bar is nothing but a junction where the power is getting distributed for independent loads.
Electrical interview question: What is the advantage of freewheeling diode in a Full Wave rectifier?
Answer: It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load
Electrical interview question: What are Motor Generator Sets and explain the different ways the motor generator set can be used?
Answer: Motor Generator Sets are a combination of an electrical generator and an engine mounted together to form a single piece of equipment. Motor generator set is also referred to as a gen set, or more commonly, a generator The motor generator set can used in the following different ways:
1.Alternating current (AC) to direct current (DC)
2.DC to AC
3.DC at one voltage to DC at another voltage
4.AC at one frequency to AC at another harmonically-related frequency
Electrical interview question: What is power quality meter?
Answer: Power Quality meters are common in many industrial environments. Small units are now available for home use as well. They give operators the ability to monitor the both perturbations on the power supply, as well as power used within a building, or by a single machine or appliance. In some situations, equipment function and operation is monitored and controlled from a remote location where communication is via modem or high-speed communication lines. So we can understand the importance of power measurement through power quality meters.
Electrical interview question: What is the different between digital phase converter and ordinary phase converter?
Answer: Digital phase converter are a recent development in phase converter technology that utilizes proprietary software in a powerful microprocessor to control solid state power switching components. This microprocessor, called a digital signal processor (DSP), monitors the phase conversion process, continually adjusting the input and output modules of the converter to maintain perfectly balanced three-phase power under all load conditions.
Electrical interview question: 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.
Electrical interview question: What is the main use of rotary phase converter?
Answer: Rotary phase converter will be converting single phase power into true balanced 3 phase power, so it is often called as single phase to three phase converter .Often the advantages of 3 phase motors, and other 3 phase equipment, make it worthwhile to convert single phase to 3 phase so that small and large consumers need not want to pay for the extra cost of a 3 phase service but may still wish to use 3 phase equipment.
Electrical interview questions: Which type of oil is used as transformer oil?
Answers: 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.
Electrical interview questions: 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?
Answers: 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
Electrical interview questions: what are the points to be consider for MCB (miniature circuit breaker selection?
Answers: I (L)*1.25=I(MAX) maximum current. MCB specifications are done on maximum current flow in circuit.
Electrical interview questions: what is the full form of KVAR?
Answers: We know there are three types of power in Electricals as Active, apparent & reactive. So KVAR is stand for ``Kilo Volt Amps with Reactive component.
Electrical interview questions: What is excitation?
Answer: Excitation is applying an external voltage to DC shunt coil in DC motors.
Electrical interview questions: In three pin plug 6 Amp. 220v AC rating. Why earth pin diameter is higher than other two pin? What its purpose?
Answer: 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)
Electrical interview questions: Different between megger and contact resistance meter?
Answers: Megger used to measure cable resistance, conductor continuity, phase identification whereas contact resistance meter used to measure low resistance like relays, contactors.
Electrical interview questions :When we connect the capacitor bank in series ?
Answers: we connect capacitor bank in series to improve the voltage profile at the load end in transmission line there is considerable voltage drop along the transmission line due to impedance of the line. So in order to bring the voltage at the load terminals within its limits i.e (+ or - %6 ) of the rated terminal voltage the capacitor bank is used in series
Electrical interview questions: What is Diversity factor in electrical installations?
Answers: Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Diversity factor is usually more than one.
Electrical interview questions: Why field rheostat is kept in minimum position while armature rheostat at maximum position?
Answers: In motors at the time of starting the armature resistance is introduced to reduce the high starting current and the field resistance is kept minimum to have high starting torque.
Electrical interview questions: Why humming sound occurred in h HT transmission line?
Answers: This 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.
Electrical interview questions: what is rated speed?
Answers: At the time of motor taking normal current (rated current) the speed of the motor is called rated speed. It is a speed at which any system take small current and give maximum efficiency.
Electrical interview questions: What is different between grounding and earthings?
Answers: Grounding means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas earthing is done in electric equipment in order to protect the equipment in occurrence of fault in the system.
Electrical interview questions: Why should be the frequency is 50 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?
Answers: We can have the frequency at any frequency you like, but then you must also make your own motors, transformers or any other equipment you want to use. We maintain the frequency at 50hz or 60hz cos the world maintains a standard at 50 /60hz and the equipment are made to operate at these frequency.
Why transformer rating in KVA instead of KW?
Any device is rated depending on losses of the machine. In a transformer the losses are iron losses which depend on voltage and copper losses which depend on current so there are no losses which depend on power factor hence transformer is rated in KVA.
There are 2 losses in transformer. One is copper loss which depends on current and the other is iron loss which depends on voltage. These two factors are not affected by the power factor. This is why transformers are rated in KVA and not KW. Single phase KVA = Amps x Volts/1000. Single phase KW = Amps x Volts x pf/1000. 3 phase KVA = Amps x Volts x 1.73/1000. 3 phase KW = Amps x Volts x 1.73 x pf/1000
yes there are two type of losses in transformer is that ohmic losses and core losses . ohmic losses due to transformer current and core losses due to transformer voltage. these losses are due to transformer voltage and current. And these losses not depends on load ie kw that's the way its rating in kva.