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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 an electric equipment in order to protect he equipment in occurence 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 than 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 equipments are 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 depends on voltage and copper losses which depends 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.. 

Q: What is meant by regenerative braking?

A:When the supply is cutt 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”. 

Q:Why is the starting current high in a DC motor?

A: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. 

Q:What are the advantages of star-delta starter with induction motor?

A 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. 

Q:Why Delta Star Transformers are used for Lighting Loads?

A: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. 

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

A: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 disconnnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

Q:Why series motor cannot be started on no-load?

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

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

A: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. 

Q: why we do 2 types of earthing on tranformer ie:body earthing & nutral earthing , what is function. i am going to instal a 5oo kva tranformer & 380 kva DG set what should the earthing value? 

A:The two types of earthing are Familiar as Equipment earthing and Sysytem earthing. In Equipment earthing: body ( non conducting part)of the eqipment shold be earthed to safegaurd 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. 

Q:What is the difference between MCB & MCCB, Where it can be used?

A:MCB is miniature circuit breaker which is thermal oprated 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. 

Q:Where should the lighting arrestor be placed in distribution lines?

A:Near distribution transformers and out going feeders of 11kv and incomming feeder of 33kv and near power transformers in sub-stations. 

Q:Define IDMT relay?

A:It is an inverse definite minumum time relay.In IDMT relay its operating is inversely proportional and also a characteristic of minumum time after which this relayoperates.It is inverse in the sense ,the tripping time will decrease as the magnitude of fault current increase. 

Q:What are the transformer losses?

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

Q:What is the difference between Isolator annd Circuit Breaker?

A:Isolator is a off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintanance 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 operated. its specification only rated current is given.But circuit breaker is onload automatic device used for breaking the circuit incase of abnormal conditions like shortcircuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current. 

Q:what is boucholz relay and the significance of it in to the transformer?

A:Boucholz relay is a device which is used for the protectionof 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 other wise it trips the circuit by its own tripping mechanism. 

Q:What is SF6 Circuit Breaker?

A:SF6 is Sulpher hexa Flouride gas.. if this gas is used as arc quenching medium in a Circuitbreaker means SF6 CB. 

Q:what is ferranti effect?

A:Output voltage is greater than the input voltage or receiveing end voltage is greater than the sending end voltage. 

Q: what is meant by insulation voltage in cables? explain it?

A: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. 

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 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 damge 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 electro magneticflux 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 bcoz 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). 

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 altenator while it is picking up or without any other load. 

Q: why the capacitors work 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 therby forming the closed contact for the flow of current. During abnormal & faulty conditions(when curent 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, 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 sychronous motor which runs in steps in either direction instead of running in complete cycle.so, in automation parts it is used. 

Q: What is Automatic Voltage regulator(AVR)?

A:AVR is an abbreviation for Automatic Voltage Regulator.It isimportant part in Synchronous Generators, it controls theoutput voltage of the generator by controlling itsexcitation current. Thus it can control the output ReactivePower of the Generator. 

Q:What is an exciter and how does it work?

A:There are two types of exciters, static exciter and rotory exciter.purpose of excitor is to supply the excitation dc voltage to the fixed poles of generator.Rotory excitor is an additional small generator mounted on the shaft of main generator. if it is dc generator, it will supply dc to the rotory poles through slip ring and brushes( conventional alternator). if it is an ac excitor, out put of ac excitor is rectified by rotating diodes and supply dc to main fixed poles.ac excitor is the ac generator whose field winding are stationary and armature rotates. intial volatge is buit up by rsidual magnetisim.It gives the starting torque to the generator. 

Q:Difference between a four point starter and three point starter?

A:The shunt connection in four point stater is providedseparately form the line where as in three point stater itis connected with line which is the drawback in three pointstater.

Q:Why use the VCB at High Transmission System ? Why can’t use ACB?

A:Actually the thing is vacuum has high arc queching property compare to air becoz in VCB ,the die electric strengthis equal to 8 times of air . That y always vaccum used as inHT breaker and air used as in LT . 

Q:what is the difference between surge arrestor and lightning arrestor?

A: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. 

Q:Give two basic sprrd control scheme of DC shunt motor?

A:1. By using flux control method:in this method a rheostat is connected across the field wdg to control the field current.so by changing the current the flux produced by the field wdg 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 propotional to Eb-IaRa the speed can be controlled. 

Q:what is the principle of motor?

A:Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movemnt is called as torque. 

Q:what is meant by armature reaction?

A:The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux. 

Q:Give two basic sprrd control scheme of DC shunt motor?

A:1. By using flux control method:in this method a rheostat is connected across the field wdg to control the field current.so by changing the current the flux produced by the field wdg 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 propotional to Eb-IaRa the speed can be controlled. 

Q:what is the difference between synchronous generator & asynchronous generator?

A:In simple, synchronous generator supply’s both active andreactive power but asynchronous generator(induction generator) supply’s only active power and observe reactivepower for magnetizing.This type of generators are used in windmills. 

Q:What is the Polarisation index value ? (pi value)and simple definition of polarisation index ?

A:Its ratio between insulation resistance(IR)i.e megger 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 . 

Q:why syn. generators r used for the production of electricity?

A: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.

Q:what is the difference between synchronous generator & asynchronous generator?

A: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 are used in windmills. 

Q:1 ton is equal to how many watts?

A: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.

Q:why syn. generators r used for the production of electricity?

A:synchronous machines have capability to work on differentpower factor(or say different imaginary pow varying the field emf. Hence syn. generators r used for the production of electricity.

Q:Enlist types of dc generator?

A:D.C.Generators are classified into two types 1)separatly exicted d.c.generator 2)self exicted d.c.generator, which is further classified into;1)series 2)shunt and 3)compound(which is further classified into cumulative and differential).

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