interview questions for eee department students

INTERVIEW QUESTIONS FOR EEE DEPARTMENT STUDENTS

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 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

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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 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.

Q: Where should the lighting arrestor is placed in distribution lines?A: Near distribution transformers and outgoing feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations.

Q: Define IDMT relay?A: 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.

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 and 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 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 operated. its specification only rated current is given. But circuit breaker is on load automatic device used for breaking the circuit incase 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.

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 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 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 Circuit breaker 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. ?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 isthe 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 theline where as in three point stater itis connected with line which isthe 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 valuefor 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 saydifferent 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 saydifferent 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 and3)compound(which is further classified into cumulative and differential)

Q: what will happen when power factor is leading in distribution of power?A: 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 verycheap to afford, &3. it also reduces over heating of tranformers.

Q:whats the one main difference between UPS & inverter ? And electrical engineering & electronics engineering ?A:uninterrupt 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 with high dc voltage.but ups start with 12v dc with 7 amp. but inverter is startwith 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup.

Qwhat is 2 phase motor?A:A two phase motor is a motor with the the starting winding and the running winding have a phase split. e.g;ac servo motor.where the auxiliary winding and the control winding have a phase split of 90 degree.

Q:Advantages of vvvf drives over non vvvf drives for EOT cranes?A:1.smooth start and stop.2.no jerking of load.3.exact posiitoning4.better protection for motor.5.high/low speed selection.6.reliability of break shoe.7.programmable break control.8.easy circutry9.reduction in controls10.increases motor life



Q:What is the significance of vector grouping in Power Transformers?A: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 phace difference betweent the current and voltage. EG. DYN11 means Delta primary, Wye Secondry and the current is at 11 o clock reffered to the voltage.

Q: which type of A.C motor is used in the fan (ceiling fan, exhaust fan, padestal fan, bracket fan etc) which are find in the houses ?A:Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

Q: why, when birds sit on transmission lines or current wires doesn’t get shock?

A:Its 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.

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:what happen if we give 220 volts dc supply to d bulb r tube light?

A: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.

Q:Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?

A C Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.

Q:what is ACSR cable and where we use it?

A:ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.

Q:What is vaccum currcuit breaker.define with cause and where be use it Device?

A:A breaker is normally used to break a ciruit. while breaking the circuit, the contact terminals will be separated. At the time of seperation 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 vaccum gas. since the air in the CB is having vaccum pressure the arc formation is interrupted. VCB’s can be used upto 11kv.

Q:How tubelight circuit is connected and how it works?A: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 intrupt the supply intermittant cycle of AC. Due to the sudden change of supply the chock will generate around 1000volts . This volt will capable of to break the electrons inside the tube to make elctrone flow. once the current passess through the tube the starter circuit will be out of part. now there is no change of supply causes choke voltage normalised and act as minimise the current.

Q: whats is MARX CIRCUIT?A: 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.

Q:What is encoder, how it function?A:An encoder is a device used to change a signal (such as a bitstream) 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.

Q: What are the advantages of speed control using thyrister?A: Advantages: 1. Fast Switching Characterestics than Mosfet, BJT, IGBT 2. Low cost 3. Higher Accuract.

Q: Why Human body feels Electric shock?? n in an Electric train during running , We didn’t feel any Shock ? Why?A: 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.

Q: what is the principle of motor?A: Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.

Q:what is electric traction?A: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 days, magnetic traction is also used for bullet trains. basically dc motors are used for electric traction systems.

Q:How can you start-up the 40w tube lite with 230v AC/DC without using any choke/Coil?A:It’s possible by means of Electronic choke.otherwise it’s not possible to ionise the particles in tube. light, with normal voltage.

Q:what is “pu” in electrical engg?A u 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 componenets (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 thecomponent 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).

Q:Operation carried out in Thermal power station?A: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

Q:why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?A:Link is provided at a Neutral common point in the circuit from which various connection 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 happen the fuse connected in the particular control circuit alone will blow off.

Q: what is the diff. between electronic regulator and ordinary rheostat regulator for fans?A:The difference between the electroic and ordinary regulator is that in electronic reg. power losses are lessi.e.for as we decrese the speed the electronic reg. give the power needed for that perticular speed but in case of ordinary rh type reg. the power wastage is same for every speed and no power is saved.In electronic regulator triac is employed for speed cntrl.by varying the firing angle speed is controled but inrheostatic ctrl resistance is decreased by steps to achieve speed control.

1. Electrical power system consist of 4 major categories:a. Generation Systemb. Transmission Systemc. Distribution Systemd. Utility System

2. Majority of electricity (65-70%) is produced by steam turbine plants and fuel is Coal or Nuclear.

3. Hydro electric generation (25-30%) forms the 2nd largest means of generating electricity. 4. Renewable sources of energy fall in a very small range (2-5%). 5. Renewable sources of energy – solar, wind, ocean, bio gas, geothermal, etc. 6. Gas Turbines are used during short periods of high demand for Peaking. 7. Large generators voltage rating or generation is @ 13.8kV to 24kV voltage levels. 8. Generator voltage is stepped up to transmission voltage level using transformers.

(Because …a. Generation and Distribution stations are far away.b. There will be huge I2R losses (transmission losses or copper losses) if the voltage level is low.c. Transformers can transform this energy to higher voltage levels, without much loss.)

9. Transmission voltage levels in the range from 115kV to 765kV. 10. Standard transmission voltages are 115kV, 138kV, 230kV, 345kV, 500kV and 765kV. 11. At distribution station, the transformer steps down the voltages. 12. Low voltage ranges from 34.5kV to 138kV at distribution station. 13. Distribution standard voltages are 4.16kV, 12.47kV, 13.2kV, 13.8kV and 34.5kV i.e.

range is from 5kV to 34.5kV.Why we cannot use higher voltages directly?



a. Difficult to have equipments with such a high insulation rating.b. Not economical.

14. Distribution transformers are used to further step down to utilization voltage levels, usually at 600V.

15. Standard utilization voltages are 480Y/277V, 460V, 208Y/120V, 240V, and 120V. 16. Higher utilization voltages – 6.9kV and 4.16kV are standard voltages for supplying large

industrial motor loads.

Q: Why field poles are laminated in DC machine ?

Field poles of a dc machine is laminated in order to reduce the eddy currents. Eddy currents are the circulating currents that are induced in the iron by the alternating magnetic field. These eddy currents produced will create a flux which opposes the main flux. So this eddy current production is considered as loss in the dc machine.Hence in order to reduce this eddy current losses thin laminations are considered and insulating coating is provided in between the laminations.

Q 6: Do commutator convert ac to dc or dc to ac ?

Commutator in a dc machine converts both ac to dc current and dc to ac current. Consider the dc generator the current produced in the armature due to the electro magnetic induction is ac. However commutator converts the bi directional current to uni directional current and supplies the current the external load circuit.

In DC motor the voltage we apply at the terminals is dc. This dc current is converted to ac and supply to the armature circuit so as to produce the rotational torque.

Hence Commutator converts both ac to dc variables and dc to ac variables.

Q: Why armature teeth of DC machine is skewed ?

The air gap length changes between the slot and teeth of the armature when the armature is rotating under constant magnetic field. This results in the production of small variation in the torque hence this results can be observed as small vibrations in the frame of the machine. So in order to avoid this vibration effect armature teeth is skewed.

Q1: Explain Armature reaction in DC machine ?

Answer:



Armature reaction is the effect of the magnetic field set up by the armature current on the distribution of the flux under the main poles of the DC machine.

Emf will be induced in the armature of the dc machine by the Fleming's electromagnetic induction principle (Conductor and magnetic field required to produce emf). As the conductors of the armature are shorted current starts flowing and this current will produce a counter magnetic filed which opposes the main field. This effect of armature flux on the main flux so as to distort or weaken the main field flux is called armature reaction.

Q 2: What is the effects of the armature reaction in a dc machine?

Answer:

Due the armature reaction the armature flux will have two impacts on the main field flux

1. It partly weakens or distorts the main field flux2. It cross magnetises or distorts the main field flux

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Q: Explain the methods to reduce the armature reaction in DC machine ?

Answer:

1. Armature reaction causes the distortion in the main field flux. This can be reduced if the reluctance of the path of cross magnetizing filed is increased.The armature teeth and air gap at the pole tips offer high reluctance to armature flux. Thereby increasing the air gap armature reaction can be reduced.

2. By special arrangements such that leading and trailing pole tips of the poles are alternatively omitted.

3. By providing compensating winding. Compensating winding will be in series with the armature winding

4. By providing Inter poles which are placed at the geometrical neutral axis at the mid way between the main poles.

Q: Why brushes of a DC machine is slightly shifted from the neutral axis by small angle?

Answer:

During no load Geometrical neutral pole (GNP) coincides with the magnetic neutral pole(MNP). This poles are required to place the brushes in neutral poles to collect the current from the armature with out the sparking effect. When the machine is loaded the Magnetic neutral pole


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shifts slightly by small angle due to armature reaction. So brushes of the dc machine is placed by slightly shifting from the neutral axis by an angle.

Q: Explain Commutation in DC machine ?

Answer:

Commutator and the brushes in a dc machine are plays a very important role:

1. It receives electrical current from the rotating armature and deliver it to the static external load in dc generator and vice verse in case of dc motor they collect the current from the terminals and deliver to the rotating armature.

2. They converts generated alternative current in the armature to dc current (Bi directional variable in to a uni directional variable) to deliver to the load and vise verse in the case of motor operation.

Rotating armature coil is connected to the commutator segments (commutator Segment is a part of the commutator which is separated with other segments by insulator). Brushes will be in contact with rotating commutator segments and are stationary.

When the armature is rotating under the magnetic field the current direction in the armature coil changes when coil of the armature passes from one pole ( say North pole) to one pole (South pole)

So in order to get uni directional current the commutator segment shifts from one brush to other when the armature current in the coil changes its direction. Under this condition the armature coil of the dc machine will be in short circuit condition for some period.

Commutation is defined as the process of reversal of current in the conductors of the armature as they pass through the brush axis is called commutation.

Commutation period is defined as period in which the armature coil of the dc machine is under short circuit condition as the commutator segment moves over the brush axis.

Good commutation means no sparking at the brushes and the commutator surface remaining un effected.Bad commutation is one such where sparks occur at the brushes and wear and tear of the commutator material is much high.

Q 6: Reasons for poor commutation ?

Answer:

Emf of the armature coil undergoing commutation will not be zero because of

1. Due to the use of short pitched or fractional pitched winding2. Due to the variation the brush contact resistance3. Due to the presence of the self inductance of the coil



Short pitched winding:

When the fractional pitch coil or short pitched coil is used the emf induced in coil at the time of commutation will not be zero and also will induce short circuit current when it is undergoing commutation results in wear out of the commutator and sparks at the brushes.

Brush Resistance variation:

Brush contact resistance variation results in the non linear variation in the coil which is undergoing the commutation.

Self Inductance of coil:

For a good commutation the current direction the coil should be changed when the coil crosses the brush axis. Due to Inductance in the coil which oppose the change in the direction of the current which results in increasing the current density at the leaving edge of the brush so sparking will occur.

Q: Explain how commutation in DC machine is improved ?

1. By using high resistance brushes2. By shift of brushes3. By use of inter poles or commutation poles

Electrical Question 1: What are the different types of DC Generators ?

Answer:

DC Generators are characterized by the method in which the flux is produced in the magnetizing circuit ( Field Winding ) of the DC generator. The flux required for the DC Generator is produced by

1. A Permanent magnet2. Field Coils excited by the external source3. Field Coils excited by the same source.

Based on the method of excitation flux produced DC Generators are Characterized by

Separately Excited DC Generator:

In Separately excited DC Generator the field winding excitation current (which produces the flux) of the DC Generator is controlled by the External source such as battery or current from a small DC machine.



Series Wound Generator:

In Series Wound Generator the field winding of the DC Generator is connected in series of the armature winding of the DC machine. The current which flows out from the DC Generator through the armature winding the same current flow through the series field winding of the DC Generator. Series field winding consists of thick coils of lesser turns.

Shunt Wound Generator:

In Shunt wound Generator the field winding of the DC Generator is connected in parallel to the armature winding of the DC Generator. In Shunt type of construction the current produced at the armature is divided between the load and field winding. Some part of the current flow through the field winding and remaining through the load. Shunt field winding consists of high resistance winding with thin coils of more turns

Compound Wound Generator:

Compound wound generators contain both series and shunt type of field windings. They are connected as long shunt and short shunt. In long shunt the shunt field winding is connected parallel to the series winding and armature winding. In Short shunt type of arrangement the shunt field winding will be placed just parallel to the armature winding alone. This type of machines has the both the characteristics of series wound machine and shunt wound machine.

Electrical Question 2: What are the Characteristics of DC Generator?

Answer:

Magnetic or Open circuit characteristics:

This is also called no load characteristics. It gives the relation between the generated emf in the armature on load and field current at constant speed.

Internal or total Characteristics:

This gives the relation between the emf generated in the armature and the armature current

External Characteristics:

This curve gives the relation between the terminal voltage and armature current under constant speed and excitation.

Electrical Question 3: Why Saturation Curve for the DC Generator does not start with zero ?

Answer:



Saturation curve or Magnetization curve in a DC Generator gives the relation between the Field Ampere turns (X axis) required to produce the flux per pole ( Y axis ). This curve starts from a point which is slightly higher than the origin representing that there is some flux produced by the field poles even no current is passed through the field windings. This flux produced is because of the property called residual magnetism which always exists in the DC Generator with out which DC Generator cannot be started.

Electrical Questions 4: What is Open Circuit Characteristics of DC Generator ?

Answer:

Magnetic or Open Circuit Characteristics of a DC Generator is the plot between the Field Current in Amps ( X axis) required to produce or build up emf in the generator terminals ( Y axis) of a DC Generator.

EMF generated E=KΦN.

Where K is the constant

Φ is the flux and

N is the speed of the DC Generator

Under constant speed the emf generated of the dc generator proportional to the flux Φ.Which is produced with the amount of field current.

Therefore it is a curve drawn between the generating emf produced under no load and field current when the machine is running at constant speed. It is the curve drawn under no load,so it is called open circuit characteristics of the machine.

Electrical Questions 5: Given the OCC curve and what happen when the field resistance of the generator increases ?

When the field resistance of the DC generator increases then the slope of the OCC characteristics of the dc generator increases, i.e, the line will move to the left towards the vertical position. This can be explained with the help of Ohms law.

Electrical Questions 6: What is Critical Field Resistance ?

Answer:

Critical Resistance of a DC Generator is defined as the maximum field resistance required to start the dc generator. Beyond this resistance the generator will not able to build up the voltage (EMF) and the motor fails to start. So care should be taken that field resistance of the dc generators should be less than the critical resistance.



Electrical Questions 7: What are the causes of failure to build up voltage in DC Generator ?

Answer:

In a DC Generator voltage will be build up in a step wise manner.

Some of the reasons for the generator failure to build up the voltage are

1. No Residual magnetism2. Reversal of Field connections ( destroys the residual magnetism when connections

reversed)3. Resistance of the field winding is more than the Critical Resistance4. 8: Explain the applications of the DC Generators ?

Answer:

1. Separately excited Generators are used in Ward Leonard Systems of speed control because self excitation at lower voltage would be unstable.

2. Series Wound generators are used in regenerative braking operation of the electrical dc locomotives. They are used in series lighting.

3. Shunt wound generators with field regulators are used in the light and power supply requirements.They have the property of providing constant voltage at any load.

4. Cummilative compound generators are used in lighting and power supply requirements5. Differential compound generators are used in applications where generators are required

related to the arc welding.

Why the exciting current of induction motor is so high as compared to the power transformer?

Unlike that of a power transformer, the magnetic circuit of the induction motor has an air-gap. Therefore, the exciting current of induction motor (3O to 4O% of full-load current) is much higher than that of the power transformer. Consequently, the exact equivalent circuit must be used for accurate results.

How transformation ratio of induction motor is different as compared to power transformer?

In a transformer, the windings are concentrated whereas in an induction motor, the windings are distributed. This affects the transformation ratio.

Why it is required to start 3-Phase Induction Motors?

The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuited secondary. At starting, the voltage induced in the induction motor rotor is



maximum ( s = 1). Since the rotor impedance is low, the rotor current is excessively large. This large rotor current is reflected in the stator because of transformer action. This results in high starting current (4 to 1O times the full-load current) in the stator at low power factor and consequently the value of starting torque is low. Because of the short duration, this value of large current does not harm the motor if the motor accelerates normally.

What is the effect of staring of induction motor on connected line?

Large starting current will produce large line-voltage drop. This will adversely affect the operation of other electrical equipment connected to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are available for this purpose.

Please describe the Methods of Starting 3-Phase Induction Motors?

The common methods used to start induction motors are:

(i) Direct-on-line starting (ii) Stator resistance starting(iii) Autotransformer starting (iv) Star-delta starting(v) Rotor resistance starting

How slip ring motors are started?

Slip ring motors are invariably started by rotor resistance starting.

What is Direct-on-line starting of induction motor?

This method of starting in just what the name implies the motor is started by connecting it directly to 3-phase supply. The impedance of the motor at standstill is relatively low and when it is directly connected to the supply system, the starting current will be high (4 to 1O times the full-load current) and at a low power factor. Consequently, this method of starting is suitable for relatively small (up to 7.5 kW) machines.

Does starting torque is higher than full load torque?

No, starting current is as large as five times the full-load current but starting torque is just equal to the full-load torque. Therefore, starting current is very high and the starting torque is comparatively low. If this large starting current flows for a long time, it may overheat the motor and damage the insulation.

What is Stator resistance starting method?

In this method, external resistances are connected in series with each phase of stator winding during starting. This causes voltage drop across the resistances so that voltage available across motor terminals is reduced and hence the starting current. The starting resistances are gradually cut out in steps (two or more steps) from the stator circuit as the motor picks up speed. When the motor attains rated speed, the resistances are completely cut out and full line voltage is applied to the rotor.



Why Stator resistance starting is not recommended?

This method suffers from two drawbacks. First, the reduced voltage applied to the motor during the starting period lowers the starting torque and hence increases the accelerating time. Secondly, a lot of power is wasted in the starting resistances. Therefore, this method is used for starting small motors only.

What is Autotransformer starting method?

This method also aims at connecting the induction motor to a reduced supply at starting and then connecting it to the full voltage as the motor picks up sufficient speed. Fig. shows the circuit arrangement for autotransformer starting.

The tapping on the autotransformer is so set that when it is in the circuit, 65% to 8O% of line voltage is applied to the motor.

At the instant of starting, the change-over switch is thrown to “start” position. This puts the autotransformer in the circuit and thus reduced voltage is applied to the circuit. Consequently, starting current is limited to safe value. When the motor attains about 8O% of normal speed, the changeover switch is thrown to “run” position. This takes out the autotransformer from the circuit and puts the motor to full line voltage.

What are the advantages of Autotransformer starting?

Autotransformer starting has several advantages like low power loss, low starting current and less radiated heat. For large machines (over 25 H.P.), this method of starting is often used. This method can be used for both star and delta connected motors.

What is Star-delta starting method of starting of 3 phase induction motor?

The stator winding of the motor is designed for delta operation and is connected in star during the starting period. When the machine is up to speed, the connections are changed to delta. The circuit arrangement for star-delta starting is shown below:


http://lh3.ggpht.com/-vzsIZ6SVuYY/ToTNiNci0JI/AAAAAAAAACg/yD8-rGYpyvM/s1600-h/clip_image0029.jpg


The six leads of the stator windings are connected to the changeover switch as shown. At the instant of starting, the changeover switch is thrown to “Start” position which connects the stator windings in star. Therefore, each stator phase gets volts where V is the line voltage. This reduces the starting current. When the motor picks up speed, the changeover switch is thrown to “Run” position which connects the stator windings in delta. Now each stator phase gets full line voltage V.

Explain in details the Starting of Slip-Ring Motors?

Slip-ring motors are invariably started by rotor resistance starting. In this method, a variable star-connected rheostat is connected in the rotor circuit through slip rings and full voltage is applied to the stator winding as shown in Fig.


http://lh4.ggpht.com/-YGeDgV8J_4c/ToTNj_qA3tI/AAAAAAAAACo/It-f1wemIhY/s1600-h/clip_image00274.jpg

http://lh3.ggpht.com/--au4OASKJME/ToTNlSylt5I/AAAAAAAAACw/5_iomUYZMzc/s1600-h/image4.png

http://lh3.ggpht.com/-yP58GJYn7Ik/ToTNmxFb9GI/AAAAAAAAAC4/E80OIH0_dM4/s1600-h/clip_image002104.jpg


At starting, the handle of rheostat is set in the OFF position so that maximum resistance is placed in each phase of the rotor circuit. This reduces the starting current and at the same time starting torque is increased.

As the motor picks up speed, the handle of rheostat is gradually moved in clockwise direction and cuts out the external resistance in each phase of the rotor circuit. When the motor attains normal speed, the change-over switch is in the ON position and the whole external resistance is cut out from the rotor circuit.

What are the advantages of slip-ring induction motors over the squirrel cage motors?

(i) High starting torque with low starting current.

(ii) Smooth acceleration under heavy loads.

(iii) No abnormal heating during starting.

(iv) Good running characteristics after external rotor resistances are cut out. (v) Adjustable speed

Is there any disadvantage of slip-ring motors?

(i) The initial and maintenance costs are greater than those of squirrel cage motors.

(ii) The speed regulation is poor when run with resistance in the rotor circuit

Q:What is inrush current?A:Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.

Q:In a Tap changing transformer where is the tap connected, is it connected in the primary side or secondary side? A:Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.

Q:Why transformer ratings are in kva?

A: Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in KWatts and include power factor.

Q: What is difference between fuse and breaker?A: Fuses are burned at the time of over current flows in the circuit but breakers are just open(not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by



multiple number of times.

Q:What is the difference between delta-delta, delta-star transformer?A:Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i,e) generally it is used where the Voltage is high & Current is low.Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena.

Q: Capacitor is load free component but why ampere meter shows current when capacitor bank breaker close?A: 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.

Q:How to determine alternating current frequency?A:Zero crossings of the sine wave to trigger a monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are "n" pulses per second, each with with a constant energy. The more pulses there are per second, the more the energy. The pulses are integrated (filtered or averaged) to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy in a second or so. Q:Why electricity in India is in the multiples of 11 like 11kv, 22kv, 33kv ?

A:Transformer Induced voltage equation contains 4.44 factor.

E=4.44*f*T*phi

E -Induced emf per phase

T -number of turns

f -frequency

phi -maximum flux per pole

From the equation we see that E is proportional to 4.4 and it is in turn multiple of 11.So always transmission voltage is multiple of 11

Q:Why we use ac system in India why not dc ?

A:Firstly, the output of power stations comes from a rotary turbine, which by it's nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to



change the voltage of AC electricity for transmission and distribution. thirdly the cost of plant associated with AC transmission

(circuit breakers, transformers etc) is much lower than the equivilant of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

Q:Which type of motor is used in trains, what is the rating of supply used explain Working principal?

A:Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.

Q:Battery banks are in connected in series or parallel and why?

A:Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same. Ex : 24 nos. 2V,200Ah batteries connected in series will give 48V,200Ah output (AH = Ampere hours)

Query:Use of switch mode power converter in real-time basis?Resolution:Switch mode power converter can be used in the following 5 different ways1) step down an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Buck Converter or Step-Down SMPS,2)step up an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Boost Converter or Step-Up SMPS,3)step up or step down an unregulated dc input voltage to produce a regulated dc output voltage,4)invert the input dc voltage using usually a circuit such as the Cuk converter, and5)produce multiple dc outputs using a circuit such as the fly-back converter.

Query:Which type of oil is used as a transformer oil?

Resolution: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.

Query: 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? Resolution: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

Query:what are the points to be consider for MCB(miniature circuit breaker selection?Resolution:I(L)*1.25=I(MAX) maximum current. Mcb specification are done on maximum current flow in circuit.

Query:what is the full form of KVAR?

Resolution:We know there are three types of power in Electrical as Active, apparent & reactive. So KVAR is stand for ``Kilo Volt Amps with Reactive component.

Query:What is excitation?Resolution:Excitation is applying an external voltage to DC shunt coilin DC motors.

Query:In three pin plug 6 Amp. 220v AC rating. why earth pin diameter is higher than other two pin? what its purpose ?

Resolution: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)

Quantities and Units :

1. When these numbers are multiplied, (6 × 103) (5 × 105), the result is


http://www.indiabix.com/electrical-engineering/quantities-and-units/


A. 3 × 108

B. 30 × 108

C. 300 × 109

D. 3,000 × 107

2. Resistance is measured in

A. henries B. ohms

C. hertz D. watts

3. The number 65,000 expressed in scientific notation as a number between 1 and 10 times a power of ten is

A. 0.65 × 104

B. 6.5 × 104

C. 65 × 104

D. 650 × 103

4. When converting 7,000 nA to microamperes, the result is

A. 0.007 µA

B. 0.7 µA

C. 700 µA

D. 7 µA

5. The number of kilowatts in 135 milliwatts is

A. 1.35 × 10–4 kW

B. 135 × 10–3 kW

C. 0.0135 kW

D. 0.00135 kW



6. The number 4.4 × 106 ohms expressed using a metric prefix is

A. 4 k

B. 4.4 k

C. 4 M

D. 4.4 M

7. The number of microamperes in 2 milliamperes is

A. 2 µA

B. 20 µA

C. 200 µA

D. 2,000 µA

8. The number of millivolts in 0.06 kilovolts is

A. 600 V

B. 6,000 mV

C. 60,000 mV

D. 600,000 mV

9. Eighteen thousand watts is the same as

A. 18 mW

B. 18 MW

C. 18 kW

D. 18 µW

10. The number 3.2 × 10–5 A expressed using a metric prefix is

A. 32 µA



B. 3.3 µA

C. 320 mA

D. 3,200 mA


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