Electronics Electrical Interview Questions, Tutorials, Circuits, Motors, Engines and more

What are the advantage of free wheeling 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

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.

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.

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.

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).

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.

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.

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.

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.

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.

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

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.

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.

How do you select a cable size (Cu & Al) for a particular load?
Answer:At first calculate the electrical current of the load, after that derate the electrical current considering derating factor(depending on site condition and laying of cable) after choose the cable size from cable catalog considering derating electrical current.After that measure the length of cable required from supply point of load to load poin. 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.

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.

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.

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

What is derating?, 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, 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 derating.

Why an ac solenoid valve attract the plunger even though we interchanges the terminal? Will the poles changes?
Answer:Yes because the poles changes for every half-cycle of ac voltage so the polarity of AC voltage is continuously changing for every half cycle. so, interchanging of terminals in ac system does not show any difference. That's why the ac solenoid attract the plunger even though it's terminals are interchanged.

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.

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.

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 genset, or more commonly, a generatorThe 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

Why series motor cannot be started on no-load?

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

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

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.

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.

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.

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.

what is ferrantic effect?

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

What is SF6 Circuit Breaker?

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

What is the difference between Isolator and Circuit Breaker?

Answer: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 onload 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.

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 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).

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 its not consuming.. hence the load current in the transformer is because of core loss so it is minimum.

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 electromagneticflux 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.

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.

what is stepper motor.what is its uses?

Answer:Stepper motor is the electrical machine which act upon input pulse applied to it. it is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle.so, in automation parts it is used.

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.

How many types of colling system it transformers?

Answer:1. ONAN (oil natural,air natural)

2. ONAF (oil natural,air forced)

3. OFAF (oil forced,air forced)

4. ODWF (oil direct,water forced)

5. OFAN (oil forced,air forced)

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 circuitArc is extinguished by the Arc Quenching media like air, oil, vaccum etc.

Why the capacitors works on ac only?

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

What happens if i connect a capacitor to a generator load?

Answer: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 alternator while it is picking up or without any other load.

What is the difference between surge arrestor and lightning arrestor?

Answer:LA is installed outside and the effect of lightning is grounded,where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Why use the VCB at High Transmission System ? Why can't use ACB?

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

Difference between a four point starter and three point starter?

Answer:The shunt connection in four point stater is provided separately form the line where as in three point stater it is connected with line which is the drawback in three point stater

What is an exciter and how does it work?

Answer: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. initial voltage is built up by residual magnetism.It gives the starting torque to the generator.

What is Automatic Voltage regulator(AVR)?

Answer:AVR is an abbreviation for Automatic Voltage Regulator.It is important part in Synchronous Generators, it controls theoutput voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

Enlist types of dc generator?

Answer: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).

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.

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 are used in windmills.

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.

the h-parameter or the hybrid parameters of a transistor helps us to analyse the amplifying action of transistor for small signal .

Why the antenna is called passive element?

Because It needs Some power input to broadcast or receive signals

what is the difference between inductance and resistance?
Inductance(L) is the property of a coil, which opposes change in current or In other words property of opposing AC.
Resistance(R) is the property of a matter or material which opposes the flow of electron.(it may be AC or DC)

Why 440volts instead of 660volts in 3 phases of supply system?

In single phase supply, 220V is voltage diff. between Line & neutral while, In 3 phase supply there are 3 supply lines phase shifted at 120deg from each other, so net voltage diff. (the phase resultant) between two phases in accordance with phase angle of 120deg is 440V.

However in 3 phase, 4 line systems having 1 neutral also, the voltage diff. between neutral and any of the phase is 220V..

this question is not any kind of technical question but if it has been asked in interview the

we can say that
1) ground wire
2) insulators

can be said as the circuit protection device

Which of the following are used to display measured quantity by digital meter?
LED,
LCD,
CRO?

differences between microprocessor and micro controller

In Microprocessor more op-codes, few bit handling instructions. But in Microcontroller: fewer op-codes, more bit handling Instructions, and also it is defined as a device that includes micro processor, memory, & input / output signal lines on a single chip.

The difference between microprocessor and microcontroller is the processor which can perform only software like operations it cannot control any parameters.hence the microcontroller will do that job.

In computers we are using the microprocessor and in industrial controls we are using microcontroller.

The microprocessor is the integration of a number of useful functions into a single IC package.

These functions are: The ability to execute a stored set of instructions to carry out user defined tasks. The ability to be able to access external memory chips to both read and write data from and to the memory.

Basically, a microcontroller is a device which integrates a number of the components of a microprocessor system onto a single microchip.
So a microcontroller combines onto the same microchip : The CPU core Memory (both ROM and RAM) Some parallel digital I/O Essentially, a microcontroller is obtained by integrating the key components of microprocessor,RAM, ROM, and Digital I/O onto the same chip die. Modern microcontrollers also contain a wealth of other modules such as Serial I/O, Timers, and Analogue to Digital Converters.

Design binary to gray code converter
It is easy to convert the binary number to gray number. First, for example take a binary number i.e ,101101.

Step 1: 101101 first write the MSB as it is i.e, darken bit i.e, 1

Step 2: add MSB and bit next to the MSB i.e, 1+0=1

Step 3: again add 0 and 1 we get i.e, 0+1=1
Step 4: again add 1 and 1 we get 1+1=0

Step 5: in previous step carry is occurred so neglect that carry . note that don't add carry to next add numbers
then again add 1+0 1+0=1

step 6: add 0+1 0+1= 1

Answer: Gray Code is : 111011

What are semiconductor devices

there are three types of materials. they are conductor , semi conductor and insulator.
where the semi conductor lies between the other two. ie. controlled conduction is possible in semi conductor.
thats why it is preferred in electronic devices.
A PN junction is formed in all the three types of materials. but in semi consuctor the junction is very thin and easy to break it with the very low voltage.

Which of the following is is used to hold ROM and RAM cards

1. Computer Bus 2. Expansion cards
3. Mother Board 4. Cache memory

The answer is mother board

what is meant by virtual ground in the op amp ?

The op amp connected in a negative feedback configuration, that is the o/p connected to the inverting terminal, tried to produce the same voltage at the inverting node as we applied to the non-inverting terminal, no matter whatever be the o/p. This happens because the opamp has a very high differential gain. so only way to not to clip or saturate the o/p in either positive or negative side is to make their both i/ps at the same potential. so when you apply the i/p to the inverting terminal and you connect the feedback to the inverting node, and your non-inverting terminal is grounded, then op amp tries to force the inverting node at the ground potential and whatever the difference between these two nodes are amplified.

How to convert binary to gray code and vice-versa?

How to measure the insulation resistance for printed circuit boards ?
By using known load impedance.

When transmitting digital signals is it necessary to transmit some harmonics in addition to fundamental frequency?
For asynchronous transmission, is it necessary to supply some synchronizing pulses additionally or to supply or to supply start and stop bit?
BPFSK is more efficient than BFSK in presence of noise. Why?
What is meant by pre-emphasis and de-emphasis?
Explain 3 dB cutoff frequency? Why is it 3 dB, not 1 dB?
Explain ASCII, EBCDIC?

What is the formula for power gain

power gain=output gain/input gain

What is D-FF?

What is the basic difference between Latches and Flip flops?

What is a multiplexer?

How can you convert an SR Flip-flop to a JK Flip-flop?

How can you convert an JK Flip-flop to a D Flip-flop?

What is Race-around problem? How can you rectify it?

Which semiconductor device is used as a voltage regulator and why?

Explain an ideal voltage source?

Explain zener breakdown and avalanche breakdown?

What are the different types of filters?

Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field. The electrical resistivity of a metallic conductor decreases gradually as the temperature is lowered. However, in ordinary conductors such as copper and silver, impurities and other defects impose a lower limit. Even near absolute zero a real sample of copper shows a non-zero resistance. The resistance of a superconductor, on the other hand, drops abruptly to zero when the material is cooled below its "critical temperature". An electric current flowing in a loop of superconducting wire can persist indefinitely with no power source.

Transponder: An automatic device that receives, amplifies, and retransmits a signal on a different frequency (see also broadcast translator).

Pull-up resistors are resistors used in the design of electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected. Pull-up resistors may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages.
The idea of a pullup resistor is that it weakly "pulls" the voltage of the wire it's connected to towards 5V (or whatever voltage represents a logic "high"). However, the resistor is intentionally weak (high-resistance) enough that, if something else strongly pulls the wire toward 0V, the wire will go to 0V.

BUS: In computer architecture, a bus is a subsystem that transfers data between computer components inside a computer or between computers. Unlike a point-to-point connection, a bus can logically connect several peripherals over the same set of wires. Each bus defines its set of connectors to physically plug devices, cards or cables together. Early computer buses were literally parallel electrical buses with multiple connections, but the term is now used for any physical arrangement that provides the same logical functionality as a parallel electrical bus.

Difference between real ground and virtual ground
Virtual ground (sometimes called virtual earth) is an important concept found in electronic circuit designs. It identifies a point in a circuit as being held close to the circuit's ground or reference level electric potential. It is called virtual since this point does not have any real electrical connection to ground. The reference may or may not be the same as the local utility ground or earth
Real ground: Voltage is a differential quantity, which appears between two points. In order to deal only with a voltage (an electrical potential) of a single point, the second point has to be connected to a reference point (ground) having usually zero voltage. This point has to have steady potential, which does not vary when the electrical sources "attack" the ground by "injecting" or "sucking" a current to/from it. Usually, the power supply terminals serve as grounds; when the internal points of compound power sources are accessible, they can also serve as real grounds

Synchronous transmission uses no start and stop bits but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signals built into each component. A continual stream of data is then sent between the two nodes. Due to there being no start and stop bits the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in protocol (computing) for sending/receiving data, so some bytes could become corrupted (by losing bits). Ways to get around this problem include re-synchronization of the clocks and use of check digits to ensure the byte is correctly interpreted and received.

Asynchronous transmission uses start and stop bits to signify the beginning bit ASCII character would actually be transmitted using 10 bits e.g.: A "0100 0001" would become "1 0100 0001 0". The extra one (or zero depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data is sent intermittently as opposed to in a solid stream. In the previous example the start and stop bits are in bold. The start and stop bits must be of opposite polarity. This allows the receiver to recognize when the second packet of information is being sent.

Signal-to-noise ratio (often abbreviated SNR or S/N) defined as the ratio of a signal power to the noise power corrupting the signal. In less technical terms, signal-to-noise ratio compares the level of a desired signal (such as music) to the level of background noise. The higher the ratio, the less obtrusive the background noise is.

What is meant by regenerative braking?

Answer: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".

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.

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.

Where should the lighting arrestor be placed in distribution lines?

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

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.

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 shouldd 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.

Common-mode rejection ratio (CMRR) of a differential amplifier (or other device) measures the tendency of the device to reject input signals common to both input leads. A high CMRR is important in applications where the signal of interest is represented by a small voltage fluctuation superimposed on a (possibly large) voltage offset, or when relevant information is contained in the voltage difference between two signals.

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 .

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 are used in windmills.

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.

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.

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 ?

Answer:Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

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 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.

Advantages of vvvf drives over non vvvf drives for EOT cranes?

Answer:1.smooth start and stop.

2.no jerking of load.

3.exact posiitoning

4.better protection for motor.

5.high/low speed selection.

6.reliability of break shoe.

7.programmable break control.

8.easy circutry

9.reduction in controls

10.increases motor life

What is 2 phase motor?

Answer: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.

whats the one main difference between UPS & inverter ? And electrical engineering & electronics engineering ?

Answer: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.

What will happen when power factor is leading in distribution of power?

Answer:If their 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 tranformers.

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

Answer: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.

what is ACSR cable and where we use it?

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

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 can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.

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.

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.

why, when birds sit on transmission lines or current wires doesn't get shock?
Answer: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.

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