networksshort2011-12pdf

8/3/2019 networksshort2011-12pdf

1/10

UNITI

1. What is the difference between network and circuit?

The interconnection of two or more circuit elements (resistors, inductors and capacitors) iscalled an electrical network. If the network contains at least one closed path, it is called anelectrical circuit. Every circuit is a network, but all networks are not circuits

2. What is the difference between a mesh and a loop?A loop is any closed path of a network. However, a mesh is the most elementary form of aloop and cannot be further divided into other loopsAll meshes are loops but all loops are not meshes

3. What are the advantages of nodal analysis over Kirchhoffs voltage law

(i) Generally less number of equation are required to a circuit by nodal analysis(ii) Nodal analysis is convenient in circuits where various voltages, referred to a common

ground, are to be determined(iii) Nodal analysis is a convenient approach to circuits that have current sources and

conductances rather than voltage sources and resistances

4. What is the difference between the active and passive element?

Active elements (voltage and current sources) are capable of delivering power to someexternal device and Passive elements (resistor,inductor and capacitors) are those which arecapable of receiving power

5. What are the difference b/w V-I characteristics of ideal, practical voltage source and

current source? An ideal source will have a constant V-I characteristic where as the practical sources will have small

droop due to its effect of internal resistance (RS)

6. What is the difference between linear and non linear element?

An element is said to be linear,if its voltage-current characteristics is at all times a straight linethrough the origin and which satisfies the principle of super position and homogenityan element is said to be nonlinear ,which does not satisfies the above principle

7. What do you mean by an ideal voltage source and ideal current source?

An ideal voltage source is one that maintains a constant terminal voltage no matter how muchcurrent drawn from it.An ideal current source is one which will supply the same current to any resistance connectedto its terminals

8. State KVL and KCL

KVL: kirchhoffs voltage law states that the algebraic sum of all branch voltagesaround any closed path in a circuit is always zero at all instants of time.

KCL: kirchhoffs current law states that the sum of the current entering into any nodeis equal to the sum of the currents leaving that node.


2/10

9. State Ohms law and write limitationsIt states that at constant temperature, potential difference V across the ends of a conductor isproportional to the current I flowing through the conductor, i.e.

IRV

IV

Limitations:

1. It is not applicable to the nonlinear devices such as diodes,zerner diodes etc.2. It does not hold for good for non-metalic conductors

10. Star to Delta and Delta to Star transformation

2

132123

c3

1

321312b2

3

213231

a1

R;

R;

R;

R

RRRRRR

RRR

RRR

R

RRRRRR

RRR

RRR

R

RRRRRR

RRR

RRR

cba

cb

cba

ba

cba

ca

11. Define charge and energy Charge is defined as the charge transferred by a current of one ampere in one second

Unit of charge is coulombsEnergy is the capacity to do the work. The work is done always at the cost of energy

Unit of energy: joules12. Why an inductive circuit should not be opened when carrying a current?

dt

diLV

When the switch is opened, Current is changing in short duration ,i.e t =0 ,dt

di

Then Voltage across the inductor is very high , For this reason inductive circuit isnot opened when carrying current

13. Why inductor opposes the sudden changes in current and write the expressions for

energy stored in L & C

If current passing through inductor changes in 0 time, infinity voltage must be appear,

which is not practicable. Hence inductor opposes sudden change of current

capacitorforCV

2

1W

InductorforLI2

1W

2

2

14. What is source transformation?

In solving networks in finding network solutions one may have to deal with energy sources.Energy sources are either voltage sources or current sources. Sometimes it is necessary toconvert a voltage source to a current source and vice versa. Source transformation technique isused to convert voltage source into current source vice versa


3/10

15. Find the net capacitance

Net Capacitance = 1F16. What is the difference between charge and current

The current is flow of electronics. Thus current can be measured by measuring how manyelectronics are passing through material per second. This can be expressed in terms of thecharge carried by those electrons in the material per second.So the flow of charge per unit timeis used to quantify an electric current

Amperest

qi

UNIT-II

1. What is power factor,If the power factor of the circuit is 0.8 lagging ,what does it mean?

The termcos in the term VIcos is called power factor.It is the cosine of the anglebetween voltage and current in any AC circtuitThe greater the p.f of a circuit ,the greater is its ability to utilise the apparent power.Thus0.8p.f of a circuit means that it will utilise only 80% of the apparent power.Lagging meansThat circuit current lags behind the applied voltage i.e the circuit is inductive

2. What is J operator?

The J operator causes the phasor to rotate through 900 in the anticlockwise direction withoutaffecting its magnitude

3. What are the values of Form factor and Peak factor for sinusoidal input?

414.1

Im707.0

Im

ValueRms

ValueMaxfactor

11.1Im637.0

Im707.0

Value

valuefactor

Peak

Avg

RmsForm

4. Define active power , reactive power and apparent power

The product of RMS values of voltage and current with the Cosine of the angle between them isCalled the reactive power in A.C circuit and is denoted by P

P = VI Cos () wattsThe product of RMS values of voltage and current with the sine of the angle between them is

Called the reactive power in A.C circuit and is denoted by QQ = VI Sin () volt-ampere-reactive(VAR)The product of RMS values of voltage and current is called the apparent power and it is denotedby S

S=VI volt-ampere(VA)5. Define the term Average Value

In general, the average value of any function v (t), with period T is given byaverage value is the total area under the complete curve divided by the distance of the curve. Theaverage value of a sine wave over one complete cycle is always zero. So the average value of a sinewave is defined over a half cycle, and not a full cycle period.


4/10

6. What is Form Factor?

Two alternating waves of same peak value and same frequency may look different, i.e. theirconfigurations are different. Form factor represents this change in configuration of periodic waveswhere the frequencies and amplitudes are same.

Form Factor is defined by the ratio of RMS value and average value.7. Define RMS value. What is the RMS value of sinusoid?

The root mean square (RMS) value of a sine wave is a measure of the heating effect of the wave. Whena resistor is connected across a dc voltage source, a certain amount of heat is produced in the resistor ina given time. A similar resistor connected across an ac voltage source for the same time ,the value of theac voltage is adjusted such that the same amount of heat is produced in the resistor as in the case of the

dc source. This value is called the RMS value. RMS value of sine wave is 0.707 Vm.8. What is Peak Factor?

The peak factor of any waveform is defined as the ratio of the peak value (maximum value) of thewave to the RMS value of the wave.

Peak factor = peak value / RMS value9. Draw the phasor diagram for an series and parallel RL circuit?

10. Draw the phasor diagram for an RLC circuit?

11. How much power is consumed in passive elements

A.C circuit can have three circuit elements ,namely R,L and C. Power consumed only in R. Land C do not consume any power. The power supplied to L or C during one half cycle isreturned in the next half cycle .Hence that net power consumed is zero

12. Draw the power triangle for inductive and capacitive loads


5/10

UNITIII

1. What is resonance and Write the condition for the resonance

A circuit is said to be resonance when the applied voltage V and the resulting current I are inphase and condition for the resonance is net reactance is equal to zero

2. Define bandwidth, quality factor and selectivity

Bandwidth:The bandwidth of any system is the ratio of frequencies for which the current orvoltage is equal to 70.7% of its value at the resonant frequency

Quality factor: Quality factor is defined as the ratio of the voltage across the inductor orCapacitor to the applied voltage.Q=VL/V =VC/V

Selectivity:Selectivity of the resonance circuit is defines as the ratio of resonant frequency tothe bandwidth of the circuit

Selectivity= Fr/B.W

3. What is Resonant frequency

The frequency at which resonance occurs

LC

2

1f

XXat

r

CL

4. What is the power factor of an RLC circuit at Resonance?

Unity,why Because at Resonance condition ,inductive reactance (XL) is equal to capacitivereactance (Xc) ,Then Z=R

5. How the series RLC circuit behaves for the frequencies above and below resonantfrequencies?

For frequencies below resonant frequency the capacitive reactance is more than the inductivereactance.Therefore the equivalent reactance is equal to capacitive and the circuit behaves like aRC circuit.For frequencies above resonant frequency the inductive reactance is more than thecapacitive reactance and circuit behaves like a RL series circuit

6. What are the differences between the series and parallel resonance?

SERIES RESONANCE PARALLEL RESONANCE

1.At resonance,power factor is unity 1.At resonance,power factor is unity2.Series resonance has minimum impedance

And maximum current at the resonantfrequency

2.parallel resonance has maximum impedanceAnd minimum current at resonant frequency

3.Series resonance magnifies the voltage byQ times ,so this circuit is also called voltagemagnification circuit

3.Parallel resonance magnifies the current byQ times ,so this circuit is also called current

magnification circuit4.Impedance at resonance is Z=R 4.Admittance at resonance is Y=1/R 5.ffr,circuit is inductive nature(lagging p.f)

5.ffr,circuit is capacitive nature(leading p.f)

6.Q=L/R=1/CR 6.Q= R/ L= CR


6/10

7.How do you get better frequency selectivity with resonant circuits?

Selectivity indicates how well a resonant circuit responds to a certain frequency andeliminates all other frequencies. The narrower the bandwidth,the greater selectivity

8. Draw the frequency response of series RLC and parallel RLC resonance circuit

9. What is locus diagram ,draw the locus diagrams for series circuits variable R,X L and R,

Variable XLA phasor diagram may be drawn and is expanded to develop a curve know as a locus. Locus diagram

are useful in determining the behavior or response of an RLC circuit when one of its parameters is varied

while the frequency and voltage kept constant. The magnitude and phase of the current vector in the

circuit depends upon the values of R, L, and C and frequency at the fixed source voltage.

Current Locus: The path traced by the terminus of the current vector when the parameters R, L, or C are

varied while frequency and voltage are kept constant is called the current locus.

10.Draw the phasor diagram of series RLC and parallel RLC circuit resonant circuits


7/10

11.Draw a graph how impedance vary with frequency in series circuit and admittance

vary with frequency in parallel circuit

12. State superposition theorem. Is superposition theorem is applicable to power

calculations?

In a linear network response in a particular branch when all the sources actingsimultaneously is equal to the linear sum of individual responses.It is not applicable for power calculations why because power is a non linear.

13. State maximum power transfer theorem. What is the percentage efficiency of maximum

power transfer theorem for D.C circuits?Maximum power is transferred in the circuit when load resistance is equal to the sourceresistance. For DC circuits power transfer is 50% at max power condition

14. State thevenins theorem

Thevenins theorem states that any two terminal linear network having a number of voltage ,current sources and resistances can be replaced by a simple equivalent circuit consisting of asingle voltage source in series with a impedance

15. State Nortons theorem

The Nortons theorem states that any linear active network with output terminals can be

replaced by a single current source in parallel with a single impedance

16. State Reciprocity Theorem.

In a linear,passive and bilateral network ,the ratio of response to the excitation is constanteven though the source is interchange from input terminals to the output terminals

17. State Tellegans Theorem.

In a linear network ,The algebraic sum of powers at any instant is zero. That is powerdelivered by some elements is equal to power absorbed by the remaining elements present ina network

18.State compensation theorem In a linear time invariant network when the resistance (R) of an uncoupled branch,carrying a

current (I) ,is changed by (R) ,the currents in all the branches would change and can beobtained by assuming that an ideal voltage source of (VC) has been connected [such that VC=I(R) ]in series with (R+R) when all other sources in the network are replaced by theirinternal resistances

19. state millmans theoremWhen a number of voltage sources (V1,V2.Vn) are in parallel having internal resistances(R1,R2Rn) respectively,the arrangement can be replaced by single equivalent voltagesource V in series with an equivalent series resistance R


8/10

UNITIV

1. Define self and mutual inductance and write the relation between them

Self inductance : It is the property of the coil due to which it opposes any increase ordecrease of a current or flux through it

I

NL

Mutual inductance : It is defined as the ability of one coil to produce an e.m.f in a nearby coil

by induction when the current in the first coil changes.

2

2

1

1

1

2I

NI

NM

Relation between L and M is 21LLKM

2. What is the purpose of dot conversion and how do you establish dots?

The sign of mutually induced voltage depends on direction of winding of the coils.But it isvery inconvenient to supply the information about winding direction of the coils.Hence dotconventions are used for purpose of indicating direction of windingIf a current enters a dot in one coil ,then mutually induced voltage in other coil is positive atthe dotted endIf a current leaves adot in one coil,then mutually induced voltage in other coil is negative at

the dotted end3. What is coefficient of coupling (k) ?

Coefficient of coupling is defined as the ratio of flux linking with one coil to the total fluxproduced by the other coil.The coefficient of coupling shows the degree of coupling to twocoils.The maximum value of K is 1

2

21

1

12

K ,When 100% of the flux lines link each coil ,then K=1

4. What is a balanced 3-phase supply system?

In which all the voltages having same magnitude & frequency and displaced from by 1200

phase angle5. The three phase system has only two types of connections i.e stat connection and delta

Connection, why?

In the three phase system, the three phases should be so connected that we get a systemthat has equal line voltages displaced 120 degrees electrical from one another and actingsimultaneously in the circuit .Only star and delta connections can satisfy these conditions.Anyattempt made to connect the three phases in a manner other than these two schemes will resultin a system of line voltages which are neither equal nor 1200 displaced. such a system will notserve the function as if three single phase circuits being connected

6. What are the advantages of 3-phase system?

The advantages of 3-phase system over a single phase system are as underi) The amount of conductor material needed to transfer same amount of power is lesser for

three phase system-thus it is more economicalii) Domestic power and industrial power can be provided from the same sourceiii) Voltage regulation of three phase system is better

7. What is the phase sequence and give the importance of phase sequence in a 3-phase

Circuits?

The order in which the voltages in the three phase reach their maximum positive values iscalled phase sequencePhase sequence is importance in 3-phase circuits because it determines

i) The direction of rotation of 3-phase motorsii) Whether one 3-phase system can be connected in parallel with another


9/10

8. Write the relations between voltages, currents and powers in and Y connected Systems

Line currents :The currents flowing in the lines are called line currents Phase currents: The currents flowing through phases are called phase currents Phase voltage : The voltage across one phase and neutral is called phase voltage Line voltage:The voltage between two lines is called line voltage

CosIV

CosIVP

II

VV

Phph

LL

PhL

PhL

3

3

3

CosIV

CosIVP

II

VV

Phph

LL

PhL

PhL

3

3

3

9. How can you reverse the phase sequence of a 3-phse system?

The phase sequence can be reversed by simply interchanging any two conductors of a 3-phLine

10. What are the advantages and disadvantages of Two wattmeter method for the

measurement of 3-ph power?

Advantages:

1) This method is applicable for balanced as well as unbalanced loads2) Neutral point for star connected load is not necessary to connect the wattmeters3) The delta connected load,need not be opened for conncting the wattmeters

4) Only 2 wattmeters are sufficient to measure total 3-phase power5) If the load is balanced not only the power but power factor also can be determinedDisadvantages:

1) Not applicable for three phase ,4 wire system2) The signs of W1 and W2 must be identified and note down correctly otherwise it may

lead to the wrong results11. Comparison between star(Y) and delta() connections

Star connection Delta connection

1.VL=3 Vph 1.VL=Vph2.IL=Iph 2.IL=3Iph

3.Line voltage leads the respective phasevoltage by 300 3.Line current lags behind the respective phasecurrent by 300

4.Power in star connection is one-third of powerin delta connection

4.Power in delta connection is 3 times of thepower in star connection

5.Three phase,three wire and three phase fourwire systems are possible

5. Only three phase three wire system ispossible

6.The phasor sum of all the phase currents iszero

6.The phasor sum of all the phase voltages isZero


10/10

12. What is the difference between balanced load and unbalanced load?

The load is said to be balanced when magnitudes of all the impedances are equal and thephase angles of all of them are equal and of same nature either all inductive or all capacitiveor all resistiveThe unbalanced load is the load in which the load impedances are not same but havingdifferent values. The values of voltages and currents are also different in each phase

13.What is the purpose of current coil and voltage coil in a Wattmeter?

Current coil : This senses the current and always to be connected in series with the load

Voltage coil: This senses the voltage and always to be connected across the supply terminals

14 .How are the variation in wattmeter readings with power factor(lagging laod)?

Prepared by,P.Venkatesh,Lecturer,Siddhartha Engineering College,Vijayawada.

networksshort2011-12pdf

Documents