23031 electrical circuit theory.pdf 1375069393

8/16/2019 23031 Electrical Circuit Theory.pdf 1375069393

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RANGANATHAN POLYTECHNIC COLLEGE

R.E.C Kalvi nagar,Viraliyur (Po) ,

Thondamuthur - (Via) , Coimbatore – 641109.

www.rpccbe.ac.in

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23031-ELECTRICAL CIRCUIT THEORY

Prepared by

D.NIRMAL KUMAR

LECT./EEE

UNIT-1

One Mark Q A


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FUNDAMENTALS OF ELECTRIC CIRCUITS

1. State Ohm’s law.

At constant temperature, the current flow through a conductor is directly

proportional to the potential difference between two ends of the conductor.[V=IR]

2. Define Resistance.

The property of a conductor which opposes the flow of current in it is known as

resistance. It is denoted by the letter “R” and its unit is ohm

3. State the relationship between KW and HP.

1KW=1.33HP (or) 1HP= 0.735KW (735.75watts)

4. Give the expression for the temperature co-efficient of resistance.

α0= (Rt-Ro)/ (Rot)

5. Write the formula for equivalent resistance of three resistors connected in

parallel.

R

-1

=R1-1+

R2-1

+……+Rn-1

6. State the properties of series circuit.

In series circuit the current through all resistors is same but the voltage drop in

each resistor is different.

7. State Kirchhoff’s first &second law.

First law: In a junction, the algebraic sum of incoming currents is equal to the

sum of outgoing currents.

Second law: In a closed circuit, the sum of applied voltage is equal to the sum of

potential drops in that circuit

8. Define Self inductance.


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The property of a coil which oppose any changes in current flow through it known

as self inductance

9. Define Electric flux density.

Electric flux per unit area is known as electric flux density D= ϕ/A

Unit wb/m2

10. Define current and mention its unit.

The flow of free electrons in conductor is known as current.

It is denoted as I. It’s unit is ampere.

11. Write the relationship between voltage, charge and capacitance.

Q=C/V (or) V= (Q/C) where Q=change,v=voltage,c=capacitance.

12. Give the formula for Electrical field intensity in terms of electric flux density

and permittivity.

E=F/Q

13. Write the formula for three capacitors connected in series.

1/c=1/c1+1/c2+1/c3

14. State the formula for energy stored in a capacitor.

E=1/2 cv2 where,c=capacitance,v=voltage.

15. Write the expression for capacitance of parallel plate capacitor.

C=ℰ0ℰr A/dwhere, ℰ0=permeability of air ℰr=relative permeability of medium

A=area D=distance between parallel plates


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16. Calculate the current and resistance of a 100w,200v electric bulb.

I=P/V=100/200=0.5Amps; R=V/I 200/0.5=400 Ω

17. Find the equivalent capacitance in the circuit.

1/c=1/2+1/3+1/6 0.5+0.33+0.17=1 so c=1µfd

18. Find the equivalent resistance (RAB).

R AB= [1/6+1/6+1/6]-1=2 Ω

19. Define Electric field intensity (E).

The electric field intensity at any point is defined as the force on a unit positive

charge placed on that point E=F/Q

20. Write the unit of inductor and capacitor.

Unit of inductor is Hendry &capacitor is Farad.

UNIT-2


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NETWORK THEOREMS

1. What is network?

Network is an arrangement of active and passive elements in an electrical circuit

2. What is node?

Node is a meeting point where two (or) more circuit elements join in a network

3. Define mesh.

Mesh is a closed circuit in a network

4. State the difference between node& junction.

Node: Two elements meeting point is node

Junction:more than two elements meeting point is junction

5. What is branch?

The part of Network placed in between two nodes is called as branch

6. Draw a simple electrical Network.

7. Give the generalized matrix for Nodal voltage Analysis.

11 1221 22 12 =

12

8. Convert the current source into voltage source.


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9.Convert voltage source into current source.

10. Write the formula for star to delta conversion.

R12=12+23+31

3

R23=12+23+31

1

R31=12+23+31

2

11. Write the formula for delta to star conversion.


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R1=12.31

12+23+31

R2=23.12

12+23+31

R3= 23.3112+23+31

12. State Super position theorem.

In an electrical circuit has more than one source the current flow through the load

resistor is equal to the algebraic sum of current flow through its individual source only

acted.

13. What is Maximum power transfer theorem?

In DC circuits the maximum power is transferred to load from source when the

load resistance value is equal to the circuit equivalent resistance [RL=RTH].

14. State the condition for maximum power transferred from source to load.

[RL=RTH]

15. State the power formula for maximum power transferred to load.

Max.power=E2/4RL

16. List the step by step procedure for the thevenin’s equivalent circuit.


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Remove load resistance(RL)

Find open circuit voltage across it(vo)

Calculate equivalent resistance (Rth)

Replace entire network with vo&Rth

Connect (RL)and find current (IL)=

+

17. List the step by step procedure for Norton’s equivalent circuit.

Remove load resistance(RL)

Find the short circuit current after short it(Ish)

Find the equivalent resistance(Rth)

Replace the circuit with ish&Rth

Connect (RL) and find current IL=Ish*

+

18. State the matrix form of mesh current analysis.


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19. Draw Thevenin’s equivalent circuit.

UNIT-3

SINGLE PHASE A.C. SERIES CIRCUITS

1. State the sinusoidal expression for instantaneous value.

Z= I m sin ω t where Im max value of current

2. Define peak factor.

The ratio of maximum value to the r.m.s. value of an alternating quantity is called

peak factor

Peak factor=Max.value/R.M.S value

3. Define frequency.


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No of cycle per second is known as frequency F=1/T in hz

4. Write the formula for impedance in AC circuit.

Z=

2 + (

− )2 (or) Z=V/I

5. Define phase angle.

The angle between voltage and current in AC circuit is known as phase angle(ϕ)

6. State the RMS value of alternating quantity.

Irms=Im/ 2 7. Draw the vector diagram of RLC series circuit.

8. What is the power in pure resistor a.c. circuit?

Power in pure resistor a.c. circuit is VI watts

PR=VI watts

9. Define power factor.


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Cosine value of phase angle is known as power factor (cosϕ )

In an a.c. circuit, the ratio between resistance and impedance is known as power

factor (cosϕ )

10. Write the formula for inductive reactance.

Xl=ω l=2π flΩ where: Xl= inductive reactance f= frequency L=inductance

11. Find the value of capacitor having capacitive reactance of 42.44Ω and supply

frequency is 50hz.

C= 1

2 = 1

2∗∗50∗42.44=75µfd

12. What is the frequency of the alternating quantity having an instantaneous

expression of v=120 sin 628 t.

F=2=

628

2 =100 Hz

13. Convert 3+j4 into polar form.

3+j4=5

14. What is KVA?

KVA is an active power.KVA=kilo volt amp

15. Describe power triangle.

KVA=real power

KW=active power

KVAR=reactive power

16. Write the expression for power in RLC series circuit.


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P=VI COS ϕ

17. Draw the wave form of voltage and current in RL series circuit.

18. Define amplitude (or) peak value.

The maximum +ve or -ve value of an alternating quantity is called amplitude (or)peak value

19. What is cycle in alternating quantity?

Both positive and negative portions of an alternating quantity is known as one

cycle

20. Define form factor.

Form factor=RMS value/Average value

UNIT-4

SINGLE PHASE A.C. PARALLEL CIRCUITS AND RESONANT CIRCUITS

1. State the conditions for series resonance.

XL= Xc where:

XL= inductive reactance

Xc = capacitance reactance

2. Define Q factor.


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Q factor= ( )

=VL/V(or)XL/R

3. Write the expression for resonant frequency in series RLC circuit.

f r = 1

2

4. State the formula for half power frequencies.

F1=fr-R/4π L and f2=fr+R/4π L

5. Define Admittance.

Admittance is defined as the reciprocal of impedance of a.c. circuit

Y=1/z mho

6. What is conductance?

Conductance is reciprocal of resistance in a.c. circuit

G=1/R mho

7. Explain susceptance.

Susceptance is reciprocal of reactance in AC circuit po=1/x mho

8. Draw the admittance triangle.

cos ϕ =G/Y

Y

9. Define dynamic resistance in parallel resonant circuit?


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Dynamic resistance=L/CR

10. What is the power factor of resonant circuit?

Power factor=unity

11. List the methods of solving parallel AC circuit

phasor method

phasor algebra method

admittance method

12. Draw the impedance triangle

Cos ϕ=R/Z

13. State the formula for current in Admittance Method

I=VY amps where: I=current in amps V=voltage in volts Y=admittance in mho

14. What is the formula for power factor in Admittance Method?

Cosϕ=G/Y

15. Draw parallel Resonance circuit.


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UNIT 5

THREE PHASE A.C. CIRCUITS

1. Write three phase power equation.

P= 3 VLIL cosϕ watts

2. What is meant by phase sequence?

The order of voltages reach their maximum is called as phase sequence

3. What are the two types of connections used in 3 phase system?

star connection

delta connection

4. Write the relationship between line and phase voltage in 3 phase star

connection.

VL= 3Vph

5. Write the relationship between line and phase current in 3 phase star

connection.

IL=Iph

6. What is line voltage?

In 3 phase system, the voltage between any two phases are caked line

voltage(VL)

7. What is line current?

In 3 phase system, the current flow through in a line is called as line current(I L)

8. Define phase voltage.


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The voltage between any one line and neutral is called phase voltage (vp)

9. Define phase current.

The current flow through a phase is known as phase current (Iph)

10. Write the relationship between line and phase voltage in 3 phase delta

connection.

VL=Vph

11. Write the relationship between line and phase current in 3 phase delta

connection.

IL= 3Iph

12. Write the expression for finding power and power factor for a 3 phase

balanced load by using two wattmeter methods.

P= 3VLILcos ϕ =W1+W2 watts

ϕ =tan-I

31−21+2

13. What is mean by balanced load?

The magnitude and angle of load impedance in each phase is same is known

balanced load.

14. Define unbalanced load.

The magnitude and angle of load impedance in each phase is not equal is known

un-balanced load.

15. In two wattmeter method, both wattmeter shows positive and equal readings,

what is the power factor of the circuit?

W1=W2, ∴ cosϕ=unity


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Power factor (cosϕ)=1

16. In two wattmeter method, both wattmeter shows equal readings and opposite

direction, what is the power factor of the circuit?

W1=-W2, ∴ cosϕ=0

Power factor (cosϕ)=zero

17. If the readings of two wattmeter method,w1=500w and w2=1000w what is the

power factor of the circuit?

W1=500w : W1=2w2

W2=1000w : ∴ cosϕ =0.866

18. If the readings of two wattmeter method,w1=500w and w2=-500w what is the

power factor of the circuit?

W1=500w : W1=-w2

W2=-500w : ∴ cosϕ =0

19. Write the formula for tanϕ in two wattmeter method.

Tanϕ = 3 1−21+2

20.The power input to a 400v,50hz,3 phase motor is measured by two wattmeter

method which indicates 300kw and 100kw respectively. Find the input power.

Input power = W1+W2

=300kw+100kw

i/p power =400kw

23031 electrical circuit theory.pdf 1375069393

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