NODAL ANALYSIS IN AC CIRCUIT

NODAL ANALYSIS

The basis of nodal analysis is Kirchoff's current law. Since KCL is valid for phasors, we can also analyze ac circuits using nodal analysis.

This are some steps to analyze AC circuits:

1. Transform the circuit to the phasor or frequency domain.

2.Solve the problem using circuit techniques (nodal analysis, mesh analysis, superposition, etc)

3. Transform the resulting phasor to the time domain.

Example:

Find Ix in the circuit using nodal analysis

We first convert the circuit to the frequency domain:

20cos4t    => 20∠0 , Ѡ=4rad/s

1H   =>   JѠL=J4

0.5H  => JѠL=J2

0.1F => 1/JѠC= -j2.5

The frequency domain equivalent circuit is as shown in figure

Applying KCL at node A

20-Va/10= Va/-j2.5 + Va-Vb/J4

or

(1+J1.5)Va+J2.5Vb=20

At node B

2Ix+Va-Vb/J4=Vb/J2

Ix=Va/-j2.5 then substitute

2Va/-j2.5+Va-Vb/J4=Vb/j2

By simplifying, we get

11Va+15Vb=0

Then Equation 1 and equation 2 can be put in matrix form as shown 

|1+j1.5              j2.5| |v1|=|20|

           |11                  11   |  |v2|=|0|             

where delta =15-j5 as i computed using matrix form

and Delta1 equals to -220 as i computed using cramers rule.

where v1=delta1/delta=300/15-j5=18.97angle18.43 V

v2=delta2/delta=-220/15-j5=13.91angle198.3 V

The current Ix is given by

Ix=V1/-j2.5=18.97angle18.43/2.5angle-90

=7.59angle108.4 A

Transforming this to the time domain,

ix=7.59cos(4t+108.4) A

PHASORS

PHASORS

-A Phasor is a complex number that represents the amplitude and phase of a sinusoid.


Phasors provide a simple means of analyzing linear circuits excited by sinusoidal sources; solutions of such circuits would be intractable otherwise. The notion of solving ac circuits using phasors was first introduced by charles Steinmetz in 1893. Before we completely define phasors and apply them to circuit analysis, we need to be throughly familiar with complex numbers.

A  complex number z can be written in rectangular form as:

                                             z=x+jy

where j=√-1; x is the real part of z; y is the imaginary part of z. In this context, the variables x and y do not represent a location as in two-dimensional vector analysis but rather the real and imaginary parts of z in the complex plane. Nevertheless, we note that there are some resemblances between manipulating complex numbers and manipulating two-dimensional vectors.

The complex number z can also be written in polar or exponential form as:

                                         z=r∠Φ
where r is the magnitude of z, and Φ is the phase of z. We notice that z can be represented in three ways:

                z=x+jy                     z=r∠Φ               z=re^jΦ



the relationship between the rectangular form and the polar form is shown in fig, 9.6, where the x axis represents the real part and the y axis represents the imaginary part of a complex number. Given x and y, we can get r amd Φ as

                              r=√󠄈x^2+y^2,                Φ=tan^-1 (y/x)



On the other hand, if we know r and Φ, we can obtain x and y as

                            x=rcosΦ             y=rsinΦ

Mathematical operations of complex numbers:

ADDITION

Z1+Z2=(X1+X2) + J(y1+y2)

SUBTRACTION

z1-z2=(x1-x2)+(y1-y2)

MULTIPLICATION

z1z2= r1r2∠Φ1+Φ2

DIVISION

z1/z2=r1/r2∠Φ1-Φ2

RECIPROCAL

1/z=1/r∠-Φ

SQUARE ROOT

square root of z=square root of r∠Φ/2

COMPLEX CONJUGATE

z*=x-jy=r∠-Φ

      

                                          

SINUSOIDS AND PHASORS

SINUSOIDS AND PHASORS

Sinusoids

- A Sinusoid is a signal that has the form of the sine and cosine function. Sinusoidal variables are of special importance in electrical and electronic systems, not only because the occur frequently in such systems but also because any periodical signal can be represented as a linear combination of a set of sinusoidal signals of different frequencies, amplitudes, and phase angles.





Sinusoidal variable can be written as:




v(t) = Vm sin(ωt + Ф)

Where;

Vm = the amplitude of the sinusoid

ω = the angular frequency in radians/s

Ф = the phase









Example 1:

Given a sinusoid, i=3sin(3t + 70) , calculate its amplitude, phase, angular frequency

Solution:

Amplitude = 3, phase =70,angular frequency = 3




Example 2:

A sinusoidal current with a frequency of 60 Hz reaches a positive maximum of 20A at . Give the expression of this current as a function of time .
Solution:
We have , , , . As cosine function reaches maximum when (or ), the phase angle should satisfy where and , i.e.,














The current is














Alternatively, the phase angle can be found as shown below:














Solving this we get , same as above.

Example 3:

Find the phase angle between i1=-4sin(377t + 25) and i2=5cos(377t – 40), does i1 lead or lag i2?

Solution:

Add 180 @ i1:

i1=-4sin(377t + 25 + 180)

i1=4cos(377t + 205)

Change: cos to sin (add 90)

Since sin(ωt + 90) = cosωt

i2=5cos(377t – 40+90)

i2=5cos(377t +50)

therefore, i1 leads i2 155o(Phase Angle)
Complex Numbers

The mathematics used in Electrical Engineering to add together resistances, currents or DC voltages uses what are called “real numbers”. But real numbers are not the only kind of numbers we need to use especially when dealing with frequency dependent sinusoidal sources and vectors. As well as using normal or real numbers, Complex Numbers were introduced to allow complex equations to be solved with numbers that are the square roots of negative numbers, √-1.

In electrical engineering this type of number is called an “imaginary number” and to distinguish an imaginary number from a real number the letter “ j ” known commonly in electrical engineering as the j-operator, is used. The letter j is placed in front of a real number to signify its imaginary number operation. Examples of imaginary numbers are: j3, j12, j100 etc. Then a complex number consists of two distinct but very much related parts, a “ Real Number ” plus an “ Imaginary Number ”.