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Friday, 30 March 2012

Sinusoidal Steady-State Analysis

Three men are my friends --- he that loves me, he that hates me, that is indifferent of me. Who loves me, teaches me tenderness; who hates m, teaches me caution; who is indifferent to me, teaches me self-reliance.                                                                                                                          -J. E. Dinger
Introduction

In this chapter, we want to see how nodal analysis, mesh analysis, Thevenin's theorem, Norton's theorem, superposition, and source transformations are applied in analyzing ac circuits. Since these techniques were already introduced for dc circuits, our major effect here will be to illustrate with examples.
     Analyzing ac circuits usually requires three steps.





Steps to Analyze AC circuits:

  1. Transform the circuits 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.


Step 1 is not necessary if the problem is specified in the frequency domain. In step 2, analysis is performed in the same manner as dc circuit analysis except that complex numbers are involved. Having done and learned Chapter 9, we are adept at handling step 3.



10.2 Nodal Analysis


     Nodal analysis is more commonly used than mesh or loop analysis for analysing
networks.  It can be used to determine the unknown node voltages of both planar
and non-planar  circuits.   Nodal equations are usually formed by applying
Kirchhoff’s Current Law to the nodes with unknown voltages, whereas equations
based on Kirchoff’s Voltage Law are used to form the mesh equations. In order to
apply nodal analysis to a circuit, the first step is to select a reference node or
datum node and then assign a voltage at each of the other nodes with respect to
the reference node.  In a circuit with dc sources, the node that has the lowest
voltage is usually selected as the reference node and then the other node voltages
would be positive.  Often the node that has the maximum elements connected to
it in a circuit tends to be the reference node.  Many electronic circuits have a
metallic chassis and the reference node, usually  the negative terminal of the dc
source present in the circuit, happens to be connected to the chassis.  It is  common
practice to connect the chassis to earth terminal of the utility supply.  Then the
reference node is at earth or ground potential and hence the reference terminal is
referred to as the ground terminal, even if it is not earthed.   In a power system, the
casing of power appliances is usually earthed and the neutral of the utility supply
remains connected to earth at the source.  The reference node in such a power
system is then at zero potential. 
In nodal analysis, the voltage at the reference node is assumed to be zero.  The
voltages at other nodes are expressed with reference to the datum node.  Since the
unknown node voltages are determined by nodal analysis, it is logical to write the
KCL equations at nodes.  The procedure can be summarized as follows.
  • Select a reference node and treat it to be at zero or ground potential.
  • Label the nodes with unknown voltages.
  • At each of these nodes, mark currentsin the elements asflowing away from the node.
  • Form KCL equations and solve the set of simultaneous equations for theunknown voltages.
     The voltage across an element can be expressed as the difference of  voltages at
nodes to which it is connected.  Use the passive sign convention and mark  the
currents to be flowing away from the node at which the KCL is applied.  Then the
mutual conductance term would have a negative sign whereas the self-conductance
at a node would be a positive value.  This aspect should become clear after a few
examples.  It is worth stressing that labelling the nodes and assigning current
direction properly are really important.  The practice of this technique would
enable one to write the KCL equations and the matrix equations by inspection.
The basis of nodal analysis is Kirchhoff's current law. Since KCL is valid for phasors, as demonstrated in Section 9.6 we can analyze ac circuits by nodal analysis. 

     This chapter has shown nodal analysis can be applied to simple circuits and   to
circuits with dependent sources. The next chapter describes the sources or signals
used for exciting an electrical circuit and their properties


10. 3 Mesh Analysis


   

      Kirchhoff's voltage law (KVL) forms the basis of mesh analysis. The validity of KVL for ac circuits was shown in Section 9.6 and is illustrated in the following examples. Keep in mind that the very nature of mesh analysis is that it is to be applied to planar circuits. 

    The two general analytical techniques used in network analysis are mesh or loop
analysis and nodal analysis.  This chapter describes mesh analysis, whereas the
next chapter is on nodal analysis. 
        
     Mesh analysis is a systematic technique to evaluate all voltages and currents in a
circuit.  It is based on Kirchoff’s Voltage Law and Ohm’s Law. This technique can
be applied to ac circuits also.  The way to form meshes or loops is described first,
followed by an example. Then a set of simultaneous equations are formed, with the
number  of these equations equaling the  number  of unknown variables to be
determined.  These unknown variables are the loop currents or the mesh currents.
The simultaneous equations can be compactly described by matrix equations, and
the solution to matrix equations can be obtained using Cramer's rule. Gaussian
elimination method can also be used to obtain the solution.  Solution by hand can
be laborious if the number of unknowns exceeds three. Some examples are
presented, to illustrate the technique of mesh analysis.  Since it is essential to be
familiar with passive sign convention, it is explained first.

    This chapter has introduced mesh analysis to resistive circuits with dependent and

independent sources.  Given a circuit with dependent sources, it is difficult to
specify a particular technique for forming the necessary equations. The number
of independent equations to be formed should equal the number of unknowns.  In
that case, the square matrix would have a non-zero determinant and then the
solution is unique.  The next chapter describes how nodal analysis can be applied
to such circuits.



10. 4  Superpositon Theorem


    Since ac circuits are linear, the superposition theorem applies to ac circuits the same way it applies to dc circuits. The theorem becomes important if the circuit has sources operating at different frequencies. In this case, since the impedances depend on frequencies, we must have different frequency domain circuit for each frequency. The total response must be obtained by adding individual responses in the time domain. It is incorrect to try to add the responses int the phasor or frequency domain.Why? Because the exponential factor e^jwt is implicit in sinusoidal analysis, and that factor would change for every angular frequency w. It would therefore not make sense to add response at different operating at different frequencies, one must add the response due to the individual frequencies in the time domain.

Importance of Superposition Theorem

     Network theorems provide insight into the behaviour and properties of electrical
circuits. Superposition theorem is  of theoretical importance, because it is
fundamental to linear circuit analysis. A circuit is linear only when it behaves in
accordance with superposition theorem.   This theorem states that the linear
responses in a circuit can be obtained as the algebraic sum of responses, due to
each of the independent sources acting alone. Thistheorem defines the behaviour
of a linear  circuit.  Within the context of linear circuit analysis, this theorem
provides the basis for all other theorems.  Given a linear circuit, it is easy to see
how mesh analysis and nodal analysis make use of the principle of superposition.

Properties of Superposition Theorem
    There are two guiding properties of superposition theorem.  The first is the
property of homogeneity or proportionality, and the second is the property of
additivity.     

Limitations of Superposition Theorem

    As stated earlier, the linear responses in a circuit  can be obtained using this
theorem as the algebraic sum of responses, due to each of the independent sources
acting alone.  Current and voltage associated with an element are linear responses.
On the other hand, power in an element is not a linear response. It is a non-linear
function, varying proportionately either with  the square of voltage across the
element or with the square of current through the element. Hence it is not possible
to apply superposition theorem directly to determine power associated with an
element.  In addition, application of superposition theorem does not normally leadto simplification of analysis.  It is not the best technique to determine all currents
and voltages in a circuit, driven by multiple of sources.    

Relationship with Mesh and Nodal Analysis

    Superposition theorem is valid for linear circuits and analysis of linear circuits is
relatively easy.  On the other hand, the principle of superposition is not valid
for non-linear circuits. And the analysis of non-linear circuits is quite complex
complex and difficult.  It is possible to apply mesh and nodal analysis to nonlinear  circuits.  However, within the  context of linear  circuits, mesh or nodal
analysis of a circuit  illustrates how the principle  of superposition is ever so
pervasive in defining the behaviour of linear circuits.   


    The analysis of linear circuits is founded on the superposition theorem.  Eventhough the direct use of superposition theorem is not always easy, it remains the
guiding principle for the behaviour of linear circuits.  Next we take up Thevenin’s
theorem.


10.5 Thevenin's Theorem


   Thevenin’s theorem is a popular theorem, used often for analysis of electronic
circuits.  Its theoretical value is due to the insight it offers about the circuit. This
theorem states that a linear circuit containing one or more sources and other linear
elements can be represented by a voltage source and a resistance. Using this
theorem, a model of the circuit can be developed based on its output characteristic.
Let us try to  find  out what Thevenin’s  theorem is  by using an investigative
approach.
   Thevenin's theorem are applied to ac ciruits in the same way as they are to dc circuits. The only additional effort arises from the need manipulate complex numbers. the frequency domain version of a Thevenin equivalent circuit, where a linear is circuit is replaced by a voltage source in series with an impedance. The Thevenin theorem equivalent circuit must be detemined at eacg frequency. This leads to entirely different equivalent circuits, one for each frequency, not one equivalent circuit with equivalent sources and equivalent impedance.


10.6 Superposition Theorem

   
    The circuits in this set of problems consist of independent sources, resistors and a meter. In 
particular, these circuits do not contain dependent sources. Each of these circuits has a seriesparallel structure that makes it possible to simplify the circuit by repeatedly   
•  Performing source transformations. 
•  Replacing series or parallel resistors by an equivalent resistor.  
•  Replacing series voltage sources by an equivalent voltage source. 
•  Replacing parallel current sources by an equivalent source source. 

    Each simplification is done in such a way that the voltage or current measured by the meter is not 
disturbed. Generally, that requires beginning the simplification at the opposite end of the circuit 
from the meter and then working toward the meter.  

   Eventually, the circuit is small enough to be easily solved using Ohm’s and Kirchhoff’s Laws. 


10.7 Summary


  1. We apply nodal and mesh analysis to ac circuits by applying KCL and KVL to the phasor form of the circuits.
  2. In solving for the steady state response of a circuit that has independent sources with different frequencies, each independent source must be considered separately. The most natural approach to analyzing such circuits is t The concept of source transformation is also applicable in the frequency domain. o apply the superposition theorem. A separate phasor circuit for each frequency must be solved independently. and corresponding response should be obtained in the time domain. The overall response is the sum of the time domain responses of all the individual phasor circuit.
  3. The Thevenin equivalent of an ac circuit consists of a voltage source Vth in series with the Thevenin impedance Zth.
  4. The concept of source transformation is also applicable in the frequency domain.

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