Presentation on theme: "Electrical Systems 100 Lecture 3 (Network Theorems) Dr Kelvin."— Presentation transcript:
1 Electrical Systems 100Lecture 3(Network Theorems)Dr Kelvin
2 Contents Thevenin’s Theorem Norton’s Theorem Superposition TheoremThevenin’s TheoremNorton’s TheoremMaximum Power Transfer TheoremMillman’s TheoremReciprocity Theorem
3 Superposition Theorem The Superposition theorem is very helpful in determining the voltage across an element or current through a branch when the circuit contains multiple number of voltage or current sourcesOne big advantage is that we do not have to use Cramer’s rule or complicated mathematical operations but simply algebraically adding solutions obtained from analysing the network with one source activated at a time
4 Superposition Theorem The superposition theorem states that:“The current through, or voltage across, an element in a linear bilateral network equal to the algebraic sum of the currents or voltages produced independently by each source”In general number of networks to be analysed is equal to the number of sources; however, it may be possible to treat the effect of two sources at a time to reduce the number of network to be analysed.
5 Superposition Theorem In removing voltage sources from the network, the voltage source is replaced by a short circuit (potential difference between the two points set to zero)In removing a current source from the network, the current source is replaced by an open circuit between the two points (current set to zero)In doing so, the internal resistance of the voltage sources and shunt resistance of current sources are preserved in the network as it was in the original network.All dependent sources must be left intact as they are controlled by circuit variables
6 Removing the effect of ideal sources Current source is replaced by a O/CVoltage source is replaced by a S/CRemoving the effect of practical sources
7 Dependent Source(a) Dependent Voltage SourceA voltage source whose parameters are controlled by voltage/current else where in the systemv = ρixCDVS(Current DependentVoltage source)v = µVxVDVS(Voltage DependentVoltage source)(b) Dependent Current SourceA voltage source whose parameters are controlled by voltage/current else where in the systemv = βixCDCS(Current DependentCurrent source)v = αVxVDCS(Voltage DependentCurrent source)For Superposition, All dependent sources must be left intact!!You can’t apply O/C and S/C on dependent sources
8 An ExampleFind i0 in the circuit shown below. The circuit involves a dependent source. The current may be obtained as by using superposition as :i’0 is current due to 4A current sourcei’’0 is current due to 20V voltage source
9 To obtain i’0we short circuit the 20V sources For loop 1i3For loop 2For loop 3For solving i1, i2, i3
10 To obtain i’’0 , we open circuit the 4A sources For loop 4i4For loop 5i5For solving i4 and i5
11 Superposition is not applicable to Power The superposition theorem does not apply to power calculations as the power is proportional to current squared or voltage squared. Consider the following :The total power must be determined using the total current not by superposition
12 Thevenin’s TheoremIt often occurs in practice that a particular element in a circuit is variable while the rest is fixed. Consider the household GPO which may be connected to various appliances. Each time a different appliance is connected the entire circuit may be required to analyse. To avoid this , Thevenin’s theorem gives a technique where the fixed part of the circuit is represented by an equivalent circuit VTH and RTH as shown:VOCVOC
13 Thevenin’s TheoremThevenin’s theorem states that a linear two terminal circuit can be replaced by an equivalent circuit consisting of a voltage source VTH in series with a resistor RTH where VTH is the open circuit voltage at the terminals a-b and RTH is the input or equivalent resistance looking at the terminals when all independent sources in the network are turned off (Voltage sources set to zero and current sources are open circuited)RthRth=8ΩVth=20VVth
14 Thevenin’s TheoremIf the circuit has dependent sources, then we need to turn off all independent sources but not the dependent sources like superposition theorem. In this case RTH can be determined as:Case 1: Applying a known voltage source v0 and measuring i0 at the terminals. The RTH is given by vo/i0.Case 2: Applying a known current source i0 and measuring v0 and then RTh is given by v0/i0
15 Thevenin’s TheoremThe load current can then be obtained as:
16 Thevenin’s Theorem Example 1 Find the Thevenin equivalent circuit of the shaded area in the bridge network shown below.Calculate VTh:Calculate the open circuit voltage across terminal a bCalculate RTH:Open circuit the current source and short circuit the voltage sourceCalculate the total resistor across terminal a b
17 Thevenin’s Theorem-An Example VTh is the open circuit voltage across a and b.VTh is calculated as:Applying KVL we get,
18 Thevenin’s Theorem Finding RTh: Short circuiting the voltage source we get the RTh as:
20 Thevenin’s TheoremExample 2Find the Thevenin equivalent circuit with respect to the terminal a and b.Finding RTh:Applying test voltage
21 Thevenin’s Theorem Finding RTh: All independent sources set to zero ITAll independent sources set to zeroApply the test voltage VTUsing NodeSubstitute Eq2 into Eq1
22 Finding VTh, Open circuit Substituting ix into the first equationVTh = 8V
23 Norton’s TheoremWe have seen earlier that every voltage source with an internal resistance has a current source equivalent. The current source equivalent of the Thevenin’s equivalent network is the Norton’s equivalent network and is determined by Norton’s Theorem.Norton’s Theorem states that:Any two terminal linear bilateral dc network can be replaced by an equivalent circuit consisting of a current source IN and a parallel resistance RN
24 Norton’s TheoremFigure below show a Linear two terminal network and its Norton’s equivalent. In the Figure it is the terminals a-b across which the Norton equivalent is to be found.INIs/cIs/c
25 Norton’s Theorem Steps to determine Norton’s equivalent: Remove the portion of the network across which Norton equivalent is to be foundCalculate RN by setting all voltage sources to zero and current sources to open circuit but keeping all internal series and shunt resistances intact. Keep all dependent sources in the circuit like superposition theorem as well. You will, note that RN = RThCalculate IN by returning all sources to their original positions and then finding current through the short circuited terminals a-b as mentioned before.Draw the Norton equivalent circuit with IN as current source and RN as parallel resistor and the portion of the circuit returned between the terminals a-b.
26 Norton’s Theorem-An Example Find the Norton equivalent circuit for the portion of the network to the left of a-b in Figure given below?Identifying the terminals of interest for Norton’s equivalent
27 Norton’s Theorem-An Example Finding RN :Finding IN:
28 Norton’s Theorem-An Example Using Superposition Theorem,Now find the contribution to IN from the current source :Looking at circuit below,
29 Norton’s Theorem-An Example The Norton equivalent circuit is then:
30 Maximum Power Transfer Theorem The maximum power transfer theorem states that:A Load will receive maximum power from a linear bilateral dc network when its total resistance value is exactly equal to the RTH of the networkMaximum power transfer is extremely important for maximum efficiency of a transmission and distribution network of an electric utility such as Western Power. The theorem also find application in electronic circuits such as matching input impedance of a speaker system to the output impedance of an amplifier.
31 Maximum Power Transfer Power is max when RL = RSRL = RS
33 Maximum Power Transfer Theorem Designing a Speaker System for your Amplifier:Consider an amplifier and a speaker and their equivalent circuit as below:
34 Maximum Power Transfer Theorem In the first circuit the power delivered to the speaker is 4.5 WattsIn the second circuit, the power delivered to each speaker is 2 WattsIn the third circuit power delivered to each speaker is also 2 Watts! Which one is better arrangement?
35 Maximum Power Transfer Theorem In the third circuit power delivered to each speaker is also 2 Watts! Which one is better arrangement?
36 Maximum Power Transfer Theorem Speakers are also available with 4 Ohm and 16 Ohm input impedance. You can use them to obtain an input impedance equal to 8 ohm to match the amplifier’s output impedance as below: