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**ELECTRIC CIRCUIT ANALYSIS - I**

Chapter 15 – Series & Parallel ac Circuits Lecture 21 by Moeen Ghiyas 19/04/2017

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**Chapter 15 – Series & Parallel ac Circuits**

TODAY’S lesson

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**Today’s Lesson Contents**

(Parallel ac Circuits) Admittance and Susceptance Parallel ac Networks Current Divider Rule Frequency Response Of The Parallel R-L Network Summary of Parallel ac Circuits 19/04/2017

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**ADMITTANCE AND SUSCEPTANCE**

In ac circuits, we define admittance (Y) as being equal to 1/Z, just as conductance (G) is related to 1/R in dc circuits. The unit of measurement for admittance is siemens (SI system), which has the symbol S. Admittance is a measure of how well an ac circuit will admit, or allow, current to flow in the circuit. The larger the value of admittance, the heavier the current flow for the same applied potential.

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**ADMITTANCE AND SUSCEPTANCE**

The total admittance of a circuit can also be found by finding the sum of the parallel admittances. The total impedance ZT of the circuit is then 1/YT

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**ADMITTANCE AND SUSCEPTANCE**

We know that conductance is the reciprocal of resistance, and The reciprocal of reactance (1/X) is called susceptance and is a measure of how susceptible an element is to the passage of current through it. Susceptance is also measured in siemens and is represented by the capital letter B.

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**ADMITTANCE AND SUSCEPTANCE**

For the inductor, . where . thus For the capacitor,

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**ADMITTANCE AND SUSCEPTANCE**

For parallel ac circuits, the admittance diagram is used with the three admittances, represented as shown in fig Note that the conductance (like resistance) is on the positive real axis, whereas inductive and capacitive susceptances are in direct opposition on the imaginary axis.

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**ADMITTANCE AND SUSCEPTANCE**

For any configuration (series, parallel, series-parallel, etc.), the angle associated with the total admittance is the angle by which the source current I leads the applied voltage E. For inductive networks, θT is negative, whereas for capacitive networks, θT is positive.

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**ADMITTANCE AND SUSCEPTANCE**

EXAMPLE - For the network of fig: Find the admittance of each parallel branch. Determine the input admittance. Calculate the input impedance. Draw the admittance diagram. Solution:

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**ADMITTANCE AND SUSCEPTANCE**

EXAMPLE - For the network of fig: Find the admittance of each parallel branch. Solution:

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**ADMITTANCE AND SUSCEPTANCE**

EXAMPLE - For the network of fig: Determine the input admittance. Calculate the input impedance. Solution: .

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**ADMITTANCE AND SUSCEPTANCE**

EXAMPLE - For the network of fig: Draw the admittance diagram. Solution:

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PARALLEL ac NETWORKS Step 1 – Determine the total impedance or admittance Step 2 – Using Ohm’s Law Calculate source current Calculate branch currents Step 3 – Apply KCL, Step 4 – Calculate Power,

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**PARALLEL ac NETWORKS Example R-L-C Circuit**

Step 0 - Converting to phasor notation

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PARALLEL ac NETWORKS Example R-L-C Circuit Step 1 – YT and ZT

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PARALLEL ac NETWORKS Example R-L-C Circuit Step 2 – Admittance Diagram

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PARALLEL ac NETWORKS Example R-L-C Circuit Step 3 – Find I, IR,IL, IC

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**PARALLEL ac NETWORKS Example R-L-C Circuit**

Step 4 – Apply KCL at nodes to verify the results or find unknown current Step 5 - Make Phasor Diagram Note that the impressed voltage E is in phase with current IR through resistor, leads current IL through inductor by 90°, and lags current IC of capacitor by 90°.

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**PARALLEL ac NETWORKS Example R-L-C Circuit**

Step 6 – Convert V and I to time domain

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PARALLEL ac NETWORKS Example R-L-C Circuit Step 7 – Sketch waveform

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**PARALLEL ac NETWORKS Example R-L-C Circuit Step 8 – Calculate power or**

Step 9 – Power factor

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CURRENT DIVIDER RULE The basic format for the current divider rule in ac circuits is exactly the same as that for dc circuits

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CURRENT DIVIDER RULE EXAMPLE - Using the current divider rule, find the current through each parallel branch of Fig Solution:

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CURRENT DIVIDER RULE EXAMPLE - Using the current divider rule, find the current through each parallel branch of Fig

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**FREQUENCY RESPONSE OF PARALLEL R-L NETWORK**

Let us now note the impact of frequency on the total impedance and inductive current for the parallel R-L network The fact that the elements are now in parallel, the element with the smallest impedance will have the greatest impact on total impedance ZT at that frequency

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**FREQUENCY RESPONSE OF PARALLEL R-L NETWORK**

XL is very small at low frequencies compared to R, establishing XL as the predominant factor in the specified frequency range In other words, at low frequencies the network will be primarily inductive, and the angle associated with the total impedance will be close to 90°, as with a pure inductor.

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**FREQUENCY RESPONSE OF PARALLEL R-L NETWORK**

As the frequency increases, XL will increase and at particular frequency it equals the impedance of the resistor in circuit which can be determined in the manner: XL = R, thus πf2L = R

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**FREQUENCY RESPONSE OF PARALLEL R-L NETWORK**

Plots of θT versus frequency also reveals the transition from an inductive network to resistive characteristics.

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**FREQUENCY RESPONSE OF PARALLEL R-L NETWORK**

A series R-C and a parallel R-L network will have an impedance level that approaches the resistance of the network at high frequencies. The (R-C series) capacitive circuit approaches the level from above, whereas the (R-L parallel) inductive network does the same from below. Conversely, for the series R-L circuit and the parallel R-C network, the total impedance will begin at the resistance level and then display the characteristics of the reactive elements at high frequencies.

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**SUMMARY: PARALLEL ac NETWORKS**

The total admittance (impedance) will be frequency dependent. The impedance of any one element can be less than the total impedance (recall that for dc circuits the total resistance must always be less than the smallest parallel resistor). Depending on the frequency applied, the same network can be either predominantly inductive or predominantly capacitive. At lower frequencies the inductive elements will usually have the most impact on the total impedance, while at high frequencies the capacitive elements will usually have the most impact. The magnitude of the current through any one branch can be greater than the source current.

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**SUMMARY: PARALLEL ac NETWORKS**

The magnitude of the current through an element is directly related to the magnitude of its impedance The current through a coil is always in direct opposition with the current through a capacitor on a phasor diagram. The applied voltage is always in phase with the current through the resistive elements, leads the current through all the inductive elements by 90°, and lags the current through all capacitive elements by 90°. The smaller the resistive element of a network compared to the net reactive susceptance, the closer the power factor is to unity.

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**Summary / Conclusion Admittance and Susceptance (Parallel ac Circuits)**

Parallel ac Networks Current Divider Rule Frequency Response Of The Parallel R-L Network Summary of Parallel ac Circuits

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19/04/2017

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