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Impedance and Admittance

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Objective of Lecture Demonstrate how to apply Thévenin and Norton transformations to simplify circuits that contain one or more ac sources, resistors, capacitors, and/or inductors.

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Source Transformation A voltage source plus one impedance in series is said to be equivalent to a current source plus one impedance in parallel when the current into the load and the voltage across the load are the same.

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Equivalent Circuits Thévenin V th = I n Z n Norton I n = V th /Z th

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Example 1 First, convert the current source to a cosine function and then to a phasor. I1 = 5mA sin(400t+50 o ) = 5mA cos(400t+50 o -90 o )= 5mA cos(400t-40 o ) I1 = 5mA -40 o

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Example 1 (con’t) Determine the impedance of all of the components when rad/s. In rectangular coordinates

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Example 1 (con’t) Convert to phasor notation

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Example 1 (con’t)

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Find the equivalent impedance for Z C1 and Z R1 in series. This is best done by using rectangular coordinates for the impedances.

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Example 1 (con’t) Perform a Norton transformation.

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Example 1 (con’t) Since it is easier to combine admittances in parallel than impedances, convert Z n1 to Y n1 and Z L1 to Y L1. As Y eq2 is equal to Y L1 + Y n1, the admittances should be written in rectangular coordinates, added together, and then the result should be converted to phasor notation.

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Example 1 (con’t)

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Next, a Thévenin transformation will allow Y eq2 to be combined with Z L2.

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Example 1 (con’t)

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Perform a Norton transformation after which Z eq3 can be combined with Z R2.

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Example 1 (con’t)

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Use the equation for current division to find the current flowing through Z C2 and Z eq4.

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Example 1 (con’t) Then, use Ohm’s Law to find the voltage across Z C2 and then the current through Zeq4.

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Example 1 (con’t) Note that the phase angles of I n2, I eq4, and I C2 are all different because of the imaginary components of Z eq4 and Z C2. The current through Z C2 leads the voltage, which is as expected for a capacitor. The voltage through Z eq4 leads the current. Since the phase angle of Z eq4 is positive, it has an inductive part to its impedance. Thus, it should be expected that the voltage would lead the current.

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Electronic Response Explain why the circuit on the right is the result of a Norton transformation of the circuit on the left. Also, calculate the natural frequency o of the RLC network.

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Summary Circuits containing resistors, inductors, and/or capacitors can simplified by applying the Thévenin and Norton Theorems. Transformations can easily be performed using currents, voltages, impedances, and admittances written in phasor notation. Calculation of equivalent impedances and admittances requires the conversion of phasors into rectangular coordinates. Use of the current and voltage division equations also requires the conversion of phasors into rectangular coordinates.

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Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 15.1 Alternating Voltages and Currents Introduction Voltage and Current.

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