Ch.9 Sinusoids and Phasors

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Ch.9 Sinusoids and Phasors
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1. Introduction

• AC is more efficient and economical to transmit
  over long distance
• Sinusoid is a signal that has the form of the
  sine or cosine function
• Sinusoidal current alternating current (ac)
• Nature is sinusoidal
• Easy to generate and transmit
• Any practical periodic signal can be represented
  by a sum of sinusoids
• Easy to handle mathematically

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2. Sinusoids

• Consider the sinusoidal voltage
• T period of the sinusoid

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Sinusoids (2)

• Periodic function
• Satisfies f(t) f(tnT), for all t and for all
  integers n
• Hence
• Cyclic frequency f of the sinusoid

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Sinusoids (3)

• Let us examine the two sinusoids
• Trigonometric identities

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Sinusoids (4)

• Graphical approach
• Used to add two sinusoids of the same frequency
• where

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Example 9.1

• Find the amplitude, phase, period, and frequency
  of the sinusoid

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Example 9.2

• Sol)

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3. Phasors

• Phasor is a complex number that represents the
  amplitude and phase of a sinusoid
• Provides a simple means of analyzing linear
  circuits excited by sinusoidal sources
• Complex number
• with

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Phasors (2)

• Operations of complex number
• Addition
• Subtraction
• Multiplication
• Division
• Reciprocal
• Square Root
• Complex Conjugate

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Phasors (3)

• Eulers identity
• with
• Given a sinusoid
• Thus, where
• Plot of the

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Phasors (4)

• Phasor representation of the sinusoid v(t)

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Phasors (5)

• Derivative integral of v(t)
• Derivative of v(t)
• Phasor domain representation of derivative v(t)
• Phasor domain rep. of Integral of v(t)

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Phasors (6)

• Summing sinusoids of the same frequency
• Differences between v(t) and V
• v(t) is time domain representation, while V is
  phasor domain rep.
• v(t) is time dependent, while V is not
• v(t) is always real with no complex term, while V
  is generally complex
• Phasor analysis
• Applies only when frequency is constant
• Applies in manipulating two or more sinusoidal
  signals only if they are of the same frequency

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Example 9.3

• Evaluate these complex numbers
• Sol)
• a)
• then
• Taking the square root

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Example

• Example 9.4
• Transform these sinusoids to phasors
• Example 9.5
• Find the sinusoids represented by these phasors

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Example

• Example 9.6
• Example 9.7
• Using the phasor approach, determine the current
  i(t)

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4. Phasor Relationships for Circuit Elements

• Voltage-current relationship
• Resistor ohms law
• Phasor form
• Inductor
• Phasor form

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Phasor Relationships for Circuit Elements(2)

• Inductor
• The current lags the voltage by 90o.
• Capacitor
• Phasor form
• The current leads the voltage by 90o.

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Example 5.6

• The voltage v12cos(60t45o) is applied to a 0.1H
  inductor. Find the steady-state current through
  the inductor
• Sol)
• Converting this to the time domain,

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5. Impedance and Admittance

• Voltage-current relations for three passive
  elements
• Ohms law in phasor form
• Imdedance Z of a circuit is the ratio of the
  phasor voltage to the phasor current I, measured
  in ohms
• When ,
• When ,

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Impedance and Admittance (2)

• Impedance Resistance j Reactance
• where
• Adimttance Y is the reciprocal of impedance,
  measured in siemens (S)
• Admittance Conductance j Susceptance

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Example 9.9

• Find v(t) and i(t) in the circuit
• Sol)
• From the voltage source
• The impedance
• Hence the current
• The voltage across the capacitor

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6. Kirchhoffs law in the frequency domain

• For KVL,
• Then,
• KVL holds for phasors
• KCL holds for phasors
• Time domain
• Phasor domain
• KVL KCL holds in frequency domain

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7. Impedance Combinations

• Consider the N series-connected impedances
• Voltage-division relationship

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Impedance Combinations (2)

• Consider the N parallel-connected impedances
• Current-division relationship

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Example 9.10

• Find the input impedance of the circuit with w50
  rad/s
• Sol)
• The input impedance is

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Example 9.11

• Determine vo(t) in the circuit
• Sol)
• Time domain ? frequency domain
• Voltage-division principle