Chapter 9 Complex Numbers and Phasors Chapter Objectives

Chapter 9 Complex Numbers and Phasors Chapter Objectives: Ø Understand the concepts of sinusoids and phasors. Ø Apply phasors to circuit elements. Ø Introduce the concepts of impedance and admittance. Ø Learn about impedance combinations. Ø Apply what is learnt to phase-shifters and AC bridges. Huseyin Bilgekul EENG 224 Circuit Theory II Department of Electrical and Electronic Engineering Eastern Mediterranean University EENG 224 1

Complex Numbers Ø A complex number may be written in RECTANGULAR FORM as: Ø A second way of representing the complex number is by specifying the MAGNITUDE and r and the ANGLE θ in POLAR form. Ø The third way of representing the complex number is the EXPONENTIAL form. • x is the REAL part. • y is the IMAGINARY part. • r is the MAGNITUDE. • φ is the ANGLE. EENG 224 2

Complex Numbers Ø A complex number may be written in RECTANGULAR FORM as: forms. EENG 224 3

Complex Number Conversions Ø We need to convert COMPLEX numbers from one form to the other form. EENG 224 4

Mathematical Operations of Complex Numbers Ø Mathematical operations on complex numbers may require conversions from one form to other form. EENG 224 5

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Phasors Ø A phasor is a complex number that represents the amplitude and phase of a sinusoid. Ø Phasor is the mathematical equivalent of a sinusoid with time variable dropped. Ø Phasor representation is based on Euler’s identity. Ø Given a sinusoid v(t)=Vmcos(ωt+φ). EENG 224 7

Phasors Ø Given the sinusoids i(t)=Imcos(ωt+φI) and v(t)=Vmcos(ωt+ φV) we can obtain the phasor forms as: EENG 224 8

Phasors Ø Amplitude and phase difference are two principal concerns in the study of voltage and current sinusoids. Ø Phasor will be defined from the cosine function in all our proceeding study. If a voltage or current expression is in the form of a sine, it will be changed to a cosine by subtracting from the phase. • Example • Transform the following sinusoids to phasors: – – i = 6 cos(50 t – 40 o) A v = – 4 sin(30 t + 50 o) V Solution: a. I A b. Since –sin(A) = cos(A+90 o); v(t) = 4 cos (30 t+50 o+90 o) = 4 cos(30 t+140 o) V Transform to phasor => V V EENG 224 9

Phasors • Example 5: • Transform the sinusoids corresponding to phasors: a) b) Solution: a) v(t) = 10 cos(wt + 210 o) V b) Since i(t) = 13 cos(wt + 22. 62 o) A EENG 224 10

Phasor as Rotating Vectors EENG 224 11

Phasor Diagrams Ø The SINOR Rotates on a circle of radius Vm at an angular velocity of ω in the counterclockwise direction EENG 224 12

Phasor Diagrams EENG 224 13

Time Domain Versus Phasor Domain EENG 224 14

Differentiation and Integration in Phasor Domain Ø Differentiating a sinusoid is equivalent to multiplying its corresponding phasor by jω. Ø Integrating a sinusoid is equivalent to dividing its corresponding phasor by jω. EENG 224 15

Adding Phasors Graphically Ø Adding sinusoids of the same frequency is equivalent to adding their corresponding phasors. V=V 1+V 2 EENG 224 16

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Solving AC Circuits Ø We can derive the differential equations for the following circuit in order to solve for vo(t) in phase domain Vo. Ø However, the derivation may sometimes be very tedious. Is there any quicker and more systematic methods to do it? Ø Instead of first deriving the differential equation and then transforming it into phasor to solve for Vo, we can transform all the RLC components into phasor first, then apply the KCL laws and other theorems to set up a phasor equation involving Vo directly. EENG 224 19