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Lesson 18 Phasors & Complex Numbers in AC

Lesson 18 Phasors & Complex Numbers in AC. Learning Objectives. Define and graph complex numbers in rectangular and polar form. Perform addition, subtraction, multiplication and division using complex numbers and illustrate them using graphical methods.

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Lesson 18 Phasors & Complex Numbers in AC

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  1. Lesson 18Phasors & Complex Numbers in AC

  2. Learning Objectives • Define and graph complex numbers in rectangular and polar form. • Perform addition, subtraction, multiplication and division using complex numbers and illustrate them using graphical methods. • Define a phasor and use phasors to represent sinusoidal voltages and currents. • Define time domain and phasor (frequency) domain • Represent a sinusoidal voltage or current as a complex number in polar and rectangular form. • Use the phasor domain to add/subtract AC voltages and currents. • Determine when a sinusoidal waveform leads or lags another. Graph a phasor diagram that illustrates phase relationships.

  3. Complex numbers • A complex number is a number of the form C = a + jb where a and b are real and j = • a is the real part of C and b is the imaginary part. • Complex numbers are merely an invention designed to allow us to talk about the quantity j. • j is used in EE to represent the imaginary component to avoid confusion with CURRENT (i)

  4. Geometric Representation C = 6 + j8 (rectangular form) C = 1053.13º (polar form)

  5. Conversion Between Forms • To convert between forms where apply the following relations

  6. Example Problem 1 • Convert (5∠60) to rectangular form. • Convert 6 + j 7 to polar form. • Convert -4 + j 4 to polar form. • Convert (5∠220) to rectangular form.

  7. Properties of j

  8. Addition and Subtraction of Complex Numbers • Easiest to perform in rectangular form • Add/subtract real and imaginary parts separately

  9. Multiplication and Division of Complex Numbers • Easiest to perform in polar form • Multiplication: multiply magnitudes and add the angles • Division: Divide the magnitudes and subtract the angles

  10. Example Problem 2 Given A =1 +j1 and B =2 – j3 • Determine A+B and A-B. Given A =1.4145° and B =3.61-56° • Determine A/B and A*B.

  11. Reciprocals and Conjugates • The reciprocal of C = C , is • The conjugate of C is denoted C*, which has the same real value but the opposite imaginary part:

  12. Example Problem 3 And now you can try with your TI!! • (3-i4) + (10∠44) ANS: 10.6∠16.1 ANS: 10.2 + 2.9i • (22000+i13)/(3∠-17) ANS: 7.3E3∠17.0 • Convert 95-12j to polar: ANS: 95.8∠-7.2

  13. Phasor Transform • To solve problems that involve sinusoids (such as AC voltages and currents) we use the phasor transform. • We transform sinusoids into complex numbers in polar form, solve the problem using complex arithmetic (as described), and then transform the result back to a sinusoid.

  14. Generating a sinusoidal waveform through the vertical projection of a rotating vector. THE SINUSOIDAL WAVEFORM

  15. Phasors • A phasor is a rotating vector whose projection on the vertical axis can be used to represent a sinusoid. • The length of the phasor is amplitude of the sinusoid (Vm) • The angular velocity of the phasor is 

  16. Representing AC Signals with Complex Numbers • By replacing e(t) with it’s phasor equivalent E, we have transformed the source from the time domain to the phasor domain. • Phasors allow us to convert from differential equations to simple algebra. • KVL and KCL still work in phasor domain.

  17. Using phasors to represent AC voltage and current • Looking at the sinusoid eqn, determine VPkand phase offset . • Using VPK, determine VRMS using the formula: • “The equivalent dc value of a sinusoidal current or voltage is 0.707 of its peak value” • The phasor is then

  18. Representing AC Signals with Complex Numbers • Phasor representations can be viewed as a complex number in polar form. E = Erms

  19. Example Problem 4 i1 = 20sin (t) mA. i2 = 10sin (t+90˚) mA. i3 = 30sin (t - 90˚) mA. Determine the equation for iT.

  20. Phase Difference Phase difference is angular displacement between waveforms of same frequency. If angular displacement is 0° then waveforms are in phase If angular displacement is not 0o, they are out of phaseby amount of displacement

  21. Phase Difference If v1 = 5 sin(100t) and v2 = 3 sin(100t - 30°), v1 leads v2 by 30°

  22. Phase Difference w/ Phasors • The waveform generated by the leading phasor leads the waveform generated by the laggingphasor.

  23. Formulas from Trigonometry • Sometimes signals are expressed in cosines instead of sines.

  24. Example Problem 5 Draw the phasor diagram, determine phase relationship, and sketch the waveform for the following: i = 40 sin(t + 80º) and v = -30 sin(t - 70º)

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