1. 1 Chapter 11 AC Power Analysis 電路學 ( 二 )

2. 2 AC Power Analysis Chapter 11 11.1Instantaneous and Average Power 11.2Maximum Average Power Transfer 11.3Effective or RMS Value 11.4Apparent Power and Power Factor 11.5Complex Power 11.6Conservation of AC Power 11.7Power Factor Correction 11.8Power Measurement

3. 3 Sinusoidal power at 2  t Constant power 11.1 Instantaneous and Average Power (1) The instantaneously power, p(t) p(t) > 0: power is absorbed by the circuit; p(t) < 0: power is absorbed by the source.

4. 4 11.1 Instantaneous and Average Power (2) The average power, P, is the average of the instantaneous power over one period. 1.P is not time dependent. 2.When θ v = θ i, it is a purely resistive load case. 3.When θ v – θ i = ±90o, it is a purely reactive load case. 4.P = 0 means that the circuit absorbs no average power.

5. 5 11.1 Instantaneous and Average Power (3)

6. 6 Example 1 Given that find the instantaneous power and average power absorbed by a passive linear network. 11.1 Instantaneous and Average Power (4)

7. 7 11.1 Instantaneous and Average Power (5) Example 2 Calculate the instantaneous power and average power absorbed by a passive linear network if: Answer:

8. 8 Example 3 Calculate the average power absorbed by an impedance Z = 30  j70  when a voltage V = 1200 is applied across it. 11.1 Instantaneous and Average Power (6)

9. 9 Example 4 A current flows through an impedance. Find the average power delivered to the impedance. 11.1 Instantaneous and Average Power (6) Answer: 927.2W

10. 10 11.2 Maximum Average Power Transfer (1) The maximum average power can be transferred to the load if X L = –X TH and R L = R TH If the load is purely real, then

11. 11 11.2 Maximum Average Power Transfer (2) Example 5 For the circuit shown below, find the load impedance Z L that absorbs the maximum average power. Calculate that maximum average power. Answer: 3.415 – j0.7317, 1.429W

12. 12 11.3 Effective or RMS Value (1) Hence, I eff is equal to: The effective of a periodic current is the dc current that delivers the same average power to a resistor as the periodic current. The effect value of a periodic signal is its root-mean-square (rms) value. The total power dissipated by R is given by:

13. 13 11.3 Effective or RMS Value (2) The average power can be written in terms of the rms values: Note: If you express amplitude of a phasor source(s) in rms, then all the answer as a result of this phasor source(s) must also be in rms value. The rms value of a sinusoid i(t) = I m cos(  t) is given by: The rms is a constant itself which depending on the shape of the function i(t).

14. 14 11.3 Effective or RMS Value (3) Example 6 Determine the rms value of the current waveform. If the current is passed through a 2- resistor, find the average power absorbed by the resistor.

15. 15 11.3 Effective or RMS Value (4) Example 7 The waveform is a half-wave rectified sine wave. Find the rms value and the average power dissipated in a 10- resistor.

16. 16 11.4 Apparent Power and Power Factor (1) Apparent Power 視功率, S, is the product of the r.m.s. values of voltage and current. It is measured in volt-amperes or VA to distinguish it from the average or real power which is measured in watts. Power factor is the cosine of the phase difference between the voltage and current. It is also the cosine of the angle of the load impedance. Apparent Power, SPower Factor, pf

17. 17 11.4 Apparent Power and Power Factor (2) Purely resistive load (R) θ v – θ i = 0, pf = 1 P/S = 1, all power are consumed Purely reactive load (L or C) θ v – θ i = ± 90 o, pf = 0 P = 0, no real power consumption Resistive and reactive load (R and L/C) θ v – θ i > 0 θ v – θ i < 0 Lagging - inductive load Leading - capacitive load

18. 18 11.4 Apparent Power and Power Factor (3) Example 8 A series-connected load draws a current i(t)=4cos(100t + 10) A when the applied voltage is v(t)=120cos(100t  20) V. Find the apparent power and the power factor of the load. Determine the element values that from the series-connected load.

19. 19 11.4 Apparent Power and Power Factor (4) Example 9 Determine the power factor of the entire circuit of the figure as seen by the source. Calculate the average power delivered by the source.

20. 20 11.5 Complex Power (1) Complex power S is the product of the voltage and the complex conjugate of the current:

21. 21 11.5 Complex Power (2) P: is the average power in watts delivered to a load and it is the only useful power. Q: is the reactive power exchange between the source and the reactive part of the load. It is measured in VAR. Q = 0 for resistive loads (unity pf). Q < 0 for capacitive loads (leading pf). Q > 0 for inductive loads (lagging pf). S = P + j Q

22. 22 11.5 Complex Power (3) Apparent Power, S = |S| = Vrms*Irms = Real power 實功率, P = Re(S) = S cos(θ v – θ i ) Reactive Power 虛功率, Q = Im(S) = S sin(θ v – θ i ) Power factor 功率因子, pf = P/S = cos(θ v – θ i ) S = P + j Q

23. 23 11.5 Complex Power (4) S = P + j Q Impedance Triangle Power Factor Power Triangle

24. 24 11.5 Complex Power (5) Example 10 The voltage across a load is v(t)=60cos(t  10) V and the current through the element in the direction of the voltage drop is i(t)=1.5cos(t + 50) A. Find: (a) the complex and apparent powers, (b) the real and reactive powers, and (c) the power factor and the load impedance.

25. 25 11.5 Complex Power (6) Example 11 A load Z draws 12 kVA at a power factor of 0.856 lagging from a 120-V rms sinusoidal source. Calculate: (a) the average and reactive powers delivered to the load, (b) the peak current, and (c) the load impedance.

26. 26 11.6 Conservation of AC Power (1) The complex real, and reactive powers of the sources equal the respective sums of the complex, real, and reactive powers of the individual loads. For parallel connection: The same results can be obtained for a series connection.

27. 27 Example 12 The figure shows a load being fed by a voltage source through a transmission line. The impedance of the line is represented by the (4 + j2)  impedance and a return path. Find the real power and reactive power absorbed by: (a) the source, (b) the line, and (c) the load. 11.6 Conservation of AC Power (2)

28. 28 Example 13 In the circuit, Z 1 = 6030  and Z 2 = 4045 . Calculate the total: (a) apparent power, (b) real power, (c) reactive power, and (d) pf, supplied by the source and seen by the source. 11.6 Conservation of AC Power (3)

29. 29 11.7 Power Factor Correction (1) Power factor correction is the process of increasing the power factor without altering the voltage or current to the original load. Power factor correction is necessary for economic reason.

30. 30 11.7 Power Factor Correction (2) Q 1 = S 1 sin θ 1 = P tan θ 1 Q 2 = P tan θ 2 Q c = Q 1 – Q 2 = P (tan θ 1 - tan θ 2 ) = ωCV 2 rms P = S 1 cos θ 1

31. 31 Example 14 When connected to a 120-V (rms), 60-Hz power line, a load absorbs 4 kW at a lagging power factor of 0.8. Find the value of capacitance necessary to raise the pf to 0.95. 11.7 Power Factor Correction (3)

32. 32 11.8 Power Measurement (1) The wattmeter is the instrument for measuring the average power. Ifand The basic structure Equivalent Circuit with load