電路分析英文Lecture8課件

1、 ECTE170 Lecture 8/111Chapter 14 BoylestadSinusoidal response of resistor, inductor and capacitorLow and high frequency response of inductors and capacitorsAverage power and Power FactorBasic Elements and Phasors2Introduction The response of the basic R, L, and C elements to a sinusoidal voltage and

2、 current will be examined with a special note of how frequency will affect the “opposing” characteristic of each element. Phasor notation will then be introduced to establish a method of analysis.3The Derivative To understand the response of the basic R, L, and C elements to a sinusoidal signal, you

3、 need to examine the concept of the derivative. The derivative dx/dt is defined as the rate of change of x with respect to time. If x fails to change at a particular instant, dx = 0, and the derivative is zero. For the sinusoidal waveform, dx/dt is zero only at the positive and negative peaks (wt =

4、p/2 and 3p/2) since x fails to change at these instants of time.45The Derivative The derivative dx/dt is actually the slope of the graph at any instant of time. The greatest change in x will occur at the instants wt = 0, p, and 2p. For various values of wt between these maxima and minima, the deriva

5、tive will exist and will have values from the minimum to the maximum inclusive. The derivative of a sine wave is a cosine wave; it has the same period and frequency as the original sinusoidal waveform.67Sinusoidal Response: ResistorFor a resistor the voltage and current are in phase and are related

6、by Ohms law8The voltage and current of a resistive element are in phase.9Sinusoidal Response: InductorsFor an inductor the current lags the voltage by 90 degreesXL = L is called the inductive reactance - unit 10Sinusoidal Response: WaveformsInductor11Sinusoidal Response: CapacitorsFor a capacitor th

7、e current leads the voltage by 90 degreesXc = 1/C is called the capacitive reactance unit 12Sinusoidal Response: WaveformsCapacitor13Sinusoidal ResponseThe current through a 5 ohm resistor is i = 40sin(377t + 30) A. Find the expression for voltage across it.14Sinusoidal ResponseThe current through a

8、 0.1H coil is i = 7sin(377t 70) A. Find the voltage across it.1516Capacitor Example1718More questions192021Low and High Frequency Response for InductorsInductors: XL = L At low frequencies, and especially DC, the reactance of a inductor is very low (zero for DC)Hence at very low frequencies, an indu

9、ctor may be considered as a short circuitAs input frequencies become very high, the reactance of an inductor approaches infinityHence at very high frequencies, an inductor may be considered as an open circuitBoylestad, Prentice Hall 200722Low and High Frequency Response for CapacitorsCapacitors: Xc

10、= 1/CAt low frequencies, and especially DC, the reactance of a capacitor is very high (infinite for DC)Hence at very low frequencies, a capacitor may be considered as an open circuitAs input frequencies become very high, the reactance of a capacitor approaches 0Hence at very high frequencies, a capa

11、citor may be considered as a short circuit23Average Power and Power Factor Second term has a zero average value over a cycle and causes no average power First term is independent of (a) time and is constant (b) whether v leads or lags i, and will be the Average Power or the Real PowerIn general v =

12、Vm sint and I = Im sin(t-) 24Average Power and Power FactorBoylestad, Prentice Hall 200725Average Power and Power Factor where V and I are rms values of the sinusoidal voltage and current respectivelyThe factor (cos ) which controls the average power flow is called the Power Factor.For a resistor th

13、e Power Factor is unityFor an inductor or capacitor Power Factor is zeroAnother way of finding the Power Factor is to use the expression26Average Power and Power Factor When the power factor is stated it is important to state whether it is leading or lagging in addition to its value (note that it li

14、es between 0 and 1.0)27Average Power and Power Factor28Average Power and Power Factor2930Power Factor FpPower Factor = Fp = cos The term leading or lagging is often written in conjunction with the power factor.They are defined by the current through the load.If the current lead the voltage then its

15、a leading power factorIf the current lags the voltage then its a lagging power factorCapacitive circuits have leading power factors, while Inductive circuits have lagging power factors31Example Power factor3233Complex NumbersAs an essential tool complex numbers will be used in solving ac circuits Re

16、ctangular form Z = a + j b where j is an operator which turns the real number b by 90 in the anti-clockwise direction on the complex plane Polar form Z = Z / Addition is convenient in rectangular form Division/multiplication is convenient in polar formajb34Complex NumbersDefining the rectangular for

17、m.35Polar formDefining the polar form.Demonstrating the effect of a negative sign on the polar form.3637Complex Conjugate38Conversion Between Forms39Complex Numbers40Complex Numbers41Maths Operations4243Polar form operations44Phasors and Phasor DiagramsOften addition/subtraction of sinusoidal voltag

18、es and currents is required in ac circuit analysis. For example consider the addition of two sinusoidal signalsv1 = Vm1 sin (wt +) and v2 = Vm2 sin wt We can add the two waveforms on a point-by-point basis as shown (a tedious process!) to obtain vT.Boylestad45Phasors and Phasor Diagrams However it is very