# A complex number W consists of real and imaginary parts a and b respectively, and the imaginary constant j which is the square root of negative one.

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1 eactance and Impedance A Voltage and urrent In a D circuit, we learned that the relationship between voltage and current was V=I, also known as Ohm's law. We need to find a similar law for A circuits, but now there is an important complication. In an A circuit, we expect the voltage and current to fluctuate, and the fluctuations may be out of phase. Therefore, there are now two relationships between current and voltage. One is the ratio of the peak voltage to the peak current; Vp/Ip. Another is the phase angle between the voltage and the current. In order to keep track of both of these quantities in as simple a manner as possible, we will use the mathematical shorthand known as complex numbers. omplex Numbers A complex number W consists of real and imaginary parts a and b respectively, and the imaginary constant j which is the square root of negative one. W = a + jb j = " (note that j = " and j = "j ) One can specify the real and imaginary parts of W explicitly: e(w) = a, Im(W) = b. Because a complex number has two components, it can be plotted on a two dimensional graph. One calls the x-axis the "real axis", and the y-axis the "imaginary axis". The diagram on the right shows a graph of the complex number W r = a + jb. Note that a complex number is really a vector, with both a magnitude and a direction. The magnitude of W is the length of the vector, W, and can be found using the Pythagorean theorem: W = a + b. The angle θ signifies the direction of the complex vector, and can be found using \$ b trigonometry: " = tan # & ' ) % a( jb Imaginary Axis W θ eal Axis a Since i is used for current, j is traditionally used instead.

2 Further trigonometry yields two more relationships: W cos" = a W sin" = b which allows the complex number W to be re-written: W = a + jb = W cos" + j W sin" = W ( cos" + jsin" ) This form is very useful, because of Euler's formula: ( cos" + jsin" ) = e j" This formula allows us to write any complex number in an exponential form: W = W e j" where as before: W = a + b and: " = tan # b a Finally, since e jθ is a unit vector in the direction of W, the following polar form notation is used, where W is magnitude and θ is angle with respect to the positive real axis. W = W e j" = W #" A vector specified as:.74 3, for example, has magnitude.74 and is oriented 3 degrees with respect to the real axis. Note that positive angles open (rotate) counterclockwise from the positive real axis and negative angles open clockwise as shown. Example 4. (omplex Vector Notation) Determine the four notational forms associated with the vector of magnitude.45 and angle 38 degrees. In polar form: W =.45! 38 In exponential form: W =.45e In trigonometric form: W =.45(cos 38 + sin 38 ) Now since: j38 a = W cos" =.45cos 38 =.45(0.788) =.93 And: b = W sin" =.45sin38 =.45(0.66) =.5 The rectangular form is: W =.93 + j. 5

3 egarding complex vector division: M "# = M \$ exp( j# ) M "# M exp(j# ) = M & exp(j# M % j# ) = M ) ( + ' * "(# %# ) And the conversion process is simplified to: M "# M "# = M "(# \$# ) where M = M M M omplex Voltage and urrent Using this complex notation, we can now write the voltage and current in a manner that keeps track of both amplitude and phase. We have already seen one way to write a sinusoidal voltage: V(t) = V 0 cos ("t + # ). Euler's formula tells us that: { } = e V 0 cos "t + # e V 0 e j ( "t +# ) { ( ) + jv 0 sin ("t + # )} = V 0 cos ("t + # ) Therefore, we may also write a sinusoidal voltage as follows: V t ( ) { } ( ) = e V 0 e j "t +# Here the imaginary part of V(t) is simply ignored. This sinusoidal-to-exponential transformation is done because it is easier perform multiplication and division with exponentials rather than sines and cosines. Moreover, it is common to make the transformation implicit by not specifying the e operator. This is indicated below for both voltage and current, each with a different phase. V( t) = V 0 cos ("t + # ) \$ V 0 e j ( "t +# ) I( t) = I 0 cos ("t + # ) \$ I 0 e j ( "t +# ) From the complex equations above, it is a simple manner to find the ratio of the peak voltage to the peak current; this ratio is simply V o I o. It is also a simple matter to find the relative phase " between the voltage and the current, simply by taking the difference " = # ". These two values are all one needs to determine the A current from the A voltage, or vice-versa, and they can be combined into one complex term known as the impedance, which is signified by "": ( ) = V o e j #" I 0 From the above equations, one can find that the general relationship between the complex voltage, the complex current, and the complex impedance, is simply: V(t) = I(t)

4 This equation looks much like Ohm's law, but be warned: these values are all complex. The rest of this chapter will concern how to calculate the impedance for a given circuit. Once this is accomplished, the real and imaginary parts of the impedance fully determine the magnitude and phase relationships between the current and the voltage for any A circuit. Impedance In general, a complex impedance vector will take the following form: = e() + jim() To find the ratio of the peak voltage to the peak current, one simply finds the magnitude of the impedance. To find the relative phase between the current and the voltage, one must find the phase angle of the impedance. Both of these equations are given here: V o = = e() + Im() " = tan # Im() I o e() Therefore, the two (real) numbers e() and Im() fully determine the relationship between the current and the voltage. These values are typically given the symbols "" and "X" respectively, yielding the equation: = + jx The value is simply the resistance of the circuit. The value X is something known as the reactance of the circuit. esistance, as we already know, determines the ratio between the voltage and current magnitudes without contributing to the relative phase. eactance, however, contributes to both the voltage/current ratio as well as the phase between them. For example, a capacitor (as we will learn in the next section) contributes a negative reactance X. The impedance diagram below corresponds to the circuit on the right. esistance vector represents the resistor and reactance vector X represents the capacitor. Because the circuit elements are in series, the impedance vector is the vector sum of these two vectors; = + X. Since impedance is a vector quantity, it is specified in terms of magnitude and direction angle. A two-dimensional plane with real and imaginary axes is used to depict impedance vectors., where resistance corresponds to the real (horizontal) axis and reactance X corresponds to the imaginary (vertical) axis..x is directed along the negative imaginary axis because it is a negative quantity.

5 Magnitude is calculated by use of the Pythagorean relation and angle is calculated using the inverse tangent relation. Note that angle is referenced to the positive real axis, and negative angles rotate in the clockwise direction. = + X " = tan! X The impedance vector allows for calculation of associated voltage and current quantities, as shown above. urrent through the circuit corresponds to current through the resistor, which is the same current that flows through the capacitor because it is a series circuit. Note that this is consistent with the fact that capacitor current changes instantaneously. The left diagram (large angle) corresponds to a circuit that is dominated by capacitive reactance, in which the current is nearly 90 degrees ahead of the voltage. The right diagram (small angle) corresponds to a circuit that is dominated by resistance, in which the current is nearly in phase with the voltage. The middle diagram corresponds to equality between capacitive reactance and resistance, in which the current lead is midway between the 90 and zero degree limits.

6 apacitive eactance One way of arriving at capacitive reactance is to examine the current through a capacitor in relation to the voltage across it. In doing so; however, we first discover a phase difference between the applied voltage and resulting current. This phase difference is graphed below. Note that current leads voltage (by 90 degrees). Voltage Source urrent Lead For the voltage source: And for the capacitor: V(t) = V 0 sin"t I(t) = dv dt = (V 0 ")cos"t = I 0 cos"t Where: I 0 = V0! Thus, a sinewave voltage across a capacitor results in cosine current through the device. Note that both signals have the same frequency (!). Now for a sinewave and cosine wave of the same frequency, the cosine is essentially a sinewave that is advanced in phase by 90 degrees. This is based on the general relation:! cos" t = sin( " t + ) In summary, it is characteristic for capacitors that current leads voltage by a 90-degree phase-shift, or alternatively, that voltage lags current by 90-degrees. But knowing the phase is not enough; we want to know the capacitive reactance. We are now in position to do so by way of the sinusoidal voltage and current equations above. The procedure is simple in that only the amplitude coefficients V 0 and I 0 are used. By finding the ratio between them, we arrive at the mathematical relation for capacitive reactance. X = V 0 I 0 = V 0 V 0 " = "

7 ombining the phase and magnitude information, we must add a minus sign (see example 4.8 for a full derivation), which gives us the reactance for a capacitor: X = " # = " \$f Example 4. (apacitive eactance alculation) For the simple A capacitor circuit, a 00-microfarad load has capacitive reactance of 6.5 ohms when subjected to a source frequency of 60 hertz. X = " #f = #(60\$00x0 "6 ) = "6.5% Example 4.3 ( ircuit Impedance) A 5-kilohertz sinewave voltage source is used to drive a series circuit comprised of a 70-ohm resistor and microfarad capacitor. The associated impedance vector has magnitude 540 ohms and angle 60 degrees. The associated capacitive reactance, calculated first, is 468 ohms. = = = " 468! 3 " # f # (5x0 )(0.068x0 ) X 6 = + X = (70) + (" 468) = 540! " = tan! X = tan!! =! 60 Example 4.4 ( ircuit Voltage and urrent) egarding the circuit of example 4., a 0-volt (rms) sinusoidal source results in 8.5 milliamps (rms) of current. Since the voltage vector is along the impedance vector, which has an angle of 60 degrees, current is advanced in phase by 60 degrees relative to the source voltage. Note that MS amplitudes are used. I MS = V MS = 0V MS 540" =8.5mA

8 Inductive eactance An inductor is the electronic complement of a capacitor. The simple A inductor circuit (on the right) can be used to formulate the following expression for inductive reactance. (See example 4.9 for a derivation.) X L = "L = #fl Inductive reactance is directly proportional to frequency and is also measured in ohms. As with capacitors, one can derive the important inductor characteristic; that current lags voltage by a 90-degree phase-shift, or alternatively, that voltage leads current by 90- degrees. At this point, we can summarize the impedances of individual resistors, capacitors, and inductors. = j = jx = " #f = j\$ L = jx L = j"fl = j#l However, knowing the impedance for a single circuit element is not enough if the circuit has more than one element. Fortunately, impedances add together very much like resistors; they add in series and add reciprocally in parallel. For two circuit elements in series, the impedances are simply added together to find the equivalent total impedance: T = + T For two circuit elements in parallel, one adds reciprocals: T = + T

9 Example 4.5 (L ircuit Analysis) The series L circuit on the right consists of a 70-ohm resistor, microfarad capacitor, and.-millihenry inductor. It is driven by a 0-volt (rms) sinewave at 6- kilohertz. The aim is to calculate associated impedance and current vectors. Because this is a series circuit, capacitive and inductive reactances are individually calculated, and then added together to obtain the total reactance X T. = = = " 90! 3 " # f # (6x0 )(0.068x0 ) X 6 X X L T = " fl = " (6x0 )(.x0 3 # 3 = X + X = " = 90! L ) = 80! The impedance vector comprises resistance and total reactance, from which magnitude and angle are calculated. efer to impedance diagram above. = + jx = 70 + j90 = + X = = 85!! = tan X = tan = 8.4 urrent through the circuit corresponds to current through the resistor, and so the current vector lies along the positive real axis, as does the impedance vector for the resistor. The magnitude (rms) of the current vector is calculated using Ohm s law. I MS = V MS = 0V MS 85" = 70.mA

10 The voltage vector corresponds to the driving source, and lies along the impedance vector, which is 8.4 degrees above the real axis. The source voltage, therefore, is advanced in phase by 8.4 degrees with respect to current. This corresponds to an inductive circuit, which is consistent in that the inductive reactance is larger than the capacitive reactance. efer to the diagram on the right. Example 4.6 (Parallel ircuit Impedance) The following impedance vector is obtained for the parallel circuit with source frequency 63.7 hertz, resistance 500 ohms, and capacitance 0 microfarads. = 00! j00 Because this is a parallel circuit, this expression is obtained by calculation of the parallel combination of individual impedances. = = 500! = jx = " j = " j50! " # (63.7)(0x0 ) 6 T = # + (500)(! j50) = 500! j "! 90 = = 4"! 63.4 = 00! j00 559"! 6.6 Example 4.7 (Impedance of a esistor) egarding a resistive load driven by a sinusoidal source: V(t) = V 0 e j"t By Ohm s law: I(t) = V(t) where: I 0 = V 0 = V 0 e j"t = i 0e j"t A resistor s impedance, therefore, is equal to its resistance: =

11 Example 4.8 (apacitive eactance Derivation) egarding a capacitor load driven by a sinusoidal source: V(t) = V 0 e j"t and: I = dv dt So: I(t) = " d dt V 0 e j#t ( ) = " j#" V 0 e j#t = \$ & % v 0 j# ' ) ( e j#t = v 0 e j#t = I 0 e j#t Where: Example 4.9 = = j = j = jx and: I 0 = V 0 j# #! f (Inductive eactance Derivation) egarding an inductor load driven by a sinusoidal source: So: I(t) = L V(t) = V 0 e j"t and: V = L di dt # V(t)" dt = # L V 0 ej\$t " dt = v 0 j\$l ej\$t = v 0 e j\$t = I 0 e j\$t L Where: L = j"l = j#fl = jx L and: I 0 = V 0 L

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