# AC RL and RC Circuits

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1 AC RL and RC Circuits When a sinusoidal AC voltage is applied to an RL or RC circuit, the relationship between voltage and current is altered. The voltage and current still have the same frequency and cosine-wave shape, but voltage and current no longer rise and fall together. To solve for currents in AC RL/RC circuits, we need some additional mathematical tools: Using the complex plane in problem solutions. Using transforms to solve for AC sinusoidal currents. 1

2 Imaginary Numbers Solutions to science and engineering problems often involve 1. Scientists define i = 1. As we EE s use i for AC current, we define j = + 1. Thus technically, j = i, but that does not affect the math. Solutions that involve j are said to use imaginary numbers. Imaginary numbers can be envisioned as existing with real numbers in a two-dimensional plane called the Complex Plane. Imaginary Axis 3j 2 nd 1 st 2j Quadrant Quadrant j rd Quadrant j 2j 3j Real Axis 4 th Quadrant The Complex Plane 2

3 The Complex Plane In the complex plane, imaginary numbers lie on the y-axis, real numbers on the x-axis, and complex numbers (mixed real and imaginary) lie off-axis. For example, 4 is on the +x axis, 8 is on the x axis, j6 is on the + y axis, and j14 is on the y axis. Complex numbers like 6+j4, or 12 j3 lie off-axis, the first in the first quadrant, and the second in the third quadrant. Imaginary Axis 3j 2 nd 1 st 2j Quadrant Quadrant j rd Quadrant j 2j 3j Real Axis 4 th Quadrant 3

4 Why Transforms? Real-world domain Problem-solving domain Transforms move a problem from the real-world domain, where it is hard to solve, to an alternate domain where the solution is easier. Sinusoidal AC problems involving R-L-C circuits are hard to solve in the real time domain but easier to solve in the ω-domain. 4

5 The ω Domain In the time domain, RLC circuit problems must be solved using calculus. However, by transforming them to the ω domain (a radian frequency domain, ω = 2πf), the problems become algebra problems. A catch: We need transforms to get the problem to the ω domain, and inverse transforms to get the solutions back to the time domain! Time Domain Inverse Transform Equations Transform Equations ω Domain 5

6 A Review of Euler s Formula You should remember Euler s formula from trigonometry (if not, ± jx get out your old trig textbook and review): e = cos x± jsin x. The alternate expression for e ±jx is a complex number. The real part is cos x and the imaginary part is ± jsin x. We can say that cos x = Re{e ±jx } and ±jsin x =Im{e ±jx }, where Re = real part and Im = imaginary part. We usually express AC voltage as a cosine function. That is, an AC voltage v(t) and be expressed as v(t) =V p cos ωt, where V p is the peak AC voltage. Therefore we can say that v(t) =V p cos ωt = V p Re{e ±jωt }. This relation is important in developing inverse transforms. 6

7 Transforms into the ω Domain Element Time Domain ω Domain Transform AC Voltage V p cos ωt V p Resistance R R Inductance L jωl Capacitance C 1/jωC The time-domain, sinusoidal AC voltage is normally represented as a cosine function, as shown above. R, L and C are in Ohms, Henrys and Farads. Skipping some long derivations (which you will get in EE 3301), transforms for the ω domain are shown above. Notice that the AC voltage ω-transform has no frequency information. However, frequency information is carried in the L and C transforms. 7

8 Comments on ω Transforms Element Time Domain ω Domain Transform 8 AC Voltage V p cos ωt V p Resistance R R Inductance L jωl Capacitance C 1/jωC Because we are studying constant-frequency sinusoidal AC circuits, the ω-domain transforms are constants. This is a considerable advantage over the time-domain situation, where t varies constantly (which is why solving for sinusoidal currents in the time domain is a calculus problem). Two other items: In the ω-domain, the units of R, jωl, and 1/jωC are Ohms. In the ω-domain, Ohm s Law and Kirchoff s voltage and current laws still hold.

9 Solving for Currents in the ω Domain Solving problems in the frequency domain: Given a circuit with the AC voltage shown, and only a resistor in the circuit, then the transform of the voltage is 10. R transforms directly as 100. Solving for the circuit current, I=V/R, or I=10/100 = 0.1 A. This current is the ω-domain answer. It must be inversetransformed to the time domain to obtain a usable answer. AC Voltage Source =10 cos (1000t) 100 Ω ω-domain voltage = V p = 10 ω-domain current = V p /R = 10/100 = 0.1 ampere 9

10 An ω Domain Solution for an L Circuit 10 The ω-domain voltage is still 10. The ω-domain transform of L = jωl = j(1000)10(10) 3. = j10. The units of the L transform is in Ohms (Ω), i.e., the ω-domain transform of L is j10 Ω. The value ωl is called inductive reactance (X). The quantity jωl is called impedance (Z). Finding the current: I = V/Z = 10/j10 = 1/j = j (rationalizing). Time-domain answer in a few slides! AC Voltage =10 cos (1000t) 10 mh ω-domain voltage = V p = 10 ω-domain current = V p /jωl = 10/j10 = j1 = j ampere

11 An ω Domain Solution for a C Circuit The ω-domain voltage still = 10. The ω-domain transform of C = 1/jωC =1/ j(1000)100(10) 6. = 1/j0.1= j10. The units of the C transform is in Ohms (Ω), i.e., the ω-domain transform of C is j10 Ω. The value 1/ωC is called capacitive reactance, and 1/jωC is also called impedance (here, capacitive impedance). Finding the current: I = V/Z = 10/ j10 = 1/ j = j1 (rationalizing) = j. Time-domain answer coming up! AC Voltage =10 cos (1000t) 100 μf ω-domain voltage = V p = 10 ω-domain current = V p /(1/jωC) = 10/ j10 = j1 = j amperes 11

12 An RL ω-domain Solution 12 The ω-domain voltage still = 10. The ω-domain impedance is 10+j10. Resistance is still called resistance in the ω-domain. The R and L transforms are called impedance, and a combination of resistance and imaginary impedances is also called impedance. Note: all series impedances add directly in the ω-domain. Finding the current: I = V/Z = 10/(10+j10) = (rationalizing) (100 j100)/200 = 0.5 j0.5. Time-domain answers next! AC Voltage =10 cos (1000t) 10 Ω 10 mh ω-domain voltage = V p = 10 ω-domain current = V p /(R+jωL) = 10/(10+j10) = 0.5 j0.5 ampere

13 Inverse Transforms Our ω-domain solutions do us no good, since we are inhabitants of the time domain. We required a methodology for inverse transforms, mathematical expressions that can convert the frequency domain currents we have produced into their time-domain counterparts. It turns out that there is a fairly straightforward inverse transform methodology which we can employ. First, some preliminary considerations. 13

14 Cartesian-to-Polar Transformations Our ω-domain answers are complex numbers currents expressed in the X-Y coordinates of the complex plane. Coordinates in a two-dimensional plane may also be expressed in R-θ coordinates: a radius length R plus a counterclockwise angle θ from the positive X-axis (at right). That is, there is a coordinate R,θ that can express an equivalent position to an X,Y coordinate. Imaginary Axis Y X 3j 2j j R θ Real Axis X j 2j 3j Y 14

15 Cartesian-to-Polar Transformations (2) The R,θ coordinate is equivalent to the X,Y coordinate if θ = arctan(y/x) 2 2 and R= X + Y. In our X-Y plane, the X axis is the real axis, and the Y axis is the imaginary axis. Thus the coordinates of a point in the complex plane with (for example) X coordinate A and Y coordinate +B is A+jB. Now, remember Euler s formula: ± jx e = cos x± jsin x Imaginary Axis Y X 3j 2j j R θ Real Axis X j 2j 3j Y 15

16 Cartesian-to-Polar Transformations (3) 16 ± jx If e = cos x± jsin x, then ± jx Re = Rcos x ± Rj sin x. But in our figure, Rcosθ = X, and Rsinθ = Y. ± jθ Or, Re = X ± jy! What this says is that when we convert our ω-domain AC current answers into polar coordinates, we can express the values in Re ±jθ format as well as R,θ format. The Re ±jθ is very important in the inverse transforms. Imaginary Axis Y 3j 2j R Y j θ j 2j 3j X Real Axis X

17 Inverse Transform Methodology We seek a time-domain current solution of the form i(t) = I p cos(ωt). where I p is some peak current. This is difficult to do with the ω-domain answer in Cartesian (A±jB) form. So, we convert the ω-domain current solution to R,θ format, then convert that form to the Re ±jθ form, where 2 2 we know that θ = arctan (Y/X), and R= X + Y. Once the ω-domain current is in Re ±jθ form (and skipping a lot of derivation), we can get the timedomain current as follows: 17

18 Inverse Transform Methodology (2) Given the Re ±jθ expression of the ω-domain current, we have only to do two things: Multiply the Re ±jθ expression by e jωt. Take the real part. This may seem a little magical at this point, but remember, Re (e jωt ) is cos ωt, and we are looking for a current that is a cosine function of time. We can see examples of this methodology by converting our four ω-domain current solutions to real timedomain answers. 18

19 Transforming Solutions In the resistor case, our ω-domain current is a real number, 0.1 A. Then X=0.1, Y= Then R X Y (0.1) 0.1, and θ = arctany/x = arctan 0 = 0. Thus current = Re {0.1 e jωt e j0 } = 0.1Re {0.1 e jωt } = 0.1 cos1000t A. Physically, this means that the AC current is cosinusoidal, like the voltage. It rises and falls in lock step with the voltages, and has a maximum value of 0.1 A (figure at right). = + = = ω-domain current = V p /R = 10/100 = 0.1 ampere Voltage Current 19

20 Transforming Solutions (2) 20 For the inductor circuit, I = j1 = j Converting to polar: R= X + Y = (1) = 1 θ = arctany/x = arctan 1/0 = arctan = 90. I ω = 1, 90 = 1e j90 = e j90. Multiplying by e jωt and taking the real part: i(t) = Re{e jωt e j90 } = Re{e j(ωt 90 ) } = (1)cos(ωt 90 ) = cos(ωt 90 ) A. Physical interpretation: i(t) is a maximum of 1 A, is cosinusoidal like the voltage, but lags the voltage by exactly 90 (plot at right). The angle θ between voltage and current is called the phase angle. Cos θ is called the power factor, a measure of power dissipation in an inductor or capacitor circuit. ω-domain current = V p /jωl = 10/j10 = j1 = j ampere Voltage Current

21 Transforming Solutions (3) 21 For the capacitor circuit, I = j A. Converting to polar: R= X + Y = ( 1) = 1 θ = arctany/x = arctan 1/0 = arctan = 90, so that I ω = 1, 90 = e j90. Multiplying by e jωt and taking the real part: i(t) = Re{e jωt e j90 } = Re{e j(ωt +90 ) } = cos(ωt+90 ) A. Physically, i(t) has a maximum amplitude of 1 A, is cosinusoidal like the voltage, but leads the voltage by exactly 90 (figure at right).. ω-domain current = V p /(1/jωC) = 1/ j0.1 = j10 amperes Voltage Current

22 Transforming Solutions (4) 22 For the RL circuit, I = 0.5 j0.5 ampere. Converting to polar: R= X + Y = (0.5) + ( 0.5) And θ = arctany/x = arctan 0.5/0.5 = arctan 1= 45 ; I ω = 0.707, 90 = 0.707e j45. Multiplying by e jωt and taking the real part: i(t) = Re{0.707e jωt e j45 } = 0.707Re{e j(ωt 45 ) } = 0.707cos(ωt 45 ) = 0.707cos(ωt 45 ) A. Note the physical interpretation: i(t) has a maximum amplitude of A, is cosinusoidal like the voltage, and lags the voltage by 45. Lagging current is an inductive characteristic, but it is less than 90, due to the influence of the resistor. ω-domain current = V p /(R+jωL) = 10/(10+j10) = 0.5 j0.5 ampere

23 Summary: Solving for Currents Using ω Transforms Transform values to the ω-domain: Element Time Domain ω Domain Transform AC Voltage V p cos ωt V p Resistance R R Inductance L jωl Capacitance C 1/jωC Solve for I ω, using Ohm s and Kirchoff s laws. Solution will be of the form A±jB (Cartesian complex plane). Use inverse transforms to obtain i(t). Convert the Cartesian solution (A±jB) to R,θ format and thence to Re ±jθ form. Multiply by e ±jωt and take the real part to get a cosineexpression for i(t). 23

24 Measuring AC Current Indirectly Because we do not have current probes for the oscilloscope, we will use an indirect measurement to find i(t) (reference Figs. 11 and 13 in Exercise 5). As the circuit resistance is real, it does not contribute to the phase angle of the current. Then a measure of voltage across the circuit resistance is a direct measure of the phase of i(t). Further, a measure of the Δt between the i,v peaks is a direct measure of the phase difference in seconds. We will use this method to determine the actual phase angle and magnitude of the current in Lab

25 Discovery Exercises Lab. 5 includes two exercises that uses inductive and capacitive impedance calculations to allow the discovery of the equivalent inductance of series inductors and the equivalent capacitance of series capacitors. Question 7.6 then asks you to infer the equivalent inductance of parallel inductors and the equivalent capacitance of parallel capacitors. Although you are really making an educated guess at that point, you can validate your guess using ω-domain circuit theory, with one additional bit of knowledge not covered in the lab text: In the ω-domain, parallel impedances add reciprocally, just like resistances in a DC circuit. (Remember that in the ω-domain, series impedances add directly). 25

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