"AC Circuit Analysis - Find i1(t) & i2(t)

In summary, the conversation discusses finding the currents through specific components in a circuit and provides a diagram and equations for analysis. After simplifying and performing node analysis, the currents are calculated and compared to the expected values, with a mistake being identified and corrected in the process.
  • #1
Incand
334
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Homework Statement


Find the currents through ##R_3, i_1(t)## and ##R_2, i_2(t)##.
I attached a diagram of the circuit (diagram.pdf).

In the original diagram the currents I'm trying to find is marked going top to bottom, there's also no directions on the voltages in the original diagram but from the currents I guess they're in the direction drawn

##u_1(t) = 2\sin(200t+30^\circ)##
##u_2(t) = 3 \cos(200t + 20^\circ)##
##R_1 = 100 \Omega##
##R_2=300 \Omega##
##R_3 = 250 \Omega##
##C=10 \mu F##
##L=300mH##

Homework Equations


N/A

The Attempt at a Solution


Lets first simplify at bit by calculating the impedance.
We then have
##Z_1 = R_3+\frac{1}{j\omega C} = 250-j500##
##Z_2 = R_2+j\omega L = 300+j60##

We then have the diagram (diagram2.pdf)
We also write the voltages in complex form
##u_1(t) = 2e^{200t+30^\circ}##
##u_2(t) = 3e^{200t+110^\circ}.## (add ##90^\circ## to write the ##\cos## in ##\sin## form.)

Node analysis:
I ground the bottom node then I use KCL on the top Node, let's call it ##A## and it's potential be ##U_A##.
We then get the equation
##\frac{U_A-u_1(t)}{R_1} + \frac{U_A}{Z_2}+\frac{U_A+u_2(t)}{Z_1}=0.##
Rearanging
##U_A\left( \frac{1}{R_1} +\frac{1}{Z_2} + \frac{1}{Z_1} \right) = \frac{u_1}{R_1}-\frac{u_2(t)}{Z_1}.##
Then ##U_A## can be calculatee and the currents are then
##i_1(t) = \frac{U_A+u_2(t)}{Z_1}## and
##i_2(t) = \frac{U_A}{Z_2}##.

To check that I didn't make any mistakes I calculated this in Matlab
Code:
R=100;
Z1 = 250-500i;
Z2 = 300+60i;
u1 = 2*exp(i*pi*30/180);
u2 = 3*exp(i*pi*110/180);
R3 =250;
Zc = -500j;

temp = u1/R-u2/Z2;
UA = temp/(1/R+1/Z2+1/Z1)
I1 = (UA+u2)/Z1
I2 = UA/Z2
A1 = abs(I1) %amplitude of i_1(t)
phi1 = 180/pi*atan(imag(I1)/real(I1)) %phase constant of i_1
A2 = abs(I2)
phi2 = 180/pi*atan(imag(I2)/real(I2))

Using this I get the output
##UA = 1.3377 - 0.0697i##
##I1=-0.0041 + 0.0027i##
##I2 = 0.0042 - 0.0011i##
##A1 = 0.0049##
##A2 = 0.0044##
##phi1 = -33##
##phi2 = -14.3##
So the currents should be
##i_1(t) =4.9 \sin (200 t -33^\circ) mA## and
##i_2(t) = 4.4 \sin (200t - 14.3^\circ mA##.

However the answer says that it should be
##i_1(t) = 5.13\sin(200t + 15.7^\circ )## mA and
##I_2(t) = 3.73 \sin (200t + 26.2^\circ )##mA.

I've been at this question for two days and I can't see where I'm going wrong.
 

Attachments

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  • diagram2.pdf
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Last edited:
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  • #2
Only thing I've noticed so far:

ACtemp.png
 
  • Like
Likes Incand
  • #3
Nice catch! That together with changing the sign of one of the voltage sources fixes it. Apparently one of them was meant to be the other way around.
 

1. What is an AC circuit analysis?

An AC circuit analysis is a method used to analyze and understand the behavior of electrical circuits that contain alternating current (AC) sources. It involves using mathematical techniques and principles to calculate the voltage, current, and impedance of different components in the circuit.

2. Why is it important to find i1(t) and i2(t) in an AC circuit?

i1(t) and i2(t) represent the currents flowing through different branches of the circuit. These currents are essential in determining the power distribution and efficiency of the circuit. Additionally, knowing these values can help in troubleshooting and identifying any potential issues in the circuit.

3. What are some common components in an AC circuit?

Some common components in an AC circuit include resistors, capacitors, inductors, transformers, and AC voltage sources such as generators or power supplies.

4. How is AC circuit analysis different from DC circuit analysis?

The main difference between AC and DC circuit analysis is that AC circuits have time-varying voltages and currents, while DC circuits have constant voltages and currents. This means that different mathematical techniques and principles are used to analyze each type of circuit.

5. How can AC circuit analysis be applied in real-life situations?

AC circuit analysis is widely used in the design and analysis of electrical systems and devices, such as power grids, motors, generators, and electronic circuits. It is also essential in industries such as telecommunications, aerospace, and renewable energy, where AC circuits are commonly used.

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