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AC analysis intro 1

Solving circuits with differential equations is hard. If we limit ourselves to sinusoidal input signals, a whole new method of AC analysis emerges. Created by Willy McAllister.

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  • blobby green style avatar for user lkchen9699
    I think the right-hand side of the equation should be dVin/dt if you define Vin as the voltage. And this equation is not KVL; it is the equation for the rate of change of voltage.
    (24 votes)
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    • piceratops ultimate style avatar for user JensOhlmann
      at Ikchen9699 you are right. The equation comes from the eqaution VL+VR+VC = Vin
      if we write the VL,VR and VC as differentials. That looks like that
      L dI/dt + R*I + 1/c *∫Idt = Vin. This equation is called intregro-differential equation because of the integral. To make this kind of equations to a differential equation take the derivative on both sides. Now we have the equitation L d^2I/dt^2 + R* dI/dt + 1/c * I = dV/dt.
      (13 votes)
  • blobby green style avatar for user samananwar
    Why inductor is used in the AC circuit?
    (2 votes)
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    • purple pi purple style avatar for user APDahlen
      Hello again Sama!

      There are a few ways of looking at this:

      First from a radio engineer's perspective: Inductors are your best friend. They along with capacitors allow us to form tuned circuits. These circuits are the building bocks upon which all radio transmitters and receivers are built. Without inductors you would not be able to tune a radio.

      From a power supply engineer: If you are using a computer be thankful we have inductors. They are the essential to the construction of power supplies. Inductors and their close cousin the transformer are found everywhere. In fact if you are using a computer you a employing dozens of inductors in the chain of power supplies that take the wall outlet power and convert it to a regulated DC voltage for the main CPU. You will also find inductors in the LED lights that light your room.

      Finally, lets see what a power grid engineer has to say: It would be nice if we didn't have inductors but they are everywhere. The large generators at the power plant act as inductors, the power lines have inductance. Yes, a simple wire will act as an inductor! Also, almost every AC motor on the grid acts as an inductor. My job is difficult as I must keep track of and supply the imaginary (aka reactive) power that all of these inductors require. This is where phase comes into the equation. All of these inductors make the delay the phase of the current relative to the voltage. That's OK by me as I can talk about cool things like phasors, real power, reactive power, VARS, VA, symmetrical components, lagging power factor, and my personal favorite the syncrophasor!

      Please leave a comment below.


      (15 votes)
  • male robot hal style avatar for user Vish
    are these voltages or rate of change of voltages?
    (5 votes)
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    • blobby green style avatar for user Tompap2007
      It really seems like it isn't KVL. When we didnt have any voltage source, the equal of KVL was zero, and if we tried to get the rate of change of the KVL equation, this was equal to d0/dt = 0. So now that the second part of the equation is Vin, we should have d(Vin)/dt, not just Vin. Could we have an explanation over this please?
      (6 votes)
  • blobby green style avatar for user samananwar
    ac signal is the function of voltage or time or current?
    (4 votes)
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    • purple pi purple style avatar for user APDahlen
      Hello Samananwar,

      Both voltage and current vary as a function of time.

      Mathematically you could talk about voltage as V(t) = Asin(2πft + Θ). Here A is the peak voltage, f is the frequency and Θ is the phase shift.

      We could make a similar equation for current.


      (4 votes)
  • blobby green style avatar for user Peter Borkowski
    plz see Professor Shankar's introduction into AC LCR circuits
    your derivations is really confusing , the results are the same but the road leading to them is very hard ,

    (0 votes)
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  • blobby green style avatar for user samananwar
    Why phase difference between two alternating quantities is more important than their actual phases?
    (3 votes)
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    • spunky sam orange style avatar for user Willy McAllister
      There really isn't a strong definition of "actual phase". Phase is always a relative measurement between a signal and a reference signal. If we have one AC signal we get to choose what time we label time = 0. It can be at a peak or trough, or where the signal crosses 0 volts. It is an arbitrary choice. When we start to think about a second AC signal, one of the important things is its phase relationship with the first signal. This is what we call relative phase.
      (2 votes)
  • blobby green style avatar for user Fabrizio Lopez
    I have an existential doubt, not about the video but about the alternating current. I do not know where to put the question, excuse me.
    Now my question: Leaving away the stationary state, how does the current to go from the positive pole of the source to the negative pole (assuming that the distance is much greater than the wavelength of the signal as in a transmission line for example or a high frequency circuit), if the polarity varies and therefore, the direction of the current also and due to this, the current should be moved same quantity of meters in one direction and the other. So, how the current can arrive to the "final" of the circuit?

    I hope someone can help me with this doubt :( thank you very much in advance.
    (3 votes)
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  • blobby green style avatar for user Balwant Singh
    in India and rest of the world we are using 50 hertz and 230 volt power supply and in america and japan they are using 60 hertz and 120 volt power supply.

    question 1. why is it so??
    question 2. why not 70 hertz, 80 hertz or less than 50 hertz. why we are choosing 50 hertz and 60 hertz only.
    (2 votes)
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  • blobby green style avatar for user kishorehari654.hk
    What is meant by positive phase sequence, negative phase sequence, and zero phase sequence in AIR CIRCUIT BREAKER
    (2 votes)
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  • blobby green style avatar for user 2T48004U
    Too bad they don't have any quizzes or unit tests for this. I got my exam this week would've been nice to get the practice other than using my assignments and labs from my class already
    (2 votes)
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Video transcript

- [Voiceover] We now begin a whole new area of circuit analysis called sinusoidal steady state analysis. You can also call it AC analysis. AC stands for alternating current. It means it is a voltage or a current that where the signal actually changes sign. It is positive sometimes. It is negative sometimes. And the conventional name for that is AC or alternating current. It could've been called alternating voltage but that's not the name. The name is AC. What I want to do in this video is we will do a quick review of what it is like to solve an equation like the one showed here. This is an RLC circuit. I'm going to show what it is like to solve this in differential equation form, which is gonna be a lot of work. I want to introduce the idea of a new point of view or a new analysis method that we refer to as the sinusoidal steady state. And it is a transformation that we are gonna do on this circuit. And it is gonna be a big reward at the end. I want to go through what's the reward and what's it gonna look like at the end. Then we are gonna know some of the math that we have to review in order to fully understand this change that we are gonna go through. This change of point of view. Let's first take a look at this circuit here. This is a circuit that's now a driven RLC circuit. So here's the drive function. It is a voltage. It is some waveform. It is driving a sequence of an inductor, a resistor, and a capacitor. Now in an earlier video we derived the natural response of this circuit and to do that, we shorted out we removed the source and shorted it out and added a little bit of energy to this circuit and saw what it did on its own. What was its natural response. Now we've upgraded this. We've added a source and now we have to solve this again including the source. If we use the differential equation technique this is how we are gonna go about it. So the first step in a circuit analysis like this is to write a KVL equation. We are gonna solve for this current right here. That's the one current that's in this. So I is our independent variable. If I write KVL as you recall when we did this for natural response, we ended up with a differential equation that looked like this. We had L times the second derivative of I plus R times the first derivative of I plus one over C times I. So these are the voltages. Each of these individual terms are the voltages across these components here. So that's the resistor voltage. This is the capacitor voltage and this here is the inductor voltage so it is inductor voltage, resistor voltage, capacitor voltage. And all those if we add those up, those have to equal VN. So this is now a forced equation, which means this is the forcing function and we are gonna have to solve this and the math for doing this is pretty difficult. It was hard enough to do the natural response and we add in this and it gets to be even more work. So as we did before, what we do now is we propose a solution and the solution we are in the habit of doing this now is gonna be sum constant times E to the sum natural frequency times T. So AE to the ST is our proposed solution for I as a function of time. You remember we called S. S is a frequency term because S times T has to have no units so S has units of one over time or frequency so that's called the natural frequency. And when we plug in I the way to tell if I is a solution is to plug this into this equation here. And we got an equation that looks like this. We ended up after factoring our I we ended up with LS squared plus RS plus one over C and all that is equal to for the natural response we put in, so we solve this equation by setting this term here equal to zero and solving for s to find out what the natural frequency is and then we go back and we find out A by looking at the initial conditions over here. Whatever initial energy was in this circuit determines the value of A here. The next step in this forced response where VN is driving the circuit is we have to set this back to VN and solve for the forced. Now if we let VN be sort of any forcing function we want, any kind of waveform, this is gonna be a really hard piece of mathematics. This is gonna be a really difficult calculation. It is gonna take a long time and basically I don't want to do it. I'm gonna wish there was some other way to do these kinds of equations and there is. The way we simplify this process substantially is we make a little limitation on ourselves on what VN can be. If we make a rule, if we basically volunteer to limit ourselves to VN equals sinusoids that means that VN is of the form cosine of omega T plus phi where phi is some angle or it could be sine of omega T plus some angle. Any waveform of this form here is called the sinusoid. So sinusoid is the general name for cosine and sine and signals that look like that. Because we don't want this math to blow up on us, with the general input over here, we are gonna develop a really elegant way to solve circuits with this we limit ourselves to sinusoidal inputs. We will take a pause right here and continue on in the next video to introduce the idea of sinusoidal analysis.