31. Alternating Current
Phasors for Inductors
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Hey guys. In this video we're going to talk about phasers and how they apply to the voltages in the currents through inductors. Alright, let's get to it. Remember guys that there are 2 functions that are very important regarding inductors in ac circuits. The current through an inductor and the voltage across an inductor at any time t. These functions are just omega t, and the at an angle omega t, and the voltage occurs at some different angle theta prime, which is omega t plus pi over 2. Because both functions occur at different angles they are said to be out of phase. Okay? The current actually lags the voltage in this case or you could say the voltage leads the current. Okay and what this means is readily apparent on phaser diagrams. In the first diagram I plot the current at its angle omega t. In the second diagram I plot the voltage at its angle omega t plus pi over 2, plus that 90 degrees. Okay? Combining these 2, we have the voltage ahead of the current by 90 degrees. Alright? It's very very important to remember that the voltage across an inductor leads the current. Okay. This is opposite to capacitors in ac circuits where the voltage lags the current. They're opposites. The voltage leads the current for inductors. The voltage lags the current for capacitors. Okay? Let's do a quick example about this. An AC source is connected to an inductor. At a particular instant in time, the current in the circuit is negative and increasing in magnitude. Draw the phasers for the voltage and the current that correspond to this instant in time. Okay? So here's my phaser diagram. Remember, what does it take for a phaser to be negative? It has to be on the left side of the graph because it's horizontal component has to be negative. What does it take for it to be increasing in magnitude? It has to be moving towards that horizontal axis. Since it rotates counter clockwise, it has to be in the second quadrant. This has to be here so that it's negative and since it's rotating counterclockwise, it's moving towards the horizontal axis. So, it's negative and increasing in magnitude. Okay? This is for the current. Remember that the current across sorry the current through an inductor lags the voltage. If you look all the way above in the green box, the voltage leads the current by 90 degrees. So I would need to draw another phaser 90 degrees ahead and that would be the voltage across the inductor. Okay and that is our phaser diagram for an inductor in an ac circuit. Alright guys, this wraps up our discussion on phasors and how they pertain to inductors in AC circuits. Thanks for watching.
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