Marc Sugrue
- Joined Jan 19, 2018
- 222
Think of it like this. An inductor is a magnetic device and energy is stored in the inductor in the form of a magnetic field.referencing this: https://www.allaboutcircuits.com/textbook/alternating-current/chpt-3/ac-inductor-circuits/
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At the beginning of the current through the inductor, the opposition to the current (positive voltage) is at it's peak - I get that. But if there is opposition, then not as much current would flow, invalidating the assumption about the amount of changing current in the beginning? I mean how can changing current induce opposition that would prevent the current from changing in the first place?
Another thing that is confusing is that when reading about inductors, they always talk about that 90° phase shift between current and voltage. But that 90° shift pertains only to the voltage/current across the inductor, induced by the inductor, not the whole circuit at hand, right? After all, I can't just take a circuit's wire, wind it around my screwdriver a couple times and suddenly the current curve of the whole circuit lags the voltage curve by 90°?
Intuitively, and with the help of circuitlab.com, I come to the conclusion that inserting a very small inductor into the circuit would not be noticeable at first. Gradually increasing the inductor in size will slowly create a lagging current curve in the circuit, while also gradually attenuating the signal, i.e. less current overall / smaller current amplitude. By the time that the inductor is so large to induce a current lag of 90° in the whole circuit, it would be so large that practically no current would flow?
Every resource I consult keeps repeating the same paradigms over and over like "the inductor wants to keep current flowing" but that does not really help me much. I wonder if I just don't get it. How would I go about plotting a circuit's voltage and current curve, knowing that the voltage that can be dropped by the inductor at di/dt times henrys equals such and such volts?
At some point (saturation) the inductor become just a piece of wire as the core can no longer store energy as its full so you have a limit to how much energy can be stored.
When you apply volts accross and inductor as long as the inductor isn't saturated the current in the inductor for a given voltage across it follows the the equation di = (V * dt) / L and the energy (joules) stored in the inductor is equal to J = 1/2*L* Isquared. Again there is only so much energy a given inductor can store before it saturates.
When the the inductor saturates the Voltage accross it become almost zero so based on this you can see that 0V accross the inductor results in no stored charge and its no longer acting as an inductor.
If you instantantanously stop the flow of current going through and inductor the collapsing magntetic field generates a EMF (voltage) which aims to continue the flow of current through the inductor. In an AC circuit where current is continually changing the inductor exhibits a characteristic called impedance or inductive reactance which is a apparent resitance caused by the changing magnetic field, this impedance is Xl = 2 * pi * F * L and is measured as Ohms. The phase shift caused by this inductive reactance causes the inductor phase current to lag due to the charging and discharging of the magnetic field impeding the current flow.
When combined with capacitors (which introduce positive phase shift) the phase elements can be pitched to cancel each other out. Where phase is important the L & C can be chosen to give very little phase shift. In areas where phase it isn't important (such as the noise portion filtered by a filter) the phase can be ignored as you don't care in this region. A common application example for this would be traditional power factor controller where a filter is designed to optimise the phase of the system current to keep it matched the mains power line voltage frequency using a L and C component.
Not sure if this helps
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