Hi all,

I understand the working of an inductor. In certain applications, it will used to generate a high voltage.
But do inductors limit current?

In this video at 8:55. it is said that we can use an inductor to limit the current?

I have never heard using an inductor to limit current. Is it right? Can someone explain in simpler terms?

Thank you.

 

An inductor can limit current BUT ONLY IN an AC circuit. An example is a choke ballast in a fluorescent light fitting.

Les.

 

To expand a little bit upon LesJones' explanation, an inductor "resists" change in current with time.

Generally, L di = dv dt
Where L is inductance
di is the change in current
dv is the change in voltage or often the voltage placed across an inductor in a DC circuit.
dt is the change in time.

You can rearrange that to say:
di = (dv/dt)/L

So if you are using a DC pulse (sounds like an oxymoron so let's change that to a unipolar pulse) a 1 henry inductor that has 1 volt placed across it for one second will ramp up the current to a maximum of 1 amp.

If placed in series with and AC voltage source, the impedance of the inductor will be XL = 2 Pi FL. In a circuit driven by a pure sine wave with only an inductive load, the RMS current will be I = V/XL.. In the case of circuits including resistance, it is I = V/Z.

For more detail you can view https://www.allaboutcircuits.com/textbook/alternating-current/

 

I understand the working of an inductor.

But do inductors limit current?

Why lead this thread off with a claim that you understand how inductors work? That's not at all helpful. Obviously you don't really understand how they work, or you wouldn't ask the rest of the questions in the post.

Don't get me wrong, it's not bad that you don't understand them - that's the whole reason for this forum. We're here to help each other learn. So just come in and say you have a question about inductors and we'll happily help... no need to pretend you have it all figured out.

Ok, with my little rant out of the way, here's my take: I admit that I still struggle to understand what's really going on inside inductors at a deep level, but the "easy" way to think about their behavior is that they resist CHANGES in current. They don't necessarily resist any particular amount of current, but they slow down changes in current. So for very short pulses or high frequency AC current, they effectively are reducing current, but for steady DC current they provide no resistance.

EDIT: I was about to write more and try to expand on the idea of resisting change as opposed to simply limiting current, but I see @DickCappels replied while I was typing with a better explanation than my own, so I'll leave it at that.

 

The equation posted is wrong, the dv needs to be replaced by v.

The correct equation is:

dI/dt = V / L

or

dI = V/L dt



Bob

 

To expand a little bit upon LesJones' explanation, an inductor "resists" change in current with time.

Generally, L di = dv dt
Where L is inductance
di is the change in current
dv is the change in voltage or often the voltage placed across an inductor in a DC circuit.
dt is the change in time.

You can rearrange that to say:
di = (dv/dt)/L

The proper equation is

V = L·(di/dt)

The total voltage (not change in voltage) is proportional to the time-rate-of-change of current.

So, in your form, it would be

L di = V dt

It would make no sense to have one differential element on one side and the product of two such elements on the other (at least I can't think of a situation where it would make sense, but there probably is one, particularly if the side with one such element also had an infinite element, but that's certainly not the case here).

 

In the context of power electronics, generally speaking a converter is either voltage-source or current-source. Most people are familiar with the voltage-source converter as its input is assumed to be an idealised source, typically with a large capacitance across it such that the input voltage is constant, one could even say "limited" as the large capacitor will not allow the input voltage to change instantaneously.

In the current-source converter, the input is assumed to be an ideal current source, typically with some large inductor in series.

Let's assume now that a fault occurs at the output of the converter, such that the load is short-circuit. This would present a serious problem to a voltage-source converter since the large capacitor would attempt to maintain the load voltage and so a very large and destructive fault current would flow. Now, let's think about the current-source converter. In this cases, when the output is short-circuit, the fault current is simply the same as the current that was flowing before since the series inductance does not allow a rapidly increasing current. Therefore, you could say that the inductor has limited the fault current in the short term. As explained by previous answers, this is only in the short term (short dt). Over a longer time, the inductor current would still ramp up.

 

Naked current has a magnetic induction process and property to itself. It is self inducing. We can sum that affect by paralleling the current with adjacent loops of current. It's summing counter EMF. Counter EMF adjustment.

A current rate choker. A simple coil of wire with a variable core, can control an AC electric furnace. Or motors, or anything that can be current controlled.

Coils and inductors can also limit rate thru External mutual flux. A "magnetic" amplifier.

 

Why lead this thread off with a claim that you understand how inductors work? That's not at all helpful. Obviously you don't really understand how they work, or you wouldn't ask the rest of the questions in the post.

Don't get me wrong, it's not bad that you don't understand them - that's the whole reason for this forum. We're here to help each other learn. So just come in and say you have a question about inductors and we'll happily help... no need to pretend you have it all figured out.

Ok, with my little rant out of the way, here's my take: I admit that I still struggle to understand what's really going on inside inductors at a deep level, but the "easy" way to think about their behavior is that they resist CHANGES in current. They don't necessarily resist any particular amount of current, but they slow down changes in current. So for very short pulses or high frequency AC current, they effectively are reducing current, but for steady DC current they provide no resistance.

EDIT: I was about to write more and try to expand on the idea of resisting change as opposed to simply limiting current, but I see @DickCappels replied while I was typing with a better explanation than my own, so I'll leave it at that.

So are you saying that during that short pulses or high frequency AC current, they inductor will limit they peak value of the signal?

 

Sorry guys. I am still not getting clarity. can someone give an example for better understanding?

 

Maybe this will help
https://electronics.stackexchange.c...-current-in-a-inductive-circuit/288384#288384

 

Hello,

Have a look here : https://www.allaboutcircuits.com/te...ysis-of-four-dc-dc-converters-in-equilibrium/
Or here : https://www.powerelectronicsnews.co...rter-power-supply-design-tutorial-section-5-1

Bertus

 

Clarity:

Two principles learned in EE.

The voltage across a capacitor cannot change instantaneously.

The current through an inductor cannot change instantaneously.

Real world inductors and capacitors are not ideal. There are parasitics involved too. Parasitics can be modeled with ideal resistances, inductances and capacitances.

 

Here's a simulation of step voltages applied to an inductor (1V for the first step and -2V for the second step).
The inductor's value is selected to be 1 Henry so 1V gives a change in current (di/dt) of 1A/second.

 

Learn and understand how a buck converter works and you will then know how an inductor can be used to drop a voltage and limit current wothout the loss that a resistor would incur.

Bob

 

Learn and understand how a buck converter works and you will then know how an inductor can be used to drop a voltage and limit current wothout the loss that a resistor would incur.

Bob

Learn how to launch a rocket to the moon, and then you'll know how a campfire can be used for warmth.

 

Seriously? A buck converter has 3 components that perform the critical function. (2 if ypu don't need a smoothing capacitor. ). If one cannot understand how it works, one really does not understand inductors.

Bob

 

Hello,

The boost converter has basicaly two stages.
When the mosfet is conducting, there is energy stored in the inductor.
When the mosfet is non-conducting, the stored energy from the inductor is passed on to the capacitor.

Bertus

 

Seriously? A buck converter has 3 components that perform the critical function. (2 if ypu don't need a smoothing capacitor. ). If one cannot understand how it works, one really does not understand inductors.

Bob

Fair enough. I must've been thinking of more complicated implementations. Sorry for the snarkiness.

 

Hi all,

I understand the working of an inductor. In certain applications, it will used to generate a high voltage.
But do inductors limit current?

In this video at 8:55. it is said that we can use an inductor to limit the current?

I have never heard using an inductor to limit current. Is it right? Can someone explain in simpler terms?

Thank you.

Hello,

Inductors, just like capacitors, are used for various purposes. Sometimes they will be used to limit current, however what they really limit is the current SURGE.
They can do this because when the voltage across the inductor changes, it takes a while for the inductor to react completely as the current ramps up, so there is less current than there would be if the inductor was not used there. The current ramps up in a somewhat slow way, and that keeps the current lower for a while. If conditions do not change though, the current will keep ramping up and thus the current could go to a very high value anyway. Usually the inductor value is chosen so that this doesnt happen if it is used to limit the current surge.

In converter circuits inductors are used as energy storage elements. That allows them to be used to provide a 'true' power conversion.

In filter circuits they are used to eliminate some frequencies and pass other frequencies.

In oscillators they are used to provide a resonant point so that the circuit can maintain oscillations at a given frequency partly determined by the inductor value.

A transformer is made up of two or more coupled inductors.