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Max Current in inductor.

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electricity86

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I'd like to apply 20Arms through inductor,
Does it mean that i need to pick inductor with at least 28.28ADC rating?

If I pass through it AC current, how do i know whats its resistance, if its written that it has max DC resistance of 1ohm?
 
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Sceadwian

Banned
If you want to pass 20amps RMS through an inductor you need an inductor that's rated for a minimum of... 20 amps =) I'm not sure where you're getting 28.28
For AC it's resistance is the least of your problems, inductors generally don't let AC pass without some reactance, which is dependent on the frequency of the AC signal you're trying to pass.
 

electricity86

New Member
The 28.2A is 20Arms * Square(2).

Its important for me to know whats the ESR of the inductor, besides the XL = wL (reactance).
Do you know what is its ESR in AC?
 

Sceadwian

Banned
28.2A is 20A RMS? What do you mean? 20 amps RMS is 20 amps... 28.2 peak amps maybe but it's RMS that's important for valuing components for AC. I'm not sure what you're talking about ESR. Reactance + Resistance = total impedence if that's what you're looking for. Without the frequency and value of the inductor we can't answer the question.
 

electricity86

New Member
I'll try to be more clear.

1.
The current limit is in DC, meaning if i apply AC current on inductor, i need to match the AC current with the DC current limit, therefore to do this match, should i take the peak value of the AC current and make sure the the peak current is under the DC current limit?

2.
In datasheet its said: MAX DC RESISTANCE IS 0.9ohm.
What is MAX DC RESISTANCE?

Thanks.
 

Sceadwian

Banned
I thought I was clear electricity, you don't use the peak value, you use the RMS value for AC, and inductor can handle MANY times it's 'maximum' current for short durations so the peak value is irrelevant as long as the RMS current is within it's specs. If the frequency allowed it you could probably put 10 times the rated current through that inductor for very short periods of time.


Datasheet for what your inductor? It sounds like the ohmic resistance of your coil is .9 ohms.

Is this a little more clear?
 
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electricity86

New Member
Hey,
Yes its more clear now.

If i need to apply 20Arms current for long periods of time, then how do i need to match the AC current (20Arms) to the MAX DC Current limit ?
meaning for 20Arms long period AC current, what should be the MAX DC current limit (they dont show in datasheet MAX AC Current limit).

Here is the info about the inductor (didnt find datasheet):
PA0135.102NLT

What is then the ohmic resistance (not reactance) in AC, 50Hz?
This ESR should be depended on frequency right?

Thanks.
 

Sceadwian

Banned
It's not usually called ESR for inductors, but yes. The ESR as you call it depends on frequency and the inductance of the coil. I'm not sure how to derate the inductor for power AC applications like that, it depends on what the core material of the inductor is, you'd have to ask the manufacturer.
The ohmic resistance is listed and you've already said it twice now. It's .9ohms resistance, AC or DC ohmic resistance is always the same.
 
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electricity86

New Member
Sorry.

I meant that the ESR is the ohmic resistance, and it shouldnt be depended on the frequency, and only the reactance XL = wL should be depended on frequency, right?

So the impedance of the inductor is Z = 0.9 + jwL ?

What i'm not sure about is long period AC current limit.
I though that the limit of long period AC current is MAX DC Current limit / SQUARE(2).

Meaning that if the MAX DC CURRENT LIMIT is 28Adc, then i can apply only 20Aac.
But you disagreed with that statement.
 

Sceadwian

Banned
You're not looking for ESR, you're looking for bulk (ohmic) resistance.
Just a language mix up =)

I wasn't thinking the DC maximum thing through sorry for shooting you down so fast. The problem with AC VS DC signal is the ripple. There's going to be heat loss in the core under AC conditions which could cause it to over heat faster than just the ohmic resistance, only the maker of the inductor can tell you what the effects of AC are. Someone else here might have a better idea of how to derate the inductor for AC use.
 

electricity86

New Member
Thank you very much for all your help and patiance :)

I know from capacitors that if the voltage rating is 353Vdc than you can apply on it 250Vrms.
In my design there is a connection like that:

AC_LIVE -------- 1N4007 -------- CAP(electrolyte)------AC_NEUTRAL.

And i was told that the CAP should be at least 250*SQAURE(2) voltage rated.

Isnt it true?
 

Sceadwian

Banned
Caps are not inductors, inductors are VERY VERY different.
 

electricity86

New Member
I know that they are the same, one keeps current continuousity, one keeps voltage's.

But why isnt it the same regarding the rating, where you take the peak of the AC and compares it to the DC limit?
 

Sceadwian

Banned
Because the physical construction of a capacitor and an inductor are VERY different not just their function.
A capacitor has a voltage limit because after a certain voltage their dielectric (internal insulators) break down and conduct, destroying the cap.
Inductors are wires wrapped around ferromagnetic cores mainly, the insulation of most inductors is significantly thicker than that of the dielectric of a capacitor so it can withstand a much higher voltage before the dielectric breaks down. A capacitor's insulator has to be thin because the closer the plates are together the higher the capacitance. With an inductor the insulator is just there to prevent short circuits. Air core inductors don't even need insulation if they're spaced far enough apart.
 
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Sceadwian

Banned
The coils resistance is always listed for an inductor, on some high value inductors it can be incredibly high.
 

MikeMl

Well-Known Member
Most Helpful Member
What SCeadwian told you about AC current through an inductor is WRONG!. The maximum current rating for an inductor states the peak current, which if exceeded, will cause the inductor's core to saturate.

If the rated peak current is 20A, then you can pass 20A dc, or 20/1.414= 14.1A rms.

If you need to pass 20 A rms, then buy an inductor rated for 28.3A dc, assuming of course that your waveform is sinosoidal.
 
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Hero999

Banned
I meant why they wrote DC resistance, and not just resistance, since it applies both for DC and AC.
That's not true, at higher frequencies the resistance will increase due to skin effect losses.
 
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