Difference between AC & DC inductors?

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Train tech

Joined Mar 10, 2024
9
I’ve read inductors are normally only used for DC. However, I have seen some used in AC circuits to filter transients. What’s the difference between ac and dc inductors and what are the different uses of them?
 

Papabravo

Joined Feb 24, 2006
21,258
Inductors are not useful in DC applications with the possible exception of their use in pairs as a common mode or differential mode choke. Inductors are basically AC, RF, and switched mode components.

Choke (electronics) - Wikipedia

The basic function of an inductor is described by the following differential equation:

\( V_l\;=\;L\cfrac{di}{dt} \)

In words, this equation reads: "the voltage across an inductor is equal to the inductance of the inductor times the time rate of change of the current through the inductor."

So if di/dt = 0, there can be no voltage across the inductor. If di/dt ≠ 0, then we are not dealing with a DC circuit.

The impedance Z, of an inductor is a complex number with a real part and an imaginary part. The real part is usually small and comes from the DC resistance of the wire and has unit of Ohms. The imaginary part comes from the inductive reactance and is dependent on frequency. It also has units of Ohms. This dependence is:

\( X_L\;=\;2\pi fL \)

This equation reads: "The inductive reactance is equal to 2 times pi times the frequency times the inductance." N.B. this is a valid relationship for sinusoidal signals ONLY. It does not apply to the fast edges of a switching waveform. Complex numbers can also be expressed in polar form by computing the magnitude and the angle of the impedance vector with the real axis.
 
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nsaspook

Joined Aug 27, 2009
13,418
I’ve read inductors are normally only used for DC. However, I have seen some used in AC circuits to filter transients. What’s the difference between ac and dc inductors and what are the different uses of them?
Just curious, where did you read that at?

In the old tube universe there were plenty of DC power supply chokes (L4) but they were there to help eliminate a AC component of the DC voltage.
1710274449417.png

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There was a difference in AC/DC core design, as the ones used for DC ripple filters were designed not to saturate from the steady DC current but there was a variation called a Saturable reactor that did use DC saturation to control AC current.
 

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MrChips

Joined Oct 2, 2009
30,925
I’ve read inductors are normally only used for DC. However, I have seen some used in AC circuits to filter transients. What’s the difference between ac and dc inductors and what are the different uses of them?
Your statement is a misrepresentation of what an inductor does.

An inductor has a property call reactance, which is proportional to frequency, i.e. the higher the frequency, the higher the reactance. At 0Hz, the reactance is zero. Hence an inductor has no effect on DC, except for the inherent resistance of the inductor.

To repeat, an inductor has zero effect on DC.
In a DC circuit, an inductor is used to filter out AC. In such applications, an inductor is also called a choke.

Inductors are used in frequency dependent circuits. For example, inductors are used in radio transmitters and receivers to tune to a given frequency.
 

BobTPH

Joined Jun 5, 2013
9,124
Think I’m getting confused. Ac inductors reduce current and dc inductors don’t, is that correct?

Anyway, are DC inductors not used on their own? To reduce interference? And how does this work. Also, what’s a dc inductor function when used with a resistor or capacitor.

And, Ac inductors are used to oppose sudden changes in current?

just looking for a basic understanding of the purposes of the two different types and how they work
There are not two different types.
 

Papabravo

Joined Feb 24, 2006
21,258
Think I’m getting confused. Ac inductors reduce current and dc inductors don’t, is that correct?

Anyway, are DC inductors not used on their own? To reduce interference? And how does this work. Also, what’s a dc inductor function when used with a resistor or capacitor.

And, Ac inductors are used to oppose sudden changes in current?

just looking for a basic understanding of the purposes of the two different types and how they work
Go back and read post #2
 

MrChips

Joined Oct 2, 2009
30,925
Think I’m getting confused. Ac inductors reduce current and dc inductors don’t, is that correct?

Anyway, are DC inductors not used on their own? To reduce interference? And how does this work. Also, what’s a dc inductor function when used with a resistor or capacitor.

And, Ac inductors are used to oppose sudden changes in current?

just looking for a basic understanding of the purposes of the two different types and how they work
I have never seen the terms "DC inductor" and "AC inductor" used.
An inductor is an inductor, is an inductor. There is no difference between a "DC inductor" and an "AC inductor".
 

Ian0

Joined Aug 7, 2020
10,001
An inductor on a DC supply will have much smaller peak-to-peak flux excursion than an inductor on an AC supply, and consequently much less core loss (core loss is approximately equal to the square root of the fifth power of the peak flux excursion).
In both cases, the design will incorporate a coil of wire around a ferromagnetic core with a gap in it, but the relationship between wire size and core size will be different.
 

MrSoftware

Joined Oct 29, 2013
2,208
I think maybe where you're getting confused is trying to separate AC and DC. In real life, circuits often have both. Or to be more precise, you can have DC with noise but the signal never crosses the 0 line (wiggly DC I guess). Or you can have AC with a DC bias, such as a sine wave with +20V/-10V, just to pull an example out of the sky. Or maybe all of the above, all mixed up, it can get messy. Check out how an FM radio works for example. Or how the output MOSFETs are DC biased in a Class A/B audio amplifier.
 

MrChips

Joined Oct 2, 2009
30,925
People are getting confused with the terms DC and AC.
In usage outside of power utilities and power supplies, DC and AC have a different meaning.
All electrical signals have DC and AC. In theory, DC refers to frequency of 0Hz. Everything above 0Hz is AC.
In real life, 0Hz does not exist. It depends on how low you want to go in frequency. Is 0.0000000001Hz DC?
 

panic mode

Joined Oct 10, 2011
2,793
inductor is a generic name for many products that make use of magnetism. there is no such thing as AC or DC inductor... all inductors are - inductors. the only case that i can think of at the moment where the difference does exist is things like solenoids/relay coils since inductance is not a factor in DC. so solenoid rated for 48VAC but connected to 48VDC would smoke.
 

MisterBill2

Joined Jan 23, 2018
18,934
Certainly Inductors are used in both types of circuits, for quite different purposes, although in most cases it is to provide inductive reactance, which affects alternating (rapidly varying) current. (Understand that "alternating" can mean varying magnitude without reversing the direction.)

Besides that is the entire realm of RESONANT CIRCUITS, which use both inductors and capacitors. But that is quite an additional topic in itself.
 

nsaspook

Joined Aug 27, 2009
13,418
I have attached two pictures of inductors being used that I want to understand. They are both used after rectification and are on a ‘DC link’ that supplies an inverter supplying 3 phase motors.
Why should we guess (even if our guesses are likely to be true) the function of those components in isolation from the reset of the schematic? This is a IMO ridiculous method of asking questions.
 

Papabravo

Joined Feb 24, 2006
21,258
To be helpful? I cannot share the whole schematic
Great. I guess you're SOL. Ask the guy who decided it was proprietary to solve the problem. You might already be in hot water for revealing the existence of the proprietary device. I knew a guy who got canned for just such an offense. He was a fresh out and asked a former professor for help. Turns out the professor was a consultant for a competitor. Lawyers had a field day with this poor SOB.
 
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