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I have seen many textbooks that say consider a capacitor which is polarised (DC capacitors) and non-polarised capacitors also exist, but why don't inductors have AC inductors and DC inductors? Sorry for the silly question, but I want to know the answer.

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    \$\begingroup\$ polarised capacitor exist only because there is a chemical process that forms the insulating layer, in one polarity, and dissolves it in the other polarity. The polarisation is a chemical thing, nothing to do with the desired electrical properties. The capacitor itself is not polarised - it just fails if used for long with the wrong polarity. \$\endgroup\$ – Henry Crun May 18 '18 at 4:10
  • \$\begingroup\$ Why should it be polarized is the question you should ask. For example, you also wouldn't ask "why do normal cars don't have guns mounted on them, because I saw a tank once, and that tank had a gun"! The principle you should be applying has a name in philosophy: Occam's razor and it means that the general assumptions used to understand something should be minimal. Things needing to be polarized is the opposite of a minimal assumption! \$\endgroup\$ – Marcus Müller Jun 7 '18 at 14:44
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There are inductors with polarity. They have a permanent magnet to bias the inductor opposite to the expected DC current. It's generally not worth it, but it has been done.

See, for example, DOI: 10.1109/ESTS.2011.5770892 Permanent magnet inductor design.

Abstract: Permanent magnet inductors (PMI) are useful in dc biased applications. They utilize the flux produced by a permanent magnet (PM) to partially offset the flux established by current in the inductor winding. The addition of permanent magnets allows for decreased inductor mass for a given inductor current and inductance rating. Herein, the Pareto-optimal front between loss and mass is established for a PMI inductor and is compared to that of a traditional EI core inductor. It is shown that the PMI inductor is significantly less massive than the EI core inductor for a given loss.

Polarized capacitors (which is a significant limitation on their application) are used because a polarized capacitor can be made significantly smaller and lighter than a non-polarized capacitor of the same value. If non-polarized capacitors were the same size, cost and performance as polarized capacitors of the same capacitance and voltage rating, there would be no reason to use the latter.

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  • \$\begingroup\$ Interesting find on PMI design. Thanks. Also, I think "lighter than a polarized capacitor" might want to be "lighter than a non-polarized capacitor". Typo, I know. \$\endgroup\$ – jonk May 18 '18 at 3:57
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    \$\begingroup\$ @jonk. Thanks. I have a vague recollection such a device was used in a consumer product (Japanese-designed TV set maybe). \$\endgroup\$ – Spehro Pefhany May 18 '18 at 4:01
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    \$\begingroup\$ If there is one export from Russia that I like, it's sci-hub for getting articles behind paywalls. Scientists want their work to be seen, never say no when I ask for a copy by email, and broadly speaking do NOT approve of Elsevier's (who over the years has bought itself a solid monopoly) policies much. Now I can just pull it. So I've been asking less, I admit, and reading more. \$\endgroup\$ – jonk May 18 '18 at 4:10
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No this is an interesting question and I learned things myself while answering this question :) There are some instances where inductors may be polar sensitive. For instance, let's look at mutual inductance.

enter image description here enter image description here

Credit goes to Fundamentals of Electric Circuits (6th edition) by Matthew Sadiku, chapter 13, page 557.

As you can see, polarity does matter when you're talking about transformers and mutual inductance. But this is more of reference polarity and magnetic field polarity. But you place the inductors upside down and it won't change anything. In other words, the inductors themselves don't have polarity.

In the case of regular inductors that are not meant to be used for its mutual inductance, it's just a coil... and no matter what direction that the current travels, it will always follow the proper path that the laws of physics define it to be. You can ask the same question about resistors. Why aren't resistors polar sensitive? It's because current travels the same way regardless how you place the resistor and the current behavior is always the same through the resistor. The same is true about an inductor.

As Iganacio pointed out, the reason why capacitors may have polarity is that of the material inside of a capacitor that defines it as polarization. On one side, there's dielectric material and on the other side, there's electrolytic material... You don't have that with inductors...

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  • \$\begingroup\$ It's worth mentioning the flyback transformer here. It has similar properties to the transformer but includes an airgap where the energy is stored, just like a regular inductor does. You can use a regular transformer/CMC as a flyback but the core does not store much energy so it's suitable for low-power circuits where isolation is desired. \$\endgroup\$ – Barleyman May 18 '18 at 9:35
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Maybe you are a bit confused. An inductor has no polarity until a coil of wire around it gives it a DC polarity because it produces a polarized magnetic field by using DC current, or use AC current of a frequency within the cores resonate range and you get an AC magnetic field of the same frequency.

Polarized cores for relays were tried for a few years then pulled from the market, as the pre-existing magnetic field tended to make them change state with extreme vibration on the z-axis.

If a core were pre-magnetized and a strong AC magnetic field was applied it would loose its pre-magnetized field and have no magnetic field unless AC or DC current flowed in the coil around it.

Inductors in general do NOT retain any magnetic field once the current is turned OFF. They are made of different materials than those used to make electromagnets or permanent magnets.

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    \$\begingroup\$ I think miniature relays still are: By using the highly efficient polar magnetic circuit “seesaw balance mechanism”, a nominal operating power of 140 mW (minimum operating power of 79 mW) has been achieved panasonic-electric-works.com/pew/de/downloads/… . I always assumed this meant they have a magnet inside. \$\endgroup\$ – Henry Crun May 18 '18 at 4:18
  • \$\begingroup\$ Latching relays are generally magnetised, they are not considered an inductor first though even though they are a solenoid coil, their function may require passing current through in a specific direction and this current may behave the same electrically though the mechanical effect differs depending on direction. \$\endgroup\$ – KalleMP May 18 '18 at 7:18
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We can also remember that while capacitors are not theoretically polarised, though electrolytically formed ones fail if incorrectly connected so similarly a general inductor is also theoretically not polarised unless magnetically biased (well described in other answer).

However in practice they may be constructed in ways that require taking note of which terminal is which. Mostly this relates to the capacitive coupling to cores and cases or the electrostatic shielding from the outside casing or layers.

Ganged capacitors and mutual inductors are also polarised in relation to other sections of the component (as mentioned in other answer).

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