Parallel Inductors - aiding, opposing and confusing.. - Page 1
Forum section: electronics / beginners

author: Quantumplate
author_rank: Regular Contributor
timestamp: June 09, 2022, 03:16:56 pm
content: Looking at my badly drawn diagram, it seems to me it depends on how close the inductors are, that determines whether they aid or oppose in their mutual inductance.So in no.1 they are so close that they oppose - see the opposing arrowheads thru the two cores. And in no.2 they aid in the space between them. Only problem is, I read examples online that state for this arrangement (coil polarity/dot convention), it classes as aiding only.Hmmm.

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2022, 03:23:04 pm
content: The "dot" convention refers to the polarity of the mutual conductance between the two coils.Self-inductance is positive-definite, but the mutual conductance can be either polarity.Therefore, the "aiding" result for coils with the indicated "dots" is true by definition.

author: Quantumplate
author_rank: Regular Contributor
timestamp: June 09, 2022, 03:47:49 pm
content: Quote from: TimFox on June 09, 2022, 03:23:04 pmThankyou, but I don't understand why it's true by definition, as mutual is, as you said either polarity. I'm looking for an explanation that takes into account flux lines - a pictorial answer really.Also, on this web page why has he classed these two inductors in diagram 2.1.4 as opposing? If they are wound the same direction then the dots would be both in the south end. I know they are in series this time, but still a physically parallel arrangement.https://learnabout-electronics.org/Oscillators/osc21.php

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2022, 04:01:37 pm
content: The dots are assigned after determining the polarity of the mutual inductance. That is the definition of the dots in a circuit drawing.An interesting experiment: start with two "flat" coils, where one can be shifted along a plane slightly spaced from the other coil.With the coils coaxial, the mutual inductance has one polarity, but as you shift one away from the other, the mutual inductance goes through zero (when they overlap slightly) and then goes negative as you shift it farther.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2022, 04:05:02 pm
content: Heh, you're mixing schematic notation -- the dot says which way is up, period -- with a physical representation (field lines between literal solenoid coils?).As it happens, adjacent (non-coaxial) solenoids have very little coupling, like 3% or something like that, varying with length and obviously spacing. So it's not a very strongly motivated example, if meant as a physical diagram.You can feel this mechanically by playing with magnets, notice how the force is weaker on the side of the magnet than on the poles. It's also changing more rapidly with position (say if you probe with a paperclip along the side, it's drawn strongly to the poles, and fairly weak in the middle).Anyway, note that the field lines are opposing each other, meaning these would be in opposing configuration. Again, you might not notice the difference, if you're, say, measuring the inductance of the pair. Consider what happens if you slide and rotate one coil along the field lines, closer to the other: eventually you'll have them butted up together end-to-end, and the lines shown are opposing (and also now coupled much more tightly, say 10-40% depending on length/diameter ratio). It's now very easy to see why they're opposed: one is simply connected/wound the opposite direction. Contrast with a single (double-long) solenoid cut in the middle, the two halves wired up identically (top to top, bottom to bottom). Or take it further and assume one can slide within the other, perhaps screwed together ("bifilar" / double helix), or their diameters differing slightly: now you have very tight coupling (>90%?) and it's like one single winding of double the turns, or of two layers!Tim

author: emece67
author_rank: Frequent Contributor
timestamp: June 09, 2022, 04:58:23 pm
content: .

author: sigma_xi
author_rank: Contributor
timestamp: June 12, 2022, 08:25:10 am
content: As pointed out by @T3sl4co1l, be mindful about notation! The inductor symbol (with or without the dot) is considered a lumped element per definition, therefore all field lines are contained "within" the device, and must not be drawn outside of it.If your drawing is meant to be a physical representation -- I assume it is -- be aware that you need to define the geometry of the inductors (Area etc.) in order to calculate mutual induction. Furthermore there are infinitely many field lines* extending far beyond either of the coils, so (1) and (2) in your drawing are pretty much identical, except that you chose to draw a different amount of field lines.Finally, I can only repeat what @emece67 said: It is the flux linkage that matters, i.e. the amount of magnetic flux caused by one coil that goes through the other coil.* ignoring the flux quantum here

author: Quantumplate
author_rank: Regular Contributor
timestamp: June 12, 2022, 09:03:39 am
content: Thanks for chipping in! I need all the help on this one. I have a page in my notebook that I think has caused me a lot of problems as it's wrong...I think. Can you confirm that for the coil winding directions drawn, that the sec. output dot and +polarity should be at the bottom not the top? The sec. current arrows would be wrong as well in that case.

author: mag_therm
author_rank: Frequent Contributor
timestamp: June 12, 2022, 12:35:29 pm
content: In your sketch of reply 7, I think you have shown the windings as being both identical and with same magnetic orientation.That is by imagining the sec winding being slid down along the bottom limb and up to the same limb as the pri.However your sec dot and voltage polarity are shown reversed I think. The sec current will flow OUT of the lower leadout as drawn.(in setting up the kirchoff loops for a transformer, the mutual inductance voltage sources are negative in the two loops)I used to do inverter c-core and toroidals in the kAmp range with windings connected in series and parallel combinations by busbars.Double checking it all for no drama at switch on.....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2022, 05:48:44 pm
content: Yeah winding's reversed, think of it this way: the end of wire sticking out the same side of the core, gets the same polarity. So the top-left and bottom-right are the same. Imagine shimmying them around so the coils overlap: which way could you connect them in parallel with no effect (if they're also equal turns, of course)?Another fact easy to miss: one pass through the hole is one turn. Even if it's not bent wire per se. So a ferrite bead on a cable, is one turn. This connects with the fact that all current loops must close: it doesn't matter the shape, it will eventually loop back. Doesn't matter if it's threaded onto a perfectly straight wire then returns around infinity, or threaded onto one of two parallel lines connected at the far end, etc. The added inductance due to the core will still be the same.Riddle: for an EI core, what is "one turn"? "Half turn"? (Actually getting a practical half turn, is a more complicated question actually; but this much ought to suggest the correct answer for the first.)Tim

author: Quantumplate
author_rank: Regular Contributor
timestamp: June 12, 2022, 07:22:17 pm
content: Quote from: T3sl4co1l on June 12, 2022, 05:48:44 pmThanks for the confirmation and the extra info which I didn't know.As for the riddle....er, is one turn just going thru one loop like a bead then?And half a turn the same but at a funny angle?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2022, 09:30:01 pm
content: A funny angle..? Consider the comment about closed current loops. It also applies to closed magnetic loops -- it's actually a topology problem!Tim

author: Quantumplate
author_rank: Regular Contributor
timestamp: June 13, 2022, 04:22:56 pm
content: Quote from: T3sl4co1l on June 12, 2022, 09:30:01 pmOk, so I googled it. My poor brain's just updating the knowledge of how a transformer actually works - none of the dozen books I've read so far bother talking about flux, opposition and the dynamic dance that occurs when a load is attached.So a turn is around the middle leg which is twice the cross-sectional area of the outer legs, and if you want a half-turn then go off script and do a quick loop around an outer leg, making sure you wind it so it gives flux in that leg in the opposite direction to the middle core.Good question!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 13, 2022, 07:19:13 pm
content: Yup. I actually have... oh yeah these are quite old hah, but visualization nonetheless. I think someone was asking with a particular phrase, well, that's fine as a turn:This is the more usual way, keeping the ends close together to minimize stray inductance. Both are one full turn around the center leg.Note that the flux from a half-turn (i.e. around just one leg) has two paths to go -- so in general it won't divide evenly between center and opposite legs. There's an extra degree of freedom there, and the equivalent circuit acts something like two transformers in series, where you can have different voltages on the two side legs.Which gives us an explanation for the riddle of the half-turn inductor, where the center leg is gapped: most of the half-turn's flux flows around the outside (ungapped, low reluctance), and therefore can saturate very easily. Which in turn has weird effects on the inductor overall.Which I also have visualizations (and measurements!) of here, and a more detailed explanation.https://seventransistorlabs.com/DC-DC_RE/index.htmlThere's also a TI (née Unitrode) appnote somewhere out there that discusses the proper way to do fractional turns, when you need just slightly different voltages on a transformer, say; they explain using straps between legs (including those rare 4-leg kinds) to enforce flux balance, and then main windings can be placed wherever you like while getting the expected voltages.Tim