special inrush control transformer? - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: June 15, 2018, 02:18:55 am
content: Does something like this exist? Since a normal E? transformer will pass like 10X its rating, and torroidal even more..What criteria am I looking for exactly for a protection standpoint? Thick windings for the current but small core? Special geometry? What side effects does it have? Are there special magnetic materials with a very sharp knee that are not necessary optimized for common use? Would it require special cooling fins or something?I am guessing it would distort signals being near saturation and act like a mixer? So bad from a EMI standpoint. But power semiconductors might explode less and appreciate it.Don't have anything in mind but I wonder if its possible to hone in on something weird. It would be intrinsically safer so the fuses and other protection would take less of a beating and all that good stuff.

author: Circlotron
author_rank: Super Contributor
timestamp: June 15, 2018, 03:15:33 am
content: If you use a transformer with a primary voltage rating of double your nominal mains voltage you will never have any inrush current. For example, say you normally use a 120V-10V transformer. Change it to a 240V-20V tranny (same current rating) but still run it on 120V. End of problem. Only down side is the tranny is twice the size it otherwise needs to be.

author: JS
author_rank: Frequent Contributor
timestamp: June 15, 2018, 12:40:52 pm
content: I was going to say similar, big core, write just thick enough, more turns than needed. You could also introduce some leakege inductance by winding the thing "wrong"Quote from: Circlotron on June 15, 2018, 03:15:33 amJS

author: CopperCone
author_rank: Super Contributor
timestamp: June 15, 2018, 01:52:44 pm
content: Arent you just using resistance by using a 240v design since it uses thinner wire?I wanted to handle the issue in the magnetic domain somehow.What would be wrong winding?Its the morning but dont you get harder to saturate with a big core? I dont follow.

author: Gyro
author_rank: Super Contributor
timestamp: June 15, 2018, 01:56:47 pm
content: No, it's not the resistance (well not significantly anyway). It's the larger than normal core that resists saturation in the first half cycle, particularly if the switch point happens to be a voltage zero crossing.Saturation = Inrush current spike.

author: CopperCone
author_rank: Super Contributor
timestamp: June 15, 2018, 01:59:24 pm
content: Ooo i think we are talking about different things. When the core saturates you heat the primary. I dont care about the primary i want toprotect the secondary. If the primary explodes the electronics are still protectedTo define better: turn the transformer into a circuit element that stops big ass transients from hitting triacs and rectifiers and capacitors unprotected by anything else.The saturated core is no longer passing energy

author: Gyro
author_rank: Super Contributor
timestamp: June 15, 2018, 02:05:37 pm
content: The only transformers that do that sort of thing (I think) are the ones that have inbuilt magnetic shunts - eg. MOTs and Neon sign lighting transformers. They limit current but as a pseudo constant current output rather than just inrush limiting.Edit: Other than that, a 'Reactor' in series with the secondary, as used in power transmission networks...https://en.wikipedia.org/wiki/Current_limiting_reactor

author: David Hess
author_rank: Super Contributor
timestamp: June 15, 2018, 02:32:50 pm
content: You might be describing aferroresonant transformer.

author: CopperCone
author_rank: Super Contributor
timestamp: June 15, 2018, 03:59:33 pm
content: Hmm i forgot about cvt. I just wanted something to prevent an inrush explosion of faulty parts etc. I guess i am looking for somekind of combination of the reactor and the transformer in one.I wonder if its possible to engineer something here that has a.favorable weight and size.A constant current device might make sense with a very wide buck boost converter that can handle it, but it would be something like a solar panel matching circuit i think.A current limited cvt would be better i guess but a cvt has a capacitor who knows what can happen to that part.Also with the cvt i feel like im picking a fight with a switching voltage regulatorSeems like it would be a nice thing for products to custom design your transformer and reactor for the specification so you can get away with using something like an ultrafast fuse. I feel that in general things engineered to survive surges are compromised in other ways, regardless of the engineering discipline.

author: CopperCone
author_rank: Super Contributor
timestamp: June 15, 2018, 04:25:25 pm
content: Also if you saturate the magnetic path the harmful energy never breaches the magnetic isolation so it cant bypass some protection by a short.Trying tothink if it matters where youplace a line reactor, on primary or secondary

author: james_s
author_rank: Super Contributor
timestamp: June 15, 2018, 04:30:26 pm
content: It sounds like what you want is a fuse. It's not inrush that's going to do damage, it's the steady state current drawn by shorted components.

author: David Hess
author_rank: Super Contributor
timestamp: June 15, 2018, 04:41:30 pm
content: Quote from: CopperCone on June 15, 2018, 03:59:33 pmThe solution I use is a pair of back to back power transformers wired as a two stage isolation transformer. The added series impedance limits short circuit current but probably not to the extent you want.An online UPS can provide very fast short circuit protection. My home built isolation transformer is powered by an online UPS.A bunch of incandescent lamps wired in parallel and then placed in series with the AC power input may be suitable.A linear AC constant current limiter circuit is possible but not trivial because of peak power and secondary breakdown issues. I have often thought of designing one for my own use. Secondary breakdown and peak power can be handled with foldback current limiting. Place a DC constant current limiter circuit inside of a diode bridge for AC operation.A step up from an AC constant current limiting circuit is an AC inverter with current limiting. What about one of the various commercial AC power supplies?QuoteCVTs being entirely passive are the toughest of the various options but I am not sure how they behave under overload conditions. They operate in saturation however so I think they would do what you describe. The capacitor is not something to worry about.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 15, 2018, 04:43:57 pm
content: Faulty parts are, by definition, those that can't handle inrush. Note that this definition doesn't have to have actively failed, to be faulty; it only needs to be a problem waiting to happen.Anything made of iron and copper (namely, your last several threads) is way, way down on the list of concerns, regarding mains transients (both voltage and current) and the things connected thereto.Tim

author: CopperCone
author_rank: Super Contributor
timestamp: June 15, 2018, 04:56:27 pm
content: I know commercial power supplies are pretty good but i want to explore it.. Call it being a reliability nutI read something about cvt causing harmonics problems or something on this forum before.I like the idea of double transformers, it seems practical off the shelf and fast but i wanted to eNgineer something. For some reason i always found the mains filter and protection circuits interesting, it can be a fairly large pile of crap that you can completely bypass but it makes something feel nice and for some reason they look visually pleasing (telephone filter is one of my favorites)Wikipedia actually has a ton of references for reactor placement that i want to read through now.Can you explain what you mean by putting the circuit in a diode bridge?I am also wondering if these extra reactors can cause some kind of resonance issues. Itreminds me of the old tube bridge rectifier filter circuits that use a inductorI am having serious difficulty imagining your ac constant current linear regulator in a bridge idea

author: woodchips
author_rank: Frequent Contributor
timestamp: June 15, 2018, 05:42:47 pm
content: Put some air gap in the magnetic circuit. Standard way to limit the inrush to large welding transformers. One I just scrapped had about 2.5mm of airgap material for a 450A welder.

author: CopperCone
author_rank: Super Contributor
timestamp: June 15, 2018, 10:53:06 pm
content: Is that so they can use a commercial off the shelf transformer and just mod it?

author: james_s
author_rank: Super Contributor
timestamp: June 15, 2018, 11:00:17 pm
content: Huh? Nothing is modded, it's built that way from the start. It's so they can have a transformer that does what an arc welder needs to do, provide a reasonably constant current to the arc, the voltage of which will vary depending on the distance from the tip of the rod to the work.

author: JS
author_rank: Frequent Contributor
timestamp: June 15, 2018, 11:36:12 pm
content: Saturating the core is opposes to what you want, with high load the core comes out of saturation and gives even more power. They do CVish kind of thing, not CV. Gap sound more like it or serial resistance. The DC CC inside the bridge is a good idea as it could be low loss and low current limit, given the minimun voltage presented by it is low enough compared to the total voltage.JS

author: CopperCone
author_rank: Super Contributor
timestamp: June 15, 2018, 11:38:13 pm
content: How does the core saturating in a transformer increase power? I think of it as magnetic impedance being high, so magnetic energy is being restricted to the secondary.To me it makes sense that you don't want a serially connected reactor to saturate (since at that point its like a short circuit with air inductance?), but saturation is how a transformer stops current limiting?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2018, 01:44:25 am
content: A ferroresonant transformer goes from inductive to capacitive reactance* as load or supply voltage increases.*To some extent, at least somewhere inside the transformer. There may be enough primary magnetizing current to balance this at nameplate nominal operation, I don't know.The saturating element is the secondary side core, and the secondary (resonant tank) is fed by a series inductance (leakage, implemented with gapped magnetic shunts). Saturation shifts the resonant frequency, such that constant flux is delivered. If the mains frequency remains constant, then so does the output voltage.Under heavy load conditions (above rated), secondary voltage drops out, and the mains input goes inductive again -- more voltage is dropped across the leakage (which acts in series) than across the secondary winding.Thus, FR transformers have very reduced fault current compared to normal ones, possibly enough that you may have trouble fusing one at nameplate amperage, if your goal is to prevent secondary side fault current from being delivered.Tim

author: David Hess
author_rank: Super Contributor
timestamp: June 16, 2018, 03:00:48 am
content: Quote from: CopperCone on June 15, 2018, 04:56:27 pmI mean that a DC current limit circuit becomes an AC current limit circuit when placed inside of a diode bridge.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: June 16, 2018, 05:15:04 am
content: Rambling midnight answer. How much current is being drawn in normal operation after the inrush is complete?One method I've encountered is to have a small resistor with a high pulse power rating in series with the input, which is in parallel with a relay to short out the resistor after some amount of time. For instance, after a large capacitor bank is initially charged. The relay releases when power is turned off.For instance, let's say you have a 10A fuse, perhaps one of these:https://www.digikey.com/product-detail/en/eaton/BK-MDA-10-R/283-2722-ND/954302It has 0.01 Ohm element resistance. If everything else were perfect, initial surge to a capacitor bank might be 120 V / 0.01 Ohm = 12000 A. Realistically it will be quite a bit less, but if a 1 Ohm resistor were is series such as this:https://www.digikey.com/product-detail/en/vishay-huntington-electric-inc/FVT20020E1R000JE/FVT200-1.0-ND/257610Then surge current would be 120 V / 1 Ohm = 120 A for a surge power of 14400 W. However, for this 225 W resistor its surge rating is 10x for 5 seconds, which may be enough to be fine as the capacitors charge.The relay would then close and get latched using some kind of timing circuit:https://www.digikey.com/product-detail/en/omron-electronics-inc-emc-div/G7L-2A-BUBJ-CB-DC24/Z2258-ND/369373

author: CopperCone
author_rank: Super Contributor
timestamp: June 16, 2018, 03:31:02 pm
content: yea I use stuff like that, like a 555 timer to short out a resistor after some time period, but its not really the point of this thread, I wanted an intrinsic solution not a gimmick, those come later when the best intrinsic solution is found, which will likely not have good margins since it uses passive components and no feedback

author: woodchips
author_rank: Frequent Contributor
timestamp: June 16, 2018, 04:43:23 pm
content: The air gap in the welding transformer just limits inrush current, but does make for a very high magnetising current.To vary the welding current you use a reactor, by varying the DC current through it effectively makes the AC impedance vary, more DC current then more AC current, the reactor is saturated then.Clever things, beat a pile of semis into a cocked hat!