Variacs DO short their windings
Вставка
- Опубліковано 30 вер 2024
- A variac is basically a variable auto-transformer that allows you to adjust the voltage between zero and the local mains supply voltage, or more if tapped mid-winding.
They find use in specialist applications where it's useful to ramp the mains voltage up slowly for regulation or testing. Some applications would be inverter, motor drive repair, soft start of vintage electronics and some specialist lighting applications where lights need to be dimmed without phase angle noise.
These units do not provide isolation from the mains supply, as they are just a single winding with an adjustable tap.
I've sometimes wondered how they can seamlessly jump from one winding to another without bridging loops and crating shorted transformer turns. Since I had my unit open for fitting a new digital meter, I thought I'd test that with a thermal imaging camera.
If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:-
www.bigclive.co...
This also keeps the channel independent of UA-cam's advertising algorithms allowing it to be a bit more dangerous and naughty.
I just had a look at the brush on my 10 amp variac.
It's a 10x10x10mm cube of brass, with a small T-profile length of graphite set into it, about 10x5x5, the edge that contacts the winding being about 1.5x the width of the winding pitch. The graphite measures about 1.5 ohms in the winding-to-winding axis, and 0.3 ohms from the brass to the contact face - the whole length of the graphite insert is effectively all in parallel to keep the resistance to the brass low, while having enough resistance in the winding-to-winding distabnce to minimise heat from shorting turns.
I know someone who tried replacing a broken variac brush with brass. it didn't end wel...
Hey Mike, have you ever popped into the synth-diy mailing list? I think you might find it interesting A lot of analog electronics design happening.
Likewise on my 2A one. Mine came with a spare brush, which I have never needed and occasionally find lying about the workshop (it's supposed to live in the bottom of the Variac cabinet). I notice that the windings are actually set slightly apart and embedded in epoxy so that the contact surface is quite flat and the 'dwell' is very short. I imagine if and when the leg of the T wears down it will be another story, but it will probably outlive me.
Someone else mentioned a tiny brass roller that rode across the windings. I tried measuring mine, but didn't see significant results with my ordinary meter. Maybe I should have tried my installation tester for a higher resistance test current.
@@bigclivedotcom I have a heavy one, that has five ! ~1 cm carbon sliders side by side so with spacing the entire takeoff area is ~6 cm , using the height of the wingdings on the outer circumference. (for normal use I have a smaller 2Amp one though)
i have second hand ones with copper cored graphite wheels, but they got stuck and have been rubbed flat on one side
This is my variac. There are many others like it, but this one is mine.
Full Insulated Metal Jacket.
I picture electroboom with a crazy look in his eye and a big capacitor ready to blow.
This is me bottle o' scrumpy. There are many like it, but this one is... MINE! :D
Where do you get one?
I have never seen a variac like that. What’s the name and model? I’m interested in a low current one with dc possibility my variac will run up to 20 amps and I have a extension cord I made for testing ac powered stuff with an incandescent bulb wired in series on one leg. That way if there’s a short the bulb gets bright instead of seeing the magic smoke coming out of the equipment that you’re fixing. One last question do they still have that motorcycle race on The Isle of Man? I remember that when I was younger and thought it was awesome going around an island as fast as possible. Thanks for your help and time.
Hmmm... I imagine that they can get away with shorting the windings because the voltage difference between the adjacent windings is very small. I'm commenting this before your measurement on the HOPI, but if there is a 80mm diameter variac and a 3mm wide contact, the maximum voltage across the contact would be 240 * 3/(3.14*80) = 2.8V. Obviously if it's shorted, the voltage is ~0V, but this shows the scale of things relative to 240V.
/\ this /\
Small voltage drop = small current dissipation.
Good 'nuff engineering.
@Flat Sign Big Clive mentioned he checked for that, and it seemed to be normal carbon with the same resistance in all directions.
@Flat Sign He tested this in this video. Some fancier high-powered variacs might use this, but this one doesn't. Also, if there are multiple vertical contacts, which one is connected to the actual wiper?
@@LazerLord10 Depending on the physical preciseness of the contact - I would say the 1st wire that makes contact is the effective one.
the problem is not voltage but magnetic flux
It never occurred to me that connecting adjacent windings would do that, but it makes sense
I've wondered about the shorted turns in the past on variacs in the past. Thanks for actually measuring one.
Years of armchair theorizing destroyed in under 10 minutes with a thermal camera and hopi.
I really like the direct relation between turning the dial and the analog meter moving. I've got too used to watching digital meters dance around as their electronics fuss over what the actual value should be.
My Variac is Digital, but the display and the dial are not at all accurate. !!! It's important to use a Volt Meter unless you want to blow up something !!!!
Interesting that you want to replace the analog meter with a digital one. I used to have digital ones but have the odd feeling, that it is much faster and more intuitive to adjust the voltage/current to a desired spot. That will be due to our instincts, we have a hardwired closed-loop in our brains for moving things to where we want it. There is no such instinctive support for 3 digits of numbers.
I added digital volt and amp meters to a variac that never had a readout. Except for the last few volts I never attempt to set a voltage from the meters, they are too slow. The variac has a 0-100 scale, and I've marked the actual output voltage at a couple of interesting points like 115V and 125V, so I can go directly there, or estimate some other voltage as a fraction of those voltages. Once I'm close I can be precise with the meter, if it matters.
It looks like a cheap eBay meter with a slow update rate too.
@@Agent24Electronics It might be relatively fast. I used eBay meters when I did my variac probably 10 years ago, and they update once or twice a second. Not wonderful, but not horrible either.
I just like analogue meters. Another advantage is if the load is varying, a meter shows it better than wandering digits...
@@lwilton twice a second is slow. 3Hz is typical
maybe would be interesting to slide a thin piece of plastic under the brush and see how much of the loss is due to the turns being shorted by it, and what remains
I remember as a kid having one of these for the lights on the Christmas tree, you could dial down the brightness when the room was dark. Of course being a kid I had to plug all sorts of things into it to see how they were affected.
That was likely a rheostat, not a variac. resistor as opposed to transformer. Rheostats were very common for lighting dimmers in the day.
The windings on these variacs aren't always with fine wire. I've got about a dozen old Statco 20 Amp, 0 to 120 VAC variacs, and the windings look like 14 GA. The carbon brush looks like it contacts more than one winding as well, but I guess it doesn't hurt it. Does dim the lights in the room for an instant when the power switch is first turned on, even with no load connected.
That sounds like the classic behaviour of a toroidal transformer.
Yeah, I got my hands on an old Grundig RT5A combined Variac/Isolation transformer that can output up to 3,5A, and it actually has a "soft start" mechanism so it does not blow the breaker when you switch it on. It actually needs it, when I first got it I forgot to reset the power switch and turned it on at the outlet - and suddenly it was dark ^^ )
Built like a brick outhouse, really heavy and the wires on the secondary are really thick don't know what gauge, though. But must be able to take afair bit of heat.
(www.radiomuseum.org/r/grundig_regel_trenn_transformato_2.html)
Now I am kind of intrigued to pull the cover off again and look at it with my cheapo thermal imager.
Magnetizing inrush current. It happens to all transformers, but it goes larger as the transformer size.
Power transformers at substations makes crazy sounds when started because of this.
My General Radio W20 Variac is probably very similar in construction, and yeah, its windings are about that size. It tends to pop the circuit breakers in my apartment if I energize it without a load. But it helps if I place a light resistive load on it, say a 40-W incandescent lamp, and the knob around 20-30%.
FWIW, the large aluminum disk that rotates and holds the brushes is referred to in GenRad/IET documentation as the "radiator". The whole disk is electrically 'hot' during operation -- found that one out the hard way! 🤣
That's how my 14 amp Staco Energy one is. Wound with coarse windings that are just ground flat on top, and the brush is the width of the core thickness. These things are ridiculously expensive, too. Got mine in new condition for free, but I think it was like 400-500 USD new. The sudden inrush is due to the core saturating since there is no magnetic field present at turn-on to resist the incoming AC. Turning the variac on near the zero crossing causes the windings to pretty much only present their DC resistance once saturation is reached, and draw LOTS of current until the field builds up. I have a large Denon receiver that does the same thing.
When I have a print adhere too well to the build plate, I unclamp the build plate and print from the printer and put them in the freezer for about half an hour, so far that's always let me get the print off without damaging the glass.
You tell it to chill out 🥶👍
I find my glass bed sometimes barely sticks so I use glue stick and that makes things way too hard to pry off. Hard to find a balance really, I used to have the flexible magnetic sheet that came with the printer but I was an idiot and scraped it with the scraping tool too much and it wore it down. I guess painters tape is something to try out next
@@waldevv Why not just buy another magnetic sheet?
@@waldevv Painter's tape worked well for me with PLA, but with ABS, the tape would melt into the print and was near-impossible to remove. The other problem was that I had to reapply it regularly. Kapton tape worked better with abs, but it had to be reapplied often as well, and it was very hard to get it on without wrinkles. A sheet of glass and a glue stick has been the best so far, but sometimes the prints stick too well, then I put the whole sheet in the freezer, and the print pops right off.
this is also what you are told to do with troublesome prints that come off of FDM printers that cost 5-6 figures
That glass delaminating thing is very common with PETG, but that was PLA. I had it happen at a small scale once. Maybe it was cheap glass?
I've had pla delaminate on PEI sheet. Temp wasn't high enough to get good bonding. Increased first layer temp and problem solved.
I use cheap soda glass, and delaminating is never a problem because of the high coefficient of expansion. Once it cools the prints quite literally pop themselves off.
Borosilicate glass is much more expensive and is normally recommended because it has a very small coefficient of expansion. I got 3mm glass from a local place for about $2. I got two for a spare, but I've not broken any of them in over a year.
So, I'm thinking that the better the glass (eg borosilicate) and if the surface is textured (ie sandblasted) so the adhesion is better, that's when you are more likely to have problems if the print sticks really well and there's no contraction of the surface to help get it off.
If I leave the bed to cool down completely I literally hear when the print releases. I've also had prints complete while I'm out, and come back to find they've popped off so forcefully they are on the other side of the bed.
@@Chris_the_Muso you should set up some targets and see how many you can pop.
Yep, I had it happen on PLA as well. Tore off a good chunk of the glass bed.
@@Chris_the_Muso haha, that's amazing. I use cut mirrors (I found some really thick 1/4 inch ones), and a smearing of sugar water to keep things attached. It works unreasonably well.
Found my variac in the e waste bin at my university. Not much was wrong with it. Fixed it up. Works like a charm.
I can never see or hear about a Variac without thinking of Andy from photonicinduction cranking them silly in his loft. I hope the guys doing well and can make an appearance again. I miss his "I ain't havin' it attitude"
Dave from EEVblog posted a community post here on youtube a while back saying he was in good health and working on some videos to be posted soon
@@michaellynch1018 yeah,. I tried to follow it best I could a while back and saw his wife was in the UK finally. Last I saw was about wanting to make videos "soon". Guess he needed more time than he thought
Yep, the brush is designed to bridge at least two turns. But it is carbon so it has some resistance, and thus it isn't a dead short, it is just a low resistance across a small voltage.
The brush is designed to bridge turns so that it will work effectively as a rheostat and the output of the variac will not step in voltage, but will be a more or less smooth increase or decrease as the arm is moved. The rheostat effect comes about as the area of contact changes from one winding turn to the next.
BTW, in place of that simple digital volt meter, you might want to look on eBay for a Peacefair model PZEM-021 or model PZEM-022 meter. This will do everything the Hopi will, and fits in about the same hole shape as one of those digital LED voltmeters. They claim to have a minimum input voltage of 80V because they are powered from the same line they are measuring. However, a little looking at the PC board will show you where you can cut a trace and separate the power input from the measuring voltage input. At that point it is fairly easy to adapt either one for use with a variac. They use the same chips as utility-supplied electric power meters, so they are pretty accurate. Search terms are "digital power watt meter monitor voltage voltmeter ammeter" and the like.
Interesting. I've got an old RS Components (I think) metal cased variac from the early~mid 1980s at work. I now have the desire to find it and open it up for a look-see :)
It's worth reminding viewers that this does NOT provide insulation from the mains, it is not an isolation transformer, so needs to be used with care.
I've added that in the description.
I have an old GE variable reluctance transformer. It has a rotatable core in a cylindrical stator. It creates variable voltage without the steps associated with brushed windings. It's much heavier than an equal powered Variac.
I've always wondered this since photonicinduction started posting videos of his variacs and now I know. Although as I've always been warned not to accidentally short a normal toroidal transformer with the mounting lug in the middle I imagined there to be a fair bit more heating.
I also have some of those ZnSe (Zinc Selenide) lenses for my FLIR and they do slightly affect the readings, it can go both ways due to temperature of the lens but it's usually reporting lower than reality by 1 or 2 degrees. The pure germanium ones don't suffer from this problem but are 3x the price.
I believe it is a actually an autoformer . A transformer would have at least two independent magnetically coupled windings and this has only one winding
@@lwilton "I know you are but what am I ?"
@@robstorms "Transformer" just doesnt tell you anything about how many windings something has. Autotransformer, however, does.
@@Basement-Science Just funnin with you ! Thanks for the clarification !
Eddie Van Halen used a Variac to control the volume on his amp.... RIP.
Yep, he turned it down to around 90V. Some say he had a 240V Marshall amp originally, and the tubes were biased too high.
I had the same incident with my 3D printer a few times. I discovered that adding a very thin layer of PVA glue to the bed helps with adhesion on those zones where bed level is not perfect and, at the same time, stops the plastic from fusing with the glass bed.
To apply the PVA I use a generic Dealz (Poundland) spray bottle filled with Dealz liquid PVA lightened with distilled water (filtered is good enough) and a bit IPA, to help the water to evaporate faster.
Everybody has a slightly different solution for this issues, so you'll get lots of suggestions, my recommendation would be to test and find the one that works better for you.
I uses Kapton/polyimide tape for my 3D printer or those flexible reusable plastic beds. No more scraping.
you don’t need to add anything to a glass bed to induce adhesion. I print fine on mine without anything.
Prrit stick
@@reme2493 It depends a lot on the printer and the material you are using. Ideally, you don't need to add anything to the bed, you are right, but reality is a bit more complicated. I personally was a huge fan of the "nothing better than a perfectly clean bed", until I started doing more and more prints and started having more and more different issues.
Some printers, specially the ones with bigger beds, almost always have issues with the bed not being perfectly flat, which affects levelling and adhesion, even when using ABL (that I strongly recommend). Please, keep in mind that I'm talking about machines on the price range of 300 to 500 €.
On the other hand, some materials really love glass, specially PETG, but also some blends of PLA. In that case, the problem is the opposite, your piece will literally solder to the bed, that is what happened to Clive here and that has happen to me too. In my own experience, adding a layer of "something" between the glass and the piece that you are printing helps a lot.
Of course, I'm talking about my own accumulated experience, that is only about 4 years, I'm no expert here, just another amateur. Every individual experience may and most likely will vary, that is why I'm suggesting to keep trying different techniques until you find the one that works the best for you, that is what really matters at the end of the day.
I have heard of people adding a waft of aerosol hairspray. Adds an easily breakable bond.
Finally we got to see your Variac. Cute small one, my 5kva one is a pain to move!
I've got a 2KVA (120V 17A), and it turns easily.
@@michaelmoorrees3585 It turns super easy, but moving it from one place to another is a pain. That beast is heavy. It also loves to pop breakers sometimes if I don't hook it up to the soft-start. :D
@@DrakkarCalethiel You could attach some caster feet
to it. Did the same with my 10kVA 60kg monstrosity : D works well
@@aulus4163 60kg 10kVA, what a beast! My 5kVA one is just around 16kg with all the control stuff.
When I'm going to pair it with a couple beefy transformers on the output, everything will get mounted on a rollable platform. Still need to find some tranies that don't cost an arm and a leg new...
Unsmoothed rectified is useful for measuring very large inductances in the presence of DC... maybe you wanted to test chokes?
I patiently waited for “Let’s take it to bits.” 😉
I think you need shirts. That bearded outliney-sunglassey icon.... w your dotcom, and a couple choices of sayings. Don’t print them yourself, stock, and ship. Do some kind of third party host thing. Don’t want you tied to regular scrambles to the post.
A smear of glue stick on the print surface will help as a release agent for your 3D prints
My first acquaintance with this was when I took apart a model railway transformer. I had read about transformers in the Great World Encyclopaedia of Science, and it was immediately obvious how the wiper selected a different number of coils in the output stage. It was linear rather than circular, and there was a long streak on the copper windings where the wiper had been wiping!
My brother used to take apart his Christmas presents of electric train set transformers so much so that he was given this Dr Frankenstein Chemistry set / welding kit instead next years because they were less costly.
I was wondering how they constructed that linear variac you seen. Was the magnetic circuit completed like a square toroid of transformer steel plates or was there a long airgap with coils wound on group of rectangular transformer steal plates, but not completing the magnetic circuit?
@@kreynolds1123 that 'linear' thing may have been a high power variable resistor with sliding contact, not a 'variac' or transformer of any kind
@@andygozzo72 Thank you. It is a little difficult finding many examples of linear autotransformers.
I had a Märklin one and it was my first "lab equipment". I was around 9 and my dad had to get a resettable porcelain fuse in the fuse box in our apartment. The fuse blew so often when I set fire to steel wool and tried arc welding with sewing needles. It sure had some punch 😁
Love how you can tell how new the video is by how scuffed looking the desk and BigCliveDotCom sticker is.
Clive, you will probably find that your digital meter does not measure below ~70V. You can modify it by adding a third wire, disconnect the supply from measured voltage. In my case it was removing one 0ohm resistor
I'll check that out to see where it taps its monitoring supply from.
I’ve never noticed before that the Hopi has Frequency misspelt as ‘Frequence’.
I think that’s French.
@@5roundsrapid263 The other labels are all in English, even “Annual Power Consumption”, so I don’t see why that one would be in French.
The potential difference of two neighbour windings seems to be small?
The dog licked my one and it broke. 🤣 As always a big fat 👍 for the COVID free big Clive! 👏🇬🇧
I hope you can fix your dog.
Everything is made better by a *FULL BRIDGE RECTIFIER!*
Someone's got to tell tell him that it's a "full WAVE bridge rectifier" Half the planet gets it wrong because of his humour.
@@gatekeeper65 I could go on about correct punctuation of that, or the base name of the device, but I won't.
Just have fun and don't stress over a man who is literally making electricity and electronics fun to learn about.
@@ConstantlyDamaged Yeah, I know, but sometimes it just gets to me. So many people don't even know basic grade school grammar. I sort of meant it in a light hearted way that doesn't always come through in text.
@@gatekeeper65 Heh, well, I never attended "grade school"-I'm Australian, we don't have those here.
As for grammar, I'm a writer by trade, so I like to think I'm reasonably on top of that.
Uh oh. A Variac ,is he going photonic
It's only a 0-100% variac. It can't overvolt stuff any differently than he already could by plugging a 120v thing into 240v.
@@Ferro_Giconi More importantly, that thing is NOT big enough for any photonic level fun.
I think it's more to do with the resistance of the brush in general.
If you assume the brush to have no resistance and do the calculations, a 10A variac would dissipate hundreds of watts in a single shorted turn. (Assuming 1.2 ohms end-to-end resistance, i.e. 5% impedance, and about 240 turns. This gives 5 milliohms per turn, put 1V across that and you get 200W!)
If you expect around the same impedance again in the overall brush (from winding to the output connection), and pull a figure out of the air to say that the turn-shorting resistance would be 25% of that, then you burn 3.33W in the brush and 55mW in the shorted turn.
Which is bupkis.
It doesn't make any sense that the brush doesn't look hot in your thermal camera images, so I may be totally wrong.
Edit: When I say transformer impedance, I mean the one that ruins your voltage regulation... not the one to do with transmission line characteristic impedances etc.
Edit 2: Ah yes, you tested your wee lil bugger with HOPI. I expect that figure is dominated by iron loss, just like any transformer.
My variac is wired to go from 0-110% , I'm in Canada, so that translates to 0-140VAC from the 120 line. I find it handy for experimenting with brown outs and also slight over voltage situations. My case is a, umm, a bit different. www.flyinglow.ca/flyinglow/2009/08/case-mod-for-a-variac.html
We used to carry a 20A Variac when I toured with a sound system (1980's). This would allow us to adjust the voltage at venues that were out of spec (for the console and other sensitive gear. Nowadays with better power supplies this is not as much of an issue.) For the connections to the windings there were actually 4 connections, 0%, about 10%, about 90% and 100%. We would connect the hot at 90% and the neutral at 0%. This would allow us to move the output wiper past the hot connection at 90% and boost the output voltage above the input. Very handy when we were at a venue with low voltage. Being from the US, we would also bring this variac along when touring in 240 volt lands, etc., allowing easy adjustment. (We would find some places where 240 was actually 260+, being able to tweak as needed was handy.
My ZnSe 20mm (aliexpress, ~$5) lens attenuates the reading with about 2°C on the Seek Compact.
Made my Seek *very* useful with SMD work.
As a young lad, many years ago, I had a model train speed controller. Imagine my delight when, after taking it to bits (as one does) I found it was in fact a variable transformer from about 6 volts to 25 ish. The variable part was an isolated secondary. Amazing for a toy. Needless to say it was never connected to the train again.
I've seen a system much like that, including a fixed tap for 12v AC lighting! Very cool stuff.
@@jercos Yes, I remember that now, made by the German Märklin model railways. Much better than the Hornby stuff that used DC and a tacky resistance to control the speed.
I have a few variacs of different power capabilities. On the small one (1 amp) the contact brush shows no lengthwise/widthwise resistance differences, but the ones I have that are over 1 amp have a definite difference in resistance. Maybe the small ones are too small to notice the issue? I'd also like to mention that my taps are all terminated to one wire. Those wires are brought out, given a few twists, and soldered to.
As i heard "Big Variac" i instantly thought of Photonicinduction...wondeer what happend to him.
He's fine. He'll be back.
Want to know why its named a H0PI?
When you touch bare live wire, you get a Shocki, that makes one go hopi.
No Idea where that came from, lol.
Lol
The brush seems to create a virtual single turn secondary winding with the turns it shorts out. Much like the shading rings do in a shaded pole motor. That can muchly explain why they heat up just in a small spot. It's a secondary short circuit. The shading rings in a shaded pole motor also get rather hot.
I also made the mistake of having an unsmoothed DC output on my variac. I didn't leave enough space for the capacitor and so now I always have to add a cap on the output if I'm using DC
the one I have has the live connecter a small distance from the end, in other words the end is floating so when you wind it to max you get up to 260Vac with 220V going in. When you was playing around with those 047mf caps 2 weeks ago to vary the voltage on the led bulbs I was going to ask if you couldnt do it with a variac, I couldnt see you not having one of them
Mine has the same function (except it's 140 from 120). The live connection to enable the boost would probably connect to the coil right near Clive's mystery shorted winding. I'm guessing that's why.
I've been wondering about this for decades, thanks for the answer. I guess the windings have a good thermal bond to the transformer iron, so the shorted turns don't overheat. As 99% of the coils are unshorted, the overall losses are low.
Dearest Clive! I never regret when you digress in your videos, and I think many of us like the content you make. ^^
As a small child, I always thought that Variacs and Celeriac were somehow related. Such things are confusing to a 7 year-old!
Easy way to tell. You can lick Celeriac.
@@bigclivedotcom You could also lick your Variac, just make sure your camera is running!
@@paulstubbs7678 LoL Beat me to it. 🤣
Protip: be sure to have an extra set of boxers ready, if the Variac is plugged in.
Lick your variac? ElectroBOOM says "hold my beer!" 😂
Very neat video! I've wondered about that same thing, seeing how a variac doesn't have any "dead" areas between windings as the control is swept across the entire scale.
Makes me think of a power transformer with an "on load tap changer" which momentarily bridges out a segment of the winding during tap changes. Those have a more complex system than a variac, but the idea is similar. There is no off state between taps. A low-value resistor is used in series with each winding for a very brief moment during tap changes.
I've got two Chinese variacs which have developed some dead spots and audible arcing. Should tear them down and inspect to see what went wrong. Both of them are less than 2 years old and followed the same path to failure. New out of box, there were many dead spots, cleared up by repeatedly sweeping the knob with power off. Then after several hours of use, dead spots reappeared at the more commonly used ranges such as 100V and the very low range used to check for signs of life in very old equipment before ramping up.
Both of these now take very little effort to rotate the knob and I expect that winding varnish was on the end turns where the brush rides, causing the initial problem; and that now the brush has become stuck due to more gumming up from varnish.
"I'm not sure why I did that ... but i did." This is exactly me when I'm working on a small Arduino project at the moment 😂
Well since noone seems to mention it...:
The reason it's not a big deal is simple. P=U^2/R, where U is the voltage of a single turn and R is its resistance.
Both U and R are proportional to the number of turns you short out. But because it's U^2, the power dissipation becomes a lot bigger if you short more turns
Clive kinda did mention it. He showed the power measurement at no load. Just didn't mention why.
Thank you for doing this expose'. I have always wondered about this exact phenomena. I guess the carbon brush is the "trick". A little lossy on the contact prevents high currents to flow in the shorted winding.
Clive, you are amazing. I love your videos and you make complicated things very accessible and even friendly with your kind delivery. We use variacs in the music industry to get the “brown sound” by slightly lowering the voltage for guitars amps. I have wondered for years how these things worked. I knew it was “like a transformer” but couldn’t figure out how you would build something like this. You, as always, really connected the dots! Thank you.
Someone has more info about those lenses he used?
Oh that's just brilliant, there was me thinking about having a lazy week but instead, I've now got to take apart my Variac and examine it with my Seek 😁 camera.
well thats a late video, were you awake till now or was that just the upload time? xD
with the glass, I heared you can get cheap glass at window makers, they even can cut it for you , if you would need a source for a piece of glass, happend to us all the time at our internship place (it was a schoolinternship where we we learned about electrical engineering, informatics and metalworks and we had 3d printers since they also used gcode as our milling machine and since its kinda neat to have for electrical engineering and metalworks) our teacher told me that he gets the glass from windowmakers
What material did you use?. PETG is known to adhere too well and pull up chunks of the build plate. PLA and ABS don't have that issue. I use cheap hair spray to help ABS and PLA adhere and I also use it for PETG as a release agent so it can't destroy the glass.
I use PLA.
@@bigclivedotcom I'm impressed it stuck that well. I can't get PLA to stick to bare glass ever. Try the aerosol hairspray trick. I use the cheap aquanet and it works well.
Odd, Clive mentions the variac was shipped, indicating that he had bought it from a previous owner. At the end of the video Clive states "I'm not sure why I put that in there", Indicating that he had built the device. SO.... Damaged in shipping must mean Clive himself broke the variac whilst transporting it home, from where now? Noone may care but I think it's fun to speculate.
Shipped by me to my new home. Well packed, but so heavy that when thrown by a shipping company, the plastic cracked.
I have a few variable autotransformers the largest of which is 30A I believe, and by necessity they ALL short 2 windings (required to avoid loss of contact with even 1 which would be catastrophic). These wires must be 14AWG and I've always wondered how that works. Are you saying that the transformer truly is shorted but the resistance of that winding is high enough to prevent overheating of the turn? That doesn't make sense to me as that would be well over 100A even at 1V... (280V max and probably around 300 turns I'd estimate.) I'm still confused.
It's possible the carbon brush does add at least a small resistance.
Use a glue stick on the build plate for the printer. It allows the print to stick firmly and releases easier when done. Or a flexible plate would work too
You’re a very wise and naughty man..I truly wish I understood what exactly you’re speaking of but I’m hoping to absorb things subconsciously in the end..
"Shorted windings is a common cause of failure" Er, I'd have thought that what ever caused the windings to short was the cause....
P'r'aps 'twas but the work of fate all along.
so.. is there a face in the center on the thermal image or what..
The volt per winding is very very small so there's a very very small eddy current in two adjacent shorted windings.
Another thing to consider: A carbon brush has more resistance than copper has. Two wires being shorted through a brush results in less "shorted" current than if the brush were made of copper. But, as pointed out, the brush may sometimes span more than two windings.
@belly tripper Kindly, while "Were" is used in the second person singular and plural and first and third person plural, it is also used in the subjunctive mood to indicate unreal or hypothetical statements.
www.grammarly.com/blog/was-vs-were/
@belly tripper Past tense "Jack was early to work", vs subjunctive unreal or hypothetical "if the goose were to lay a golden egg."
@belly tripper ok, roger that.👌
I'd like to propose that you brush up some. Chech out the section on 'The Subjunctive Mood with "Be" and "Were" ' www.merriam-webster.com/words-at-play/getting-in-the-subjunctive-mood.
If you are to be a grammar nazi, you might want to be an informed grammar nazi.
Please have a nice day.
@belly tripper With all due respect, given your two statements ' your grammar is horrible, you mean ""if the brush **was** copper"" ', please understand that when one wants to criticize another person's grammar, one should know that an informed grammar nazi would not start either sentance with a lower case letter. Nor would they end either statement with either a comma or nothing. Nor would a grammar nazi use "" where a single " is required.
Lighten up on people please. Putting them down on something does not lift you up. And when you are the one that falls short on the same measuring stick you used to measure another person, the heaping dish of crow pie doesn't taste very good.
In the case that the last idiom is not common wherever you are, I'll share this from wikipedia "eating crow is a colloquial idiom, used in some English-speaking countries, that means humiliation by admitting having been proven wrong after taking a strong position.The crow is a carrion-eater that is presumably repulsive to eat in the same way that being proven wrong might be emotionally hard to swallow."
When I heard back in the 80's that Eddie Van Halen used one on his amp, I got a variac and put it on an old Yorkville Sound amp. Since it wasn't really meant for guitar specifically, it didn't have a distortion circuit. But the variac, when turned down, allowed me to crank the volume "up to 11" which overdrove the tubes something spectacular, turning that amp into an insta-Marshall. Seriously dangerous project, but it worked amazingly well.
Is this what spinal tap used to make their amps go to 11?
Yet another great myth-busting video Clive. Thank you! There are many assumed theories about the graphite brushes on variacs, but it's obvious that they simply short out a few turns of the coil which after all amounts to only a few volts precisely as you say. I have an 8 amp variac and it dissipates 13 watts quiescent AND it has a finned heatsink on the graphite brush. That's where the quiescent power goes.
CLIVE!!!
Talking to Andy (Photonicinduction) and when you've got more than one variac (like in Andy's 3ph one's). You need a smoothing ballast(s).
I'm not sure what there actually called but the outputs go into a ballast which have a couple of really thick turn's and then you get the output from the centre tap.
So if one's at 100V and the others at 95V. Then the full current is going between the windings, which will fry your variac's. Which is something nobody wants as decent one's ain't cheap ☹️
It's easy for anyone to see this fact just by looking at the brush. The brush is wider than 2 turn surfaces. Note also that they turn / mill the windings flat for the brush; this is obviously necessary for brush movement, and while it may increase the wire's contact area, it drastically reduces the circular cross-section. So the classic name brand units at least (Superior, Staco, and GenRad) always seemed to use quite large wire for a given current rating. I've always assumed it was to compensate for that very problem. And the cheap chinese units do NOT do this, I've noticed they all run hotter. There's a reason used 50A name-brand units (very much in demand among Tesla coil builders, among a handful of other uses) are still breathtakingly expensive; there's a boat-load of copper in them!
The question is: WHY do they short the windings when clearly it is preventable? It's a current density problem, localized heating at the brushes. There isn't sufficient contact surface even after milling the wires to get the rated current into and out of the windings by using only one turn. So designers accepted the heating of one turn because it prevents even higher localized heating at the brushes.
@Railgap Esoterica, while I am a Luddite, this is the explanation that I have been waiting for. I hope that educated corroborators chime in... :-)
OMG Clive it's tiny! Does it output 240 or 270?
If you go on my channel you'll see one of my Variac's. Obviously as mines bigger, the tap off's are just on one winding. The windings on that one are 3mm or bigger.
I have one too but it doesnt have a case, just have to be careful not to touch the brown coloured wires
Deterrence against curious burglars!
I recently had a massive 15 amp / 120v one burn out just sitting there. I'd been using it to control a Harbor Freight spot welder, for which it was very useful. I forgot and left it energized for half an hour (spot welder inactive) and then there was that smell in the room - and about 3 turns at the position of the brush were black... The variac was collected by my Dad in the 50's, no idea of the actual vintage. Things like those stay in the family for generations, as many know, they don't grow on trees and the big ones cost real money. Luckily, we've been avid collectors...
i have an old (30 ish years, i think?) claude lyons 'regavolt' variac , its winding is quite 'coarse' , but would be as its a 4 amp type, never yet used it, as i need to make a case for it first.. its brush is quite thin vertically to minimize turns shorting
Use Rustoleum 2X or 3X clear, 3-4 coats at 10 minutes between each; you can make 3D parts that look like they belong to a concert grand piano. Hides all the print lines and brings out the color. Gloss hurts to look at. Semi-gloss still looks glossy, but not so dazzling. Have not tried the flat or satin yet.
That Digital panelmeter normally begins to work and show voltage from 60a65V, but you can hack it to show voltage from 0 to 240V.
Lift one side of the brown capacitor and use that for measuring the voltage, use a separate 12V DC and connect this parallel to the black electrolytic capacitor.
It's the same hack that is used on the Peacefair panelmeters.
A neighbor gave me one of those from probably the 50's. Big heavy thing. the wire has cracks but it still works. I've always wondered how it works, so thanks for showing me.
I read about putting your build plate in the freezer to get stubborn, stuck prints unstuck for years before I tried it. Works every time. You even get an audible "pop" when it comes off. Build surfaces are consumables IMO.
Bigclivedotcom, They aren't suppose to terminate the end of the windings like they did, the end is suppose to loop around and connect to a terminal, a screw for example. What I'm seeing in yours looks like some amature had their grubby hands on it and didn't really know what they were doing.
WIDE BLUE PAINTERS MASKING TAPE on the build plate.
Never worry about the glass again. Bonus -The texture is oddly satisfying.
3d printer - clean the glass with acetone, apply a thin layer of 'ABS juice' before printing. ABS Juice is just clear abs dissolved in acetone. 50g acetone to 5g ABS. Works very well on PLA, TPU, PETG and ABS. Maybe other materials.
750mA ?
BABY !
My variacs are all 8A 0-115%
And Clive, if you think that's big, you should see my resistor - I'm guessing it's going to be around 0·01 ohm and rated in kA. I'm guessing it's in an oil bath which has a water supply - pipe diameter approx 3/4" It's years since I've seen it but I know where it is !
1.04...sounded like..." when you irritate the knob" ha ha ha
So my question is......How is it? Is it irritating you? Lol
One could actually make a pretty decent LAB variable PSU with a VARIAC followed by an isolation transformer with a NPN or NMOS capacitance multiplier. Very low heat dissipation. Especially useful for audio amplifiers and such.
Hey, thats hand controller from my Scalectrix Slot Car racing set! ;)
My Variac is like 1stone (16 lb). It don't short no windings. But its circa 1910 Russian. I can dim the street lights in my street :)
Yah, flickery hopi is back 8)
The reason it needs to be shorted is because of "switch bounce". If it was make and brake with each winding you would have so much noise in your AC it would be ridiculous
maybe the variac has multple paralleled wingdings to limit the possibility of shorts?
it does not. You even need to short at least one winding at a time for it to deliver a steady output as well.
@@Basement-Science well if you have 2 alternating windings you don't get a short but just a small voltage drop between them
Used a variac in our shop. Rated for 15A and the windings were much more 'course' as it was about 14 AWG wire. As you turned the knob, you could 'feel' the bumping as the brush moved from one turn to the next. They are a sort of auto-transformer (uses the same winding for primary and secondary) with 'taps' on every turn of winding.
Nice video. And some VERY nice FLIR photos.
Got myself a 2 KVA thingamabob....heavy as sh@t.....and huge...Brush is the size of my thumb...
Clive It was a typing error. My Variac is about 300v max. It`s 2,000 watts. I`m always getting watts mixed up with sieverts and cosmic strings !!!!
I have a couple variacs. One is 240 input with 5-240 output. Another is 115V input with 0-150V output. The third is 110v input and 6. 14. 24. and 0-40v output. Variacs are just basically an auto transformer with one tap that can move. Great bit of kit though.
3:24 Where can i find those lenses and/or more information about them?
I'm not sure where Christoph got them from. eBay may be a good start.
@@bigclivedotcom It's like he was scrubbed off the internet and put in witness protection program. 👀 Can't find 💩
RE 3D Printing Stuck to the build plate: Tip! After printing spray the item with 50/50 water/Isopropyl and wait 1 min, the 3d print will come off so easy you may not have to recalibrate next time.
I have one of these capable of 4A. has pretty beefy wire to it as a result. I received it loose, and installed it into an enclosure I found in a stream. Doesn't have a meter, but its faceplate voltage indications are accurate enough for government work. I can use my own Hopi on the output if I need higher accuracy. I tapped it to accept American 115v and output up to 280v at full scale, but I have to be careful with the current, as I could hit 4A on the input pretty easily when stepping it up that far.
The idea of unsmoothed DC output is interesting! I might have to build a FULL BRIDGE RECTIFIER for mine. Probably as a module I can attach to the output cable...
I've started using Kapton Tape on my glass bed for printing PETG, PLA, and ABS...No glue needed. Just a quick wipe with Isopropyl. I'll NEVER go back to anything else.
Why would decreasing the resistance of a section of winding cause all that heat. It seems like there should be enough resistance from all the remaining un-shorted windings to limit the current as if the short was absent. Like shunting a 2 ohm resistor that lives in series with a 1k ohm resistor. Is this not what's going on?
The no-load dissipation of your Variac seems high for its output power, possibly due to the shorted turns at the end connections.
My 1400 VA (10 A 140 V) Superior Electric unit draws 2 W and 2 VA with no load. The little 300 VA unit is 1 W and 2 VA unloaded. But none of my Variacs have shorted turns at the winding connections. Either the winding wire end is connected to a terminal board, or the tap connection is soldered to a single turn of the winding.
It's funny, I've been eyeing a variac at the only REAL electronics shop still around in my area, some of the components look like they've been hanging on the peg since their grand opening, there's unopened bottles of chemicals that are empty or nearly empty from years upon years of slow evaporation through the sealed cap, they've had a surge of patrons due to the beer virus, good in a sense, I like walking the isles choosing components but I digress, I've been trying to decide🤔, should I buy a variac or wait, I don't have a need right now, but prices only seem to increase, even though scamazon has them pretty cheap I prefer to buy something I can touch taste and smell before I buy it,
Oh I actually got to meet the inventor of the toggle switch😉(warning sarcasm ahead) WOW gee you look great for 124 years old AND your still driving🙄😩 not all squirrels live in trees✌
Thank you BC. I'd been aware of this for a very long time but it's good to have it highlighted by you in your usual professional way. My 8 amp variac dissipates 30 watts on standby and the brush has an heatsink which gets quite warm on standby. When you think of it, there are only a few volts between each turn of the winding so the current flowing as the brush shorts them out is minimal on a miniature variac but it gets significant on a large variac with a correspondingly large brush hence the brush's heatsink on my 8 amp one.
LOL I cleaned the shit out of my heated bed, put gluestick on, then spread it evenly with everclear. On top of that it was in an enclosure with no airflow, 100°C bed temp, 240°C hotend, so it stayed nice and warm and didn't warp. The adhesion I got with ABS was absolutely nuts. It was too perfect. Took me 20 mins to get my part off
Oh and it has a mirror shine on the bottom LOL
I have a 2 kva variac .Not the greatest one ,but it works exactly the same way .My friend has one that was made in the 50s it uses a phenolic ring with silver contacts the wiper makes contact with .The wires are solderd from each winding to each contact .his is only 400va and wuite heavy and more robust in build .Cant remember the brand ,its a bluish green color .The scale is amazing and the voltages are spot on pending supply voltage .
Bridge rectifier... have you been playing with generators with self exciting brushless alternators?