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Transformer Question

Started by Unknown February 2, 2017
On Saturday, February 4, 2017 at 9:18:22 PM UTC+1, John Larkin wrote:
> On Sat, 4 Feb 2017 11:58:08 -0800 (PST), tabbypurr@gmail.com wrote: > > >On Saturday, 4 February 2017 18:30:34 UTC, John Larkin wrote: > >> On Sat, 4 Feb 2017 09:32:13 -0800 (PST), tabbypurr wrote: > >> >On Saturday, 4 February 2017 17:16:45 UTC, John Larkin wrote: > > > >> >> A dial-primary (120:240 to whatever) transformer makes a good > >> >> isolation transformer. Use the two primaries and ignore the secondary, > >> >> unless you have some use for it too. > >> > > >> >as long as you don't want too much isolation. What voltage breakdown do you get with the 2 primaries wouund one directly on the other? > > > >> Kilovolts, probably. Test it for margin if you plan to float the load > >> way off ground. > > > >that's at odds with my experience of hipot testing transformers. Enamel on enamel failed below 1kV. Unfortunately I don't remember what voltage it managed. Probably over 500v, but that's not a comfortable margin. > > > > > >> It will have more inter-winding capacitance than one of those > >> extreme-expense isolation things. > > > >Yes. I don't know how much shock current that lets through. > > > > > >NT > > I made a twisted pair of #36 magnet wire and applied DC until it > arced. 1400 volts. > > Somebody should test a few ordinary dual-primary transformers for > breakdown. >
Relying on testing insulation is venturing on a slippery slope Then you need to do 100% production testing and getting approval from UL or VDE is VERY difficult and time consuming Cheers Klaus
On Sat, 4 Feb 2017 15:49:14 -0800 (PST), the renowned Klaus Kragelund
<klauskvik@hotmail.com> wrote:

>On Sunday, February 5, 2017 at 12:27:49 AM UTC+1, Spehro Pefhany wrote: >> On Sat, 04 Feb 2017 13:53:14 -0800, the renowned John Larkin >> <jjlarkin@highlandtechnology.com> wrote: >> >> >On Sat, 4 Feb 2017 00:21:41 -0800 (PST), klaus.kragelund@gmail.com >> >wrote: >> > >> >>You should not make an isolation transformer your self if you don't have extensive experience >> >> >> >>Isolation transformers must have double insulated design which involves special copper wire, >> > >> >Special copper wire? >> >> He's probably thinking of triple-insulated wire, and I don't think >> you'll find that in any 50/60Hz isolation transformers. It's useful in >> SMPS transformers where you want to put the windings as close together >> as possible to link all the flux, but you don't care much about >> primary-secondary capacitance. >> >Nope, for a transformer the basic insulation in case of an earthed laminated core or the double insulation in case of no earth depends on the wire classification > >Typical class F wire, which is approved for 155 degrees C. > >See for example section 7 in this: > >https://www.indiannavy.nic.in/sites/default/files/tender_document/Transformer.pdf > >Or first table on this link: > >https://en.wikipedia.org/wiki/Insulation_system > >During maximum power through the transformer or shorting of the secondary side, the winding gets hot, and must stay below 155 degrees to still have the insulation system in place > >Cheers > >Klaus
Insulation between primary and secondary is **not** dependent on the wire insulation. It's provided either by bobbin design (split bobbin) or by the UL/CSA approved (north america) tape between primary and secondary. That's true whether the wire is class F or lower temperature rating. --sp -- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
On Sat, 4 Feb 2017 15:52:41 -0800 (PST), the renowned Klaus Kragelund
<klauskvik@hotmail.com> wrote:

>Relying on testing insulation is venturing on a slippery slope > >Then you need to do 100% production testing and getting approval from UL or VDE is VERY difficult and time consuming > >Cheers > >Klaus
On our instruments we used to do 100% Hipot with a 60 second soak time. Dead users are bad for business. --sp -- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
On Sat, 4 Feb 2017 15:52:41 -0800 (PST), Klaus Kragelund
<klauskvik@hotmail.com> wrote:

>On Saturday, February 4, 2017 at 9:18:22 PM UTC+1, John Larkin wrote: >> On Sat, 4 Feb 2017 11:58:08 -0800 (PST), tabbypurr@gmail.com wrote: >> >> >On Saturday, 4 February 2017 18:30:34 UTC, John Larkin wrote: >> >> On Sat, 4 Feb 2017 09:32:13 -0800 (PST), tabbypurr wrote: >> >> >On Saturday, 4 February 2017 17:16:45 UTC, John Larkin wrote: >> > >> >> >> A dial-primary (120:240 to whatever) transformer makes a good >> >> >> isolation transformer. Use the two primaries and ignore the secondary, >> >> >> unless you have some use for it too. >> >> > >> >> >as long as you don't want too much isolation. What voltage breakdown do you get with the 2 primaries wouund one directly on the other? >> > >> >> Kilovolts, probably. Test it for margin if you plan to float the load >> >> way off ground. >> > >> >that's at odds with my experience of hipot testing transformers. Enamel on enamel failed below 1kV. Unfortunately I don't remember what voltage it managed. Probably over 500v, but that's not a comfortable margin. >> > >> > >> >> It will have more inter-winding capacitance than one of those >> >> extreme-expense isolation things. >> > >> >Yes. I don't know how much shock current that lets through. >> > >> > >> >NT >> >> I made a twisted pair of #36 magnet wire and applied DC until it >> arced. 1400 volts. >> >> Somebody should test a few ordinary dual-primary transformers for >> breakdown. >> >Relying on testing insulation is venturing on a slippery slope > >Then you need to do 100% production testing and getting approval from UL or VDE is VERY difficult and time consuming > >Cheers > >Klaus
I assume the op wants to hack one isolation transformer for personal use. I don't think he intends to get UL or VDE certification. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Saturday, February 4, 2017 at 4:01:09 PM UTC-5, Jasen Betts wrote:


> Microwave ovens typically have one end of the secondary grounded to > the transformer core, so, if you hook 2 together hoping for two layers > of functional isolation you need to not ground the transformer cores, > else you only get one layer. > > Microwave oven transformer cores are typically welded shut, so > changing the windings is tricky (maybe you could open one with a 1mm > thick cut-off wheel and re-close it using a MIG welder). but still > you only get two layers of functional isolation, unless you do a > complete replacement of the windings. >
If you are making a 1:1 transformer using a couple of MWT's ( MicroWave Tronsformer ), You do not use the secondary windings. You can open a MWT using an ordinary hacksaw. Been there, done that. You use the primary winding from both transformers and remove te laminations that increase the leakage. inductance. A large hose clamp will work to hold the core pieces together, or you can kludge a nicer looking clamp with a couple pieces of steel and some fairly long bolts. If you are making a low voltage transformer you can cut the secondary off , remove the flux short laminations and wind a new secondary. But I can never get a very nice secondary this way. _If you are wanting a honking big low voltage transformer you can use two ( or more ) MWT's with the original secondaries removed and wind a new secondary winding the secondary through both cores. Dan
On Saturday, February 4, 2017 at 3:18:22 PM UTC-5, John Larkin wrote:

> >> >as long as you don't want too much isolation. What voltage breakdown do you get with the 2 primaries wouund one directly on the other? > >
> John Larkin Highland Technology, Inc > > lunatic fringe electronics
Don't most dual primary transformers have a layer of insulation between the two primaries? Dan
On Sat, 4 Feb 2017 17:26:28 -0800 (PST), "dcaster@krl.org"
<dcaster@krl.org> wrote:

>On Saturday, February 4, 2017 at 3:18:22 PM UTC-5, John Larkin wrote: > >> >> >as long as you don't want too much isolation. What voltage breakdown do you get with the 2 primaries wouund one directly on the other? >> > > >> John Larkin Highland Technology, Inc >> >> lunatic fringe electronics > >Don't most dual primary transformers have a layer of insulation between the two primaries? > > Dan
I don't know. But they surely have to be insulated enough to withstand line transients. And of course the 120/240. I've done the dual-primary trick many times, and I'm still here. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Piotr Wyderski is a Dangeous MORON wrote:
> >
( snip pile of this lunatic's crap )
> > Same with the magnetizig current, which, by definition, > is the primary current *at no load*, so it cannot decrease > with load, moron. >
** But that is *exactly* what happens - you stupid fuckwit!! As I already told another poster, secondary current causes increased primary current and the increased primary current causes extra voltage drop on the primary winding due to IR losses. You know this is true but refuse to see that in particular cases the effect is quite large. Those cases include the small transformers used in most external adaptors, microwave oven transformers and nearly all transformers that operate under intermittent load like spot welders and temp controlled soldering irons. Cleary you have never monitored transformer primary current on a scope - something I do every day and have done for 20 years. There is a big VISIBLE difference between magnetising current and load current - the former having maxima at or near zero crossings and the latter having maxima at supply voltage peaks. Go try it out sometime - fuckhead. .... Phil
Tim Williams wrote:

> > > Load current is reflected in the primary circuit, which causes a voltage > drop across DCR, therefore reduces the EMF applied to the core. >
** Now consider that I mag increases *exponentially* with applied voltage when the core is being driven into saturation. It's not unusual to see a 5% increase in applied voltage produce a 500% increase in I mag. Similarly, when a load is applied to the secondary the reverse happens, with I mag dropping by 500%. ..... Phil
dca...@krl.org wrote:
> > Phil Allison wrote: > > > > > > > If you really want to spend very little, get a couple of microwave ovens > > > and use two transformers to make one isolation transformer. > > > > > > > > > > ** Completely crazy idea that cannot work. > > > > Too many reasons to bother going into here, but it is both > > stupid and dangerous. > > > > > > My experience is that it is not dangerous.
** You have no experience with any such idea.
> If you do a little searching on the internet, you will find > numerous articles on making arc and spot welders using microwave > transformers. >
** Massive red herring. Those articles require the disassembly ( grinding off weldings ) and rewinding of the transformer. You are an utter IDIOT, Dan. ..... Phil