On Thursday, 17 January 2019 02:54:29 UTC, Tim Williams wrote:> "Bert Hickman" <bert-hickman@comcast.net> wrote in message > news:3uydnW0svNh3CKLBnZ2dnUU7-InNnZ2d@giganews.com...> > Adding additional turns to the primary and knocking out the magnetic > > shunts will make these transformers more efficient in a repurposed > > application. > > Indeed. Consider adding about 20% more primary turns, and consider reducing > the total VA capacity to maybe 60% of the oven's nameplate rating. That'll > get you a transformer that runs cool, or at least as cool as the core will > allow (which by the way, isn't too bad, because the core stack is welded > across the outside -- this doesn't create shorted turns, it creates a book > with a single spine; if the book were bound twice with two spines, there > would be a shorted loop). > > A properly made ~400VA transformer will be smaller than an MOT thusly > repurposed, but you're not complaining because you got it for free out of > the trash. ;-) > > TimAre you sure about 20% more turns? IIRC nuke transformers have less than half the typical number of primary turns. Are you sure about 60% power rating? They get hot to overheating in 15 minutes under forced ventilation, so I can't imagine them running cool unfanned at 60%. If you look at similar power non-nuke transformers they're several times the size. Re-using them is possible but not trivial. NT
Make your own Power Transformer
Started by ●January 13, 2019
Reply by ●January 17, 20192019-01-17
Reply by ●January 18, 20192019-01-18
On 1/16/2019 12:41 PM, Bob Engelhardt wrote:> I have repurposed microwave oven transformers & have always had awful > hum. Is that a problem with all homemade transformers? If not, what > makes the difference? I did quiet them somewhat by using wooden wedges > between the added winding and the core. >MOTs are commonly 1500VA & I'm using my repurposed one at 400VA max, more typically at 100VA. The shunts were removed. I realized that the hum isn't from the new secondary, cause the hum is there even at no load. It's the core humming. I also realize that it hums a lot in the in the oven & that the fans obscure the hum.
Reply by ●January 18, 20192019-01-18
Bob Engelhardt wrote:> > MOTs are commonly 1500VA** No they are not. Continuous fan cooling plus *intermittent operation* allows the use of an otherwise grossly undersized transformer.> & I'm using my repurposed one at 400VA max,** IOW, right on the limit with no fan and continuous loaded.> more typically at 100VA. The shunts were removed. >** You did not rewind the primary with more turns - correct ? So the tranny draws a heavy magnetising current & runs hot off load.> I realized that the hum isn't from the new secondary, cause the hum is > there even at no load. It's the core humming. I also realize that it > hums a lot in the in the oven & that the fans obscure the hum.** Correct. .... Phil
Reply by ●January 18, 20192019-01-18
Jasen Betts wrote:> > > If you can scope the magnetising current just add turns until > saturation reduces sufficiently. >** What's the scope for ?? An AC amp meter will do the job, particularly if a variac is available. Knowing the max primary voltage the tranny is comfortable with allows easy computation of the needed additional turns. .... Phil
Reply by ●January 18, 20192019-01-18
On Friday, January 18, 2019 at 3:07:56 PM UTC-5, Phil Allison wrote:> Bob Engelhardt wrote: > > > > > MOTs are commonly 1500VA > > > ** No they are not. > > Continuous fan cooling plus *intermittent operation* allows the use of an otherwise grossly undersized transformer. > > > > & I'm using my repurposed one at 400VA max, > > > ** IOW, right on the limit with no fan and continuous loaded. > > > > more typically at 100VA. The shunts were removed. > > > > ** You did not rewind the primary with more turns - correct ? > > So the tranny draws a heavy magnetising current & runs hot off load. > > > > I realized that the hum isn't from the new secondary, cause the hum is > > there even at no load. It's the core humming. I also realize that it > > hums a lot in the in the oven & that the fans obscure the hum. > > > ** Correct.Magnetostriction in the iron? Hum should be at 100/120 Hz and not 50/60. George H. (So does it take more energy to heat a cup of water in a microwave than in the tea kettle, on an electric stove say?)> > > > > .... Phil
Reply by ●January 18, 20192019-01-18
On 1/18/19 3:32 PM, George Herold wrote:> On Friday, January 18, 2019 at 3:07:56 PM UTC-5, Phil Allison wrote: >> Bob Engelhardt wrote: >> >>> >>> MOTs are commonly 1500VA >> >> >> ** No they are not. >> >> Continuous fan cooling plus *intermittent operation* allows the use of an otherwise grossly undersized transformer. >> >> >>> & I'm using my repurposed one at 400VA max, >> >> >> ** IOW, right on the limit with no fan and continuous loaded. >> >> >>> more typically at 100VA. The shunts were removed. >>> >> >> ** You did not rewind the primary with more turns - correct ? >> >> So the tranny draws a heavy magnetising current & runs hot off load. >> >> >>> I realized that the hum isn't from the new secondary, cause the hum is >>> there even at no load. It's the core humming. I also realize that it >>> hums a lot in the in the oven & that the fans obscure the hum. >> >> >> ** Correct. > Magnetostriction in the iron? Hum should be at 100/120 Hz and not 50/60. > > George H. > (So does it take more energy to heat a cup of water in a microwave than > in the tea kettle, on an electric stove say?) >> >> >> >> >> .... Phil >An old-timey immersion heater is better still (if you don't mind burning your house down occasionally). Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●January 18, 20192019-01-18
On 2019-01-18, Phil Allison <pallison49@gmail.com> wrote:> Jasen Betts wrote: > > >> >> >> If you can scope the magnetising current just add turns until >> saturation reduces sufficiently. >> > > ** What's the scope for ??you can see distortion in the sine wave> An AC amp meter will do the job, particularly if a variac is available. > > Knowing the max primary voltage the tranny is comfortable with allows easy computation of the needed additional turns.yeah, that would be a better way to do it. -- When I tried casting out nines I made a hash of it.
Reply by ●January 18, 20192019-01-18
On Friday, 18 January 2019 20:32:30 UTC, George Herold wrote:> (So does it take more energy to heat a cup of water in a microwave than > in the tea kettle, on an electric stove say?)kettle is most efficient, nukes are middling, stoves poor. NT
Reply by ●January 18, 20192019-01-18
Jasen Betts wrote:> > > >> > >> If you can scope the magnetising current just add turns until > >> saturation reduces sufficiently. > >> > > > > ** What's the scope for ?? > > you can see distortion in the sine wave >** Sure - but obtaining a scope and interfacing the input safely to AC supply CURRENT is not a trivial task for most people. .... Phil
Reply by ●January 18, 20192019-01-18
<tabbypurr@gmail.com> wrote in message news:30925dfe-e5f4-4c42-bbc9-01696f491817@googlegroups.com...> On Friday, 18 January 2019 20:32:30 UTC, George Herold wrote: > >> (So does it take more energy to heat a cup of water in a microwave than >> in the tea kettle, on an electric stove say?) > > kettle is most efficient, nukes are middling, stoves poor. >Most efficient would be a heat pump, but only by a little; the cost would be considerable. So, for economic purposes, an immersion heater with reasonable insulation (which ever since the "space age", Dewars have been not just practical, but rather cheap even), is about as good as it gets. Another comparison might be natural gas direct (stovetop teakettle), versus power station plus distribution plus whatever kitchen appliance is used; but this isn't a very interesting comparison as a very small fraction of energy (in either form) is used for heating beverages. :-) Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
