Make your own Power Transformer

If you're just after a 1 off, you could take a mains transformer & cut the secondary wind out. Cost nothing, done in 10 minutes.

NT

Yup.

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:

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.

onsdag den 16. januar 2019 kl. 18.42.36 UTC+1 skrev Bob Engelhardt:

back when I was in school and had to wind and assemble a transformer it was dipped in lacquer and baked after assembly

I have wound a few transformers, I ask my local motor rewind company for a quart of the lacquer they use when they rewind motors. A soaked the transformer in the lacquer let it drip a bit and baked at low temp in a toaster oven. Also made a coil for a 120V generator the same way. I had some experience, I worked in a motor rewind company in the early

80s. Dirty, hard work, but I'm glad I had the experience. I got laid off in an economic turn down, they did a lot of work for the auto manufacturers, and when the slowed, we slowed, more.

Mikek

** Microwave oven transformers are very specialised for that job. Consider that they rely on constant fan cooling, operate under a heavy load ( an overload really) ALL the time and emit considerable audible hum while being used in the oven.

When repurposed and lightly loaded, the core is in permanent saturation.

OK for a C&N spot welder I guess.

.... Phil

Microwave transformers are different to all others. Yes they hum plenty. They also overheat in 15 minutes & have shorted laminations, creating inductance on the output.

I've only seldom made mains transformers, not had any hum problem.

NT

Microwave oven transformers (MOT's) are intentionally made to be inexpensive. Since the manufacturers skimp on iron and copper (or aluminum), they run close to saturation under light or no load. It's not unusual to see 3-4A of primary magnetizing current on a 120 volt MOT with no load.

One other feature of these transformers is the addition of magnetic shunts in the magnetic circuit between the primary and secondary windings in order to add significant leakage inductance. This helps to limit fault current during arcing within the magnetron or the oven cavity. If you don't remove these shunts, your repurposed transformer will have relatively poor voltage regulation under load.

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. ;-)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/

Are 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

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.

There's no typical number of turns.

larger transformers have fewer turns.

If you can scope the magnetising current just add turns until saturation reduces sufficiently. you probably want to do this at the maximum supply voltage (or add extra turns to compensate for the expected max)

saturation will do that.

yeah.

--
  When I tried casting out nines I made a hash of it.

** No they are not.

Continuous fan cooling plus *intermittent operation* allows the use of an otherwise grossly undersized transformer.

** IOW, right on the limit with no fan and continuous loaded. ** You did not rewind the primary with more turns - correct ?

So the tranny draws a heavy magnetising current & runs hot off load.

** Correct.

.... Phil

** 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

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?)

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

you can see distortion in the sine wave

yeah, that would be a better way to do it.

--
  When I tried casting out nines I made a hash of it.

kettle is most efficient, nukes are middling, stoves poor.

NT

** Sure - but obtaining a scope and interfacing the input safely to AC supply CURRENT is not a trivial task for most people.

.... Phil