Make your own Power Transformer

On Monday, 14 January 2019 02:03:23 UTC, snipped-for-privacy@myshop.com  wrote:



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

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  

On 1/16/2019 11:41 AM, Bob Engelhardt wrote:





  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

Bob Engelhardt wrote:



** 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  

On Wednesday, 16 January 2019 17:42:36 UTC, Bob Engelhardt  wrote:




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

Bob Engelhardt wrote:





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

On Thursday, 17 January 2019 02:54:29 UTC, Tim Williams  wrote:



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

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.

Jasen Betts wrote:




** 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  

you can see distortion in the sine wave



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

Jasen Betts wrote:




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





....  Phil

On 1/16/2019 12:41 PM, Bob Engelhardt wrote:





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.

Bob Engelhardt wrote:




 ** 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  

On Friday, January 18, 2019 at 3:07:56 PM UTC-5, Phil Allison wrote:

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?)

On 1/18/19 3:32 PM, George Herold wrote:

An old-timey immersion heater is better still (if you don't mind burning  
your house down occasionally).

Cheers

Phil Hobbs

On Friday, 18 January 2019 20:32:30 UTC, George Herold  wrote:



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


NT

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