Rewiinding a MOT

On 29/06/2020 03:50, Robert Roland wrote:
> The magnetron in a microwave oven has a toroidal magnet in it, which
> might look like fun to play with. The problem as that some magnetrons
> contain a ceramic-like insulator that is very toxic. It is safe if
> left intact, but if you crack it, the toxins will go flying. When
> trying to get to the magnet, it is very easy to accidentally crack
> that insulator.

I have often read people write on the internet that consumer microwave 
oven mangetrons contain beryllium oxide. I have not seen anyone cite any 
kind of source for thing other than that other people on the internet 
say it too. Other people on the internet say that it is alumina. It is 
possible that both exist, but I can't see any reason why they would use BeO.

Have you ever seen any mention of BeO on the magnetron manufacturer's 
datasheet?

I suspect strongly that it is an urban legend, (perhaps with the 
exeption of a few microwave ovens produced shortly after WWII out of 
re-purposed military radar parts that might have been full of all sorts 
of things).

I have some equipment that does contain BeO, but that has prominent 
warnings both in the instruction manual and also on big yellow stickers 
on the inside and the outside of the casing. I've never seen such 
warnings from a microwave oven manufacturer.

On 6/28/2020 8:55 PM, Phil Allison wrote:
> Robert Roland wrote:
> 
> ====================
> 
>>
>> An unloaded transformer is almost entirely inductive, so the PF will
>> be very low. As the load increases, the PF will also increase. This is
>> normal. All transformer are like that.
>>
> ** If the same tranny is undergoing core saturation, the current wave is highly distorted and maxima tend to co-incide with zero crossings - exacerbating the poor PF.
> 
> 
> 
>> One thing to look out for with MOTs, is that they are run very "hard",
>> i.e. they magnetize the core well into saturation. That way, the
>> manufacturers can save on iron, which saves on weight, which saves on
>> cost. Since the MOT is normally run at fairly low duty cycle, and with
>> fan cooling, they can get away with that. You may find that the
>> transformer with zero load will draw 100 W or more of real power. If
>> you leave it running for some amount of time, it will get very hot.
>> The remedy for this is to add some additional turns to the primary
>> winding. This will, of course, require you to add some additional
>> secondary turns to reach the output voltage you want.
>>
>> Intuitively, you might think that adding more primary turns would
>> increase the magnetization, therefore worsen the saturation situation.
>> The reason it does not, is that the added windings increase the
>> inductance of the primary winding, which results in a reduction in
>> magnetizing current. The current reduces relatively more than the
>> increase in turns.
>>
>   
> ** Magnetisation goes up and down with the number of primary turns and applied voltage while inductance follows the square of the number of turns.
> 
> 
> 
> .....  Phil
> 

  I have graphed the Voltage and Current of the original primary winding 
and after adding 37 Turns, 48 Turns and 60 Turns.
Any comments?
> https://www.dropbox.com/s/0n1y4yh5o0j2sg6/mot%20primary%20voltage%20vs%20current%202%20graphs.jpg?dl=0

                                                     Mikek


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On 28/06/2020 18:09:55, amdx wrote:
> On 6/28/2020 11:49 AM, Robert Roland wrote:
>> On Sat, 27 Jun 2020 09:31:08 -0500, amdx <nojunk@knology.net> wrote:
>>
>>> I thought I could remove the secondary and the shunts, then install a
>>> new secondary and have a low voltage transformer, maybe not.
>>
>> I am thinking of a similar thing.
>>
>>> &nbsp;&nbsp; I removed the HV secondary, Then wound 10 turns and got 8 volts.
>>> &nbsp; I loaded it with 2 ohms, all is well except the PF is 0.15.
>>> I will need to have about 25 turns to get my 20 Volts, but, do I need to
>>> do something about the PF? Why is it so low?
>>
>> An unloaded transformer is almost entirely inductive, so the PF will
>> be very low. As the load increases, the PF will also increase. This is
>> normal. All transformer are like that.
>>
>> One thing to look out for with MOTs, is that they are run very "hard",
>> i.e. they magnetize the core well into saturation. That way, the
>> manufacturers can save on iron, which saves on weight, which saves on
>> cost. Since the MOT is normally run at fairly low duty cycle, and with
>> fan cooling, they can get away with that. You may find that the
>> transformer with zero load will draw 100 W or more of real power. If
>> you leave it running for some amount of time, it will get very hot.
>> The remedy for this is to add some additional turns to the primary
>> winding. This will, of course, require you to add some additional
>> secondary turns to reach the output voltage you want.
>>
>> Intuitively, you might think that adding more primary turns would
>> increase the magnetization, therefore worsen the saturation situation.
>> The reason it does not, is that the added windings increase the
>> inductance of the primary winding, which results in a reduction in
>> magnetizing current. The current reduces relatively more than the
>> increase in turns.
>>
> 
>  &nbsp;You may have missed my last post, I added 39 turns to the approx.
> 143 turns this dropped the current from 4.5amp to 0.87 amps.
> This is the Voltage vs current curve. It seems to me I'm still in 
> saturation.
>> https://www.dropbox.com/s/5itjzorex11nxvt/Sat%20Curve.jpg?dl=0

Microwave transformers, as said before, are designed on the limit of 
power capability and loss for the minimum of resources, namely copper 
and iron.

Most transformers have a low magnetising current where at full load the 
resistive loss will dominate. Microwave transformers are either on at 
full power, or off. They are much smaller than a convention transformer 
of the same power.

I'm not surprised you have such a high magnetising current, I have 
assumed that a microwave design starting point would be to make core 
losses and copper losses equal and to reduce size until the device would 
overheat say after 15-20 minutes. Being in the forced air flow would 
assist a longer life.

I would get another transformer(s). You can put them in series, or if 
identical, in parallel.


-- 
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk

Ceramic transmitting tubes and semiconductors also use BeO. Those warning labels don't always follow equipment through production and testing. A test tech on the GRC106 ended up in the hospital when one of the Eimac finals was damaged during shipping from Mexico to Cincinnati. They were just tossing the paperwork since they couldn't read English. Luckily for that tech, the man at the next bench saw him with the damaged tube. He wrapped a plastic bag over the tech's hands and the tube. He had someone call for an a.mbulsnce as he hurried him running to a sink to wet down the dust. He saved his coworker's life. After that, the paperwork was wired to each subassembly that contained BeO. YOLO.

On Mon, 29 Jun 2020 14:28:36 +1000, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>Have you ever seen any mention of BeO on the magnetron manufacturer's 
>datasheet?

I have to admit I was parroting what I have read on the Internet. The
text stated that roughly half of the magnetrons in microwave ovens
contain a toxic substance. I have never read the datasheet of any
magnetron.
-- 
RoRo

amdx <nojunk@knology.net> wrote:
> I thought I could remove the secondary and the shunts, then install a 
> new secondary and have a low voltage transformer, maybe not.
>   I removed the HV secondary, Then wound 10 turns and got 8 volts.
>  I loaded it with 2 ohms, all is well except the PF is 0.15.
> I will need to have about 25 turns to get my 20 Volts, but, do I need to 
> do something about the PF? Why is it so low?
>  I expect Phil will set me straight on this.
> 
>                         Thanks, Mikek

You knocked out the shunts, the transformer is probably going into 
saturation. Does it overheat real fast with no load? If you can measure 
power factor,  you can can probably measure the no load wattage too.

Microwaves are high production items. They've cut every possible corner 
possible. If your original microwave said a line voltage of +/- 10% of 120 
volts was OK, you can rest assure it won't be happy 1% out of that range. 
Removing the shunts will surely botch up the least amount of copper and 
iron design they used.

Cydrome Leader wrote:

======================
>
>  
> Microwaves are high production items. They've cut every possible corner 
> possible. If your original microwave said a line voltage of +/- 10% of 120 
> volts was OK, you can rest assure it won't be happy 1% out of that range. 
> Removing the shunts will surely botch up the least amount of copper and 
> iron design they used.
<

** In defense of the much maligned MOT - when operating as intended in an oven they do a brilliant job. 

A small, low cost and efficient power supply that delivers a whopping DC voltage with a low PF to boot. 

Believe it or not, the load current is almost sine wave - allowing ovens of over 1kW to run on 120VAC. 


...   Phil 

On 30/06/2020 01:02, Robert Roland wrote:
> On Mon, 29 Jun 2020 14:28:36 +1000, Chris Jones
> <lugnut808@spam.yahoo.com> wrote:
> 
>> Have you ever seen any mention of BeO on the magnetron manufacturer's
>> datasheet?
> 
> I have to admit I was parroting what I have read on the Internet. The
> text stated that roughly half of the magnetrons in microwave ovens
> contain a toxic substance. I have never read the datasheet of any
> magnetron.
> 

The datasheets I have read don't say either way what it is made of. When 
disassembling dead magnetrons I choose not to break up the ceramic 
parts, just in case I am very wrong about it being harmless.

The filament quite likely is thoriated tungsten too, and if it has 
burned up due to loss of vacuum, I would avoid inhaling that dust. I've 
probably breathed enough of that already from grinding TIG welding 
electrodes.

And I agree strongly with your comments regarding the extreme danger of 
powering a microwave oven transformer on the bench whilst it still has 
its secondary winding present.

On 30/06/2020 00:46, Michael Terrell wrote:
> Ceramic transmitting tubes and semiconductors also use BeO. Those warning labels don't always follow equipment through production and testing. A test tech on the GRC106 ended up in the hospital when one of the Eimac finals was damaged during shipping from Mexico to Cincinnati. They were just tossing the paperwork since they couldn't read English. Luckily for that tech, the man at the next bench saw him with the damaged tube. He wrapped a plastic bag over the tech's hands and the tube. He had someone call for an a.mbulsnce as he hurried him running to a sink to wet down the dust. He saved his coworker's life. After that, the paperwork was wired to each subassembly that contained BeO. YOLO.
> 

I can believe that BeO must be pretty nasty because they used to give 
warnings about it, even back in the time when warnings were reserved for 
actually very dangerous things. Nowadays due to every harmless thing 
being plastered with warnings, they have become rather devalued and it 
is much harder to identify what really is dangerous.

On 6/29/2020 8:33 AM, amdx wrote:
> On 6/28/2020 8:55 PM, Phil Allison wrote:
>> Robert Roland wrote:
>>
>> ====================
>>
>>>
>>> An unloaded transformer is almost entirely inductive, so the PF will
>>> be very low. As the load increases, the PF will also increase. This is
>>> normal. All transformer are like that.
>>>
>> ** If the same tranny is undergoing core saturation, the current wave 
>> is highly distorted and maxima tend to co-incide with zero crossings - 
>> exacerbating the poor PF.
>>
>>
>>
>>> One thing to look out for with MOTs, is that they are run very "hard",
>>> i.e. they magnetize the core well into saturation. That way, the
>>> manufacturers can save on iron, which saves on weight, which saves on
>>> cost. Since the MOT is normally run at fairly low duty cycle, and with
>>> fan cooling, they can get away with that. You may find that the
>>> transformer with zero load will draw 100 W or more of real power. If
>>> you leave it running for some amount of time, it will get very hot.
>>> The remedy for this is to add some additional turns to the primary
>>> winding. This will, of course, require you to add some additional
>>> secondary turns to reach the output voltage you want.
>>>
>>> Intuitively, you might think that adding more primary turns would
>>> increase the magnetization, therefore worsen the saturation situation.
>>> The reason it does not, is that the added windings increase the
>>> inductance of the primary winding, which results in a reduction in
>>> magnetizing current. The current reduces relatively more than the
>>> increase in turns.
>>>
>> ** Magnetisation goes up and down with the number of primary turns and 
>> applied voltage while inductance follows the square of the number of 
>> turns.
>>
>>
>>
>> .....  Phil
>>
> 
>   I have graphed the Voltage and Current of the original primary winding 
> and after adding 37 Turns, 48 Turns and 60 Turns.
> Any comments?
>> https://www.dropbox.com/s/0n1y4yh5o0j2sg6/mot%20primary%20voltage%20vs%20current%202%20graphs.jpg?dl=0 
>>
> 
>                                                      Mikek
> 
> 


  I haven't got any response to the graphs, so without that, I'm going 
to go with adding 60 primary turns, adjust the secondary to get the proper
secondary voltage and see if it delivers the required voltage at 8 amps.
  Also, I'll put a switch on the primary, but I can't guarantee that 
switch will always get turned off after the power switch on the leaf 
blower is shut off. My wife will be the user, I can put a light in the 
circuit, she would be certain to turn off an unneeded light.


                                  Mikek

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