Rewiinding a MOT

The easy way is find a pair of the same part. Rewind the secondaries and connect the primaries in series. No saturation problems. 

Michael Terrell wrote:

=========================
>
> The easy way is find a pair of the same part.
> 
>
 ** Agreed. 

> Rewind the secondaries and connect the primaries in series.
>
>

** The secondaries need to be paired too, preferably in parallel. 

  Ensures the primaries split the applied AC voltage evenly.


.....   Phil 

"Jasen Betts" <jasen@xnet.co.nz> wrote in message 
news:rd8d4n$7dq$1@gonzo.revmaps.no-ip.org...
> You've increased the magnetic resistance by removing the shunts so now
> it's running further into partial saturation, you can maybe fix that by
> adding some primary turns,  the core is built with E and I stacked
> instead of interleaved this makes for more gap which probably isn't
> helping either.

Not really, the core cross section on the primary side is still the same. 
Shunts, well, shunt some flux, so the secondary side is probably not in 
saturation.

But also, if the secondary is heavily loaded, the shunts themselves will 
saturate and secondary current suddenly ramps up.  This gives a hockey stick 
sort of load curve.  Which I think doesn't matter in the intended 
application: a magnetron has a zener-like characteristic, so only a modest 
compliance range is needed for the current limiting.  Hence the small shunts 
are a cost-saving measure.

(For a linear application, shunts need the full cross section of the core, 
and are air gapped to set leakage inductance.  AFAIK, neon sign transformers 
are designed this way.)

Tim

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

On 6/27/2020 9:09 PM, Phil Allison wrote:
> amdx puked:
> 
> ===========
> 
>>>> 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.
>>>
>>>
>>> ** So you put a 30 watt load on a 1kW, saturating supply tranny and expected a good PF ???
>>>
>>>
>>>>    I expect Phil will set me straight on this.
>>>
>>>
>>> ** Sure thing, but not the Phil you wanted.
>>>
>>>    As Mrs Cooper famously said:  "dumb as soup" .
>>>
>>>
>>> ....  Phil
>>>
>>
>>    No you are the Phil I expected and I got the usual reaction.
>>
> 
> ** Precise and clear, unlike you.
> 
> 
>> You have made a lot of comments about transformers over the years
>> so I expected you would have some good info.
> 
> 
> ** Then use " Phil A"
>    
>>   If you could just get past your personality!
> 
> ** Take a look in the mirror, sunshine -  you have the personality and IQ of a demented chimp.
> 
> 
> ....   Phil
> 

No secondary on the core.
Original Primary 122.5V, 4.46amps, 549VA,
Primary with an additional 39 turns, 122.5V, 0.87Amps, 106VA
  Here's a curve of Voltage VS Current
> https://www.dropbox.com/s/5itjzorex11nxvt/sat%20curve.jpg?dl=0
  Is there a Standard Ratio of Magnetizing current to Full load current?

I saw a video of someone else adding turns to an MOT, he had about the 
same original current, but after adding turns his dropped to 0.44 amps, 
about half of what I have.

                                    Mikek






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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.

>   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?

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.
-- 
RoRo

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.
> 
>>    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?
> 
> 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.
> 

  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

                                    Mikek


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On Sat, 27 Jun 2020 23:03:24 -0400, bitrex <user@example.net> wrote:

>isn't there a rule of thumb about MOTs that if you have to ask you 
>probably shouldn't be messing with MOTs

The point of this statement is to express that they are dangerous. And
they definitely are.

But I would not take the statement too literally. How are you going to
learn anything about anything if you're not allowed to ask? You end up
in a "catch-22" situation.

>I have a confession to make, I don't think I've ever thought messing 
>with MOTs would be a productive use of my time. sometimes I feel like 
>those things are catnip to electronics dudes I don't get it.

I think it's about cost. A transformer is attractive to a hobbyist,
but buying one with a few hundred VA will cost a pretty penny for a
hobbyist.


I have three pieces of advice for those wanting to play with MOTs:

1. Leave the magnetron alone. The magnet is not even very strong.

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.

2. Watch out for the capacitor.

Inside the microwave oven, there is a capacitor that will charge to a
couple kV. Use insulated snips to cut the wires, one at a time. Cut
each wire twice, with as much distance as possible between the cuts.
That way, you reduce the risk of accidental reconnection. Then, if
possible, use an insulated screwdriver to short the capacitor.

3. Do not apply mains power to the transformer until you have removed
the secondary winding.

The secondary winding otputs a few kV at practically unlimited power.
It is extremely dangerous.
-- 
RoRo

On 6/28/2020 12:50 PM, Robert Roland wrote:
> On Sat, 27 Jun 2020 23:03:24 -0400, bitrex <user@example.net> wrote:
> 
>> isn't there a rule of thumb about MOTs that if you have to ask you
>> probably shouldn't be messing with MOTs
> 
> The point of this statement is to express that they are dangerous. And
> they definitely are.
> 
> But I would not take the statement too literally. How are you going to
> learn anything about anything if you're not allowed to ask? You end up
> in a "catch-22" situation.
> 
>> I have a confession to make, I don't think I've ever thought messing
>> with MOTs would be a productive use of my time. sometimes I feel like
>> those things are catnip to electronics dudes I don't get it.
> 
> I think it's about cost. A transformer is attractive to a hobbyist,
> but buying one with a few hundred VA will cost a pretty penny for a
> hobbyist.

  Ya, it's cost, not because I can't afford it, I can afford it,
because I've been frugal most of my life!
Someone posted a transformer at about $50.
More fun trying to modify one.
Yes, to being safe.
Now, on to insulating the terminals on the Variac!

                      Mikek



> 
> I have three pieces of advice for those wanting to play with MOTs:
> 
> 1. Leave the magnetron alone. The magnet is not even very strong.
> 
> 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.
> 
> 2. Watch out for the capacitor.
> 
> Inside the microwave oven, there is a capacitor that will charge to a
> couple kV. Use insulated snips to cut the wires, one at a time. Cut
> each wire twice, with as much distance as possible between the cuts.
> That way, you reduce the risk of accidental reconnection. Then, if
> possible, use an insulated screwdriver to short the capacitor.
> 
> 3. Do not apply mains power to the transformer until you have removed
> the secondary winding.
> 
> The secondary winding outputs a few kV at practically unlimited power.
> It is extremely dangerous.
> 


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Tim Williams wrote:

==================

>  the core is built with E and I stacked
> > instead of interleaved this makes for more gap which probably isn't
> > helping either.
> 
> Not really, the core cross section on the primary side is still the same. 

**  An air gap like that reducers the effective primary inductance so increases magnetising current. 


> Shunts, well, shunt some flux, so the secondary side is probably not in 
> saturation.

** Huh ? 

> 
> But also, if the secondary is heavily loaded, the shunts themselves will 
> saturate and secondary current suddenly ramps up. 

** On what planet ? 



 .....  Phil 

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