Microwave transformer, too good to throw away?

[Michael GilliganParticipant @michaelgilligan61133]

It’s a simple and effective way to stop the laminations buzzing

simple and effective = cheap = ‘a good engineering solution’

MichaelG.

[Nigel Graham 2]

On Nigel Graham 2 Said:

Don’t those welds defeat the object of the laminations? They are that so currents cannot circulate around the core.

I thought so too, and looked it up in my Transformer book.   It says laminations should be insulated from each other, and from the bolts used to clamp the laminations together.  (MOV transformers are welded rather than bolted, almost certainly to cut costs.)

My guess is the welds do allow eddy currents to flow, but they’re relatively small because the short is on the outer edge of the magnetic circuit.  The efficiency of the transformer must be reduced, but not so badly that it matters.  And domestic MOVs are only supposed to be used in short bursts, giving the transformer plenty of time to cool off.   Be interesting to measure the losses in a welded lamination transformer compared with one where the welds have been replaced with an insulated clamp, but I can’t think of a way of doing it without risking electrocution!

My Transformer Book was published just before WW2 and is incomplete in that it only covers big multi-kilowatt units, not the sub-kilowatt transformers I’ve used!

It says paper is suitable for insulating laminations, but troublesome in practice because big sheets tend to tear whilst being applied.  Japanning is electrically OK, but rejected because gluing laminations together makes the transformer difficult to repair.   The preferred option is a watery mix of flour and chalk: the insulation is adequate, and the laminations are easily separated if need be.  I wonder if that mix is still used?

Another fun part discusses the mechanical stresses on a big transformer when shorted or struck by lightning. Apparently, the massive current causes the other winding and the yoke to motor away with a thump, and a bad incident is violent enough to fling lumps of transformer about unless it’s been suitably reinforced!

Dave

 

 

 

[duncan webster 1Participant @duncanwebster1]

I suspect one stripe of weld doesn’t form a circuit, no doubt someone who knows what they are talking about will shoot me down, but at least it will have elicited a response

[William HulmeParticipant @williamhulme24010]

An acquaintance of mine obtained a transformer from a !000 watt microwave and had it rewound as a 420 volt auto-transformer by an armature rewinding tradesman. A qualified electrician connected to a small VFD in a suitable enclosure. Apparently most VFD’s will give an output of 380 volts 3 phase if supplied with a slightly higher 2 phase voltage applied to L1 and either L2 or L3 according to instructions. Please note that all voltages from the input plug to the motor are highly dangerous.

[duncan webster 1]

On duncan webster 1 Said:

I suspect one stripe of weld doesn’t form a circuit, no doubt someone who knows what they are talking about will shoot me down, but at least it will have elicited a response

Correct, see my reply earlier.  Note that the eddy currents flow in a plane perpendicular to the magnetic field so the two welds on the outside edge will not allow eddy currents.

Russell

[Howard LewisParticipant @howardlewis46836]

For anyone thinking of investigating a failed microwave.

They are DANGEROUS! LETHAL

One our speakers told of the lecturer, on a microwave course, saying “Be very careful, don’t touch this” , did, and died on the spot!

Howard

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Howard, yes they can be lethal, but only if you don’t know what you are doing, and a 13A wall socket can be just as lethal for the same reasons. So if people don’t know that you shouldn’t be poking about inside any electrical item, while they are connected to the mains, or know how to make sure the items need to be discharged before working on them, they should leave well alone. Road vehicles can also be lethal if they are not properly respected, along with many other things in life.

Regards Nick.

[Sonic EscapeParticipant @sonicescape38234]

The welds create Eddy currents but it is a more cost effective method. Anyway these transformers are designed to work close to saturation and are not meant to run for long time. Since they are designed to be as cheap as possible they might even have aluminum windings. I saw them re-used in cheap power supplies for tube amplifiers after the magnetic shunt was removed. Sometimes the primary of two transformers are connected in series to compensate for the insufficient number of turns. If I remember correctly they had an unusually high voltage drop with variable load and a big resistor was added to minimize this.
My Bosch oven keeps the fan running more than 1 min after I turn it off. The food is not the only thing that is cooking inside. Using such a transformer, at least with the original primary winding is justified only by extreme budget constrains.

 

[Robert Atkinson 2Participant @robertatkinson2]

There is lots of apparently authoritative information on the internet on the weld bead on transformers, mostly relating to microwave oven transformers. Almost all of it says it is to hold the laminations inplace  /reduce mechanical hum. That is totally incorrect. The weld can atually make hum worse.

The weld is there to increase the leakage inductance of the transformer. It forms a magnetic shunt. For most transformers you want to keep leakage inductance low. However for a microwave oven it has two benefits. Firstly it provides some current limiting. Secondly it attenuates harmonics that would otherwise be transmitted into mains network.
Two other applications are battery chargers and welders, both because of the current limiting.

Note that the increased leakage inductance also busts another MOT myth, that they produce ever increasing output making them particuarly dangerous.
It is also the reason why they are a good starting point for a spot welding transformer. As an aside. a toroidial transformer is a very poor choice for a spot welder dispite being physically easy to implement. This is because it’s leakage inductance is very low so excessive current will flow.

Robert.

[Michael GilliganParticipant @michaelgilligan61133]

I stand corrected !

MichaelG.

[Howard LewisParticipant @howardlewis46836]

Nick,

I have been lucky having withstood, many years ago, shocks from flyback EHT on a TV, Petrol ignition systems (Kettering, not the later types!) and 240 volt mains.  The top came off a 13 Amp plug as I pulled it from the socket, so finger and thumb onto Live and Neutral. I was on a 5,000 volt proof floor, so that may have helped.

All part of learning the hard way, fortunately without injury.

In those days, we used to use the Avo 7 to check our resistance, every Monday morning.  A cold would reduce resistance from 140K to nearer 100K Ohms, so made us more than usually cautious. (The HT supply to the valve instruments in the racks was 300 volt DC, so not to be touched. (Like the very early TVs with their separate mains powered EHT suuply.)

My predecessor was such low resistance, that he could feel 24 volts. A mains shock would disable him for the rest of the day, so he had to move out.

“There are bold pilots, and there are old pilots, but few bold old pilots”

It pays to be careful

Howard

[Russell EberhardtParticipant @russelleberhardt48058]

There is a lot of nonsense being posted here about eddy currents.  There is a good explanation of the effect of cores being laminated here:

https://en.wikipedia.org/wiki/File:Laminated_core_eddy_currents_2.svg

If you look at the diagram on that page you can see that connecting the laminations with a weld across the top will not allow currents to flow in larger loops as that would require a connection across the bottom of the diagram.

Leakage inductance is another matter entirely and is a way of visualising the effect of imperfect coupling of the flux in the primary and secondary windings.

Russell

[Robert Atkinson 2Participant @robertatkinson2]

Hi Russell,

What are you trying to say? while you comment on eddy currents and leakage inductance you don’t say how either apply in the case of the microwave oven transformer.

Robert.

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

Hi Russell,

What are you trying to say? while you comment on eddy currents and leakage inductance you don’t say how either apply in the case of the microwave oven transformer.

Robert.

What I am pointing out is that the posts above that say that the welds on the sides of the transformer do not result in increased eddy currents as a number of people have stated.  Eddy currents in a transformer core result in greater losses of efficiency and increased heating of the core.

Leakage inductance is a red herring that someone introduced.

Russell

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Howard, I’ve been bit once or twice by a TV HT, but not seriously, but enough to leave a little burn mark, I’ve had several jolts from stick welding in damp places, but my worst shock was when doing some home electrics once, many years ago, when I was connected with one hand on neutral and the other on live and had to throw myself off to let go, one foot went down the gap of the only floorboard that I had up, and punched a big hole in my kitchen ceiling, which gave me another job to do, that I didn’t want, I was still fairly young then and if it happens now, it would probably be lights out for good. Such things really do wake you up and smell the coffee, and make you be a lot more cautious, all though I was well aware of the dangers of mains electricity back then.

Regards Nick.

[Russell EberhardtParticipant @russelleberhardt48058]

Sorry that message was a bit garbled. What I meant to say was:

What I am pointing out is that the posts above that say that the welds on the sides of the transformer result in increased eddy currents are incorrect. Eddy currents in a transformer core result in greater losses of efficiency and increased heating of the core.

I plead an aging brain although I am sure that what I learned in my electrical engineering degree in the 1960s is still valid.

Russell

[Robert Atkinson 2Participant @robertatkinson2]

Hi Russell,

I introduced leakage inductance. It isn’t a red herring. It is a critical to the operation of a conventional mmicrowave oven. The construction of the transformer core, including laminations and their welding is part of the designed increased leakage inductance. I didn’t want to introduce complex details but one of the reasons high leakage inductance is required is becuase the magnetron has a negative resistance charactistic.
A bit more info without too much math is here:
https://resources.pcb.cadence.com/blog/2023-leakage-inductance-an-introduction
but like most discussions it mainly addresses reducing leakage inductance.

Robert.

[Russell EberhardtParticipant @russelleberhardt48058]

Hi Robert

That’s interesting.  I have never used a magnetron.  I have only used Gunn diodes to generate microwaves so wasn’t aware that they exhibited negative resistance.  As regards the weld increasing leakage inductance by acting as a magnetic shunt, that can’t be right as the weld runs perpendicular to the magnetic field.

Could the high leakage inductance required be caused by the core beginning to run into saturation at the operating current of the magnetron thus reducing the effective permeability of the core?

This transformer appears to have a gap between the windings of the secondary and the core stack which would give increased leakage inductance:

Requires a bit more copper but perhaps easier to control.

Russell

[Robert Atkinson 2Participant @robertatkinson2]

Hi Russell,

Gunn diodes are negative resistance too. It is very hard to know why a particular transformer is designed the way it is. Loose coupling certainly increases leakage inductance and welding will as you say affect saturation as well. Only the designer of the transformer knows why it was designed a certain way. And maybe not even the designer fully understands.
I am certain however the the stack is not welded solely for mechanical purposes or to reduce audible hum. Most transformers with welds that I have seen actually hum more than a conventional one.

Robert.

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