Why Microwave oven transformer is welded?

Nigel Goodwin said:

Have you tried buying one? - why do you imagine they are cheaper?.

They are lighter (as I've already said), but that's their only advantage.

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Hi,

Wasnt he talking about the *transformer* alone, not the whole microwave oven?

I got a nice one out of a big microwave a while back but have yet to rewind it.

Also, for this particular transformer, i found that they under design the primary (of course) . They dont really have enough turns for the line voltage it has to support, so there will be some near saturation in regular operation. That most likely distorts the wave shape.
Therefore i intended to us it for test equipment only not something that has to run 24/7 as it may overheat.

Welding also means you dont have to bolt it together which is the usual way if it is not welded.
Save on 4 large bolts.

Eddy currents could flow through the weld, but the cross sectional area of the weld is much less than the whole core so the current (and therefore the extra core loss) should be minimal. The reason for laminations is so that eddy currents can not flow perpendicular to the plates. If the current flowed parallel to the plates they would not do much good because they are all metal in that direction.
Butt stacks are used to keep leakage inductance higher. Interleaved laminations reduce leakage inductance, which is actually desirable in many power supply applications.

MrAl said:

Wasnt he talking about the *transformer* alone, not the whole microwave oven?

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I presumed so?, which was what I answered to - and a SMPSU for a microwave costs a LOT more than a simple transformer.

Nigel Goodwin said:

I presumed so?, which was what I answered to - and a SMPSU for a microwave costs a LOT more than a simple transformer.

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Hi again,

Oh so you are talking about the whole power supply? That's reasonable i guess.

Hey i have an interesting side question for you Nigel...
You think a regular MW oven (non inverter inside type) could be run with just a good variac?
I ask this because instead of a Panasonic inverter oven i may one day have to turn to that option.
What i noticed was that as the line voltage goes low around here as it always does in the summer months, the regular non inverter oven would always cook much much much slower, sometimes barely cook at all. That happens when the voltage goes down like 110, then 100, then 90vac. Once it gets down too low it doesnt cook at all. So i was thinking a variac could be used to force lower power levels in the oven and that would be continuous power too not on/off on/off on/off cycling.
Also, what do you think about running the Panasonic inverter inside type oven with the variac, think it could hurt the oven in any way ?

Never seen MOT's with Aluminium coils.
The 240 Volts MOT's I have worked with or burnt out all had Copper windings.

MrAl said:

Hi again,

Oh so you are talking about the whole power supply? That's reasonable i guess.

Hey i have an interesting side question for you Nigel...
You think a regular MW oven (non inverter inside type) could be run with just a good variac?
I ask this because instead of a Panasonic inverter oven i may one day have to turn to that option.
What i noticed was that as the line voltage goes low around here as it always does in the summer months, the regular non inverter oven would always cook much much much slower, sometimes barely cook at all. That happens when the voltage goes down like 110, then 100, then 90vac. Once it gets down too low it doesnt cook at all. So i was thinking a variac could be used to force lower power levels in the oven and that would be continuous power too not on/off on/off on/off cycling.
Also, what do you think about running the Panasonic inverter inside type oven with the variac, think it could hurt the oven in any way ?

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A variac is just a variable transformer, so it doesn't affect the oven in any way. You can't use a variac to adjust the oven power, as you're turning the mag heater down as well, and as you've found below a certain point it won't work any more.

However, it would be a good idea to get your faulty mains sorted out - presumably those variations are well above that allowed?.

Nigel Goodwin said:

A variac is just a variable transformer, so it doesn't affect the oven in any way. You can't use a variac to adjust the oven power, as you're turning the mag heater down as well, and as you've found below a certain point it won't work any more.

However, it would be a good idea to get your faulty mains sorted out - presumably those variations are well above that allowed?.

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Hello again,

Yes i thought that about the mains too and i'll have to look into it i guess.

But with the variac, i guess you are saying that you dont think the extra inductance or anything will hurt as the oven turns off, and i didnt think of measuring that until just now. But with the power level, it has already been shown that as the voltage drops, the power level of the cooking goes down, making it an oven that can cook at lower power continuously without an inverter inside. I was mainly worried about the extra inductance.
I would think i could turn it down a very little bit by bit and see how the input power responds, and maybe go from there.
So i am pretty sure the power level setting idea would work, but what do you think about the extra inductance?

Yes, it's more expensive, but people have been known to use variacs for making other things in the kitchen like maybe beer? Cant remember now.

Thorn 3000, saw one of those in a museum.

I get through a few motors being an industrial leccy, I only fit high efficiency ones now which are copper.

There was a scrap yard right next door to us, the owner wass the son of my old mans mate, we used to get on well, now we have a unit full of dead machines, transport cost is more than scrap weight.

dr pepper said:

There was a scrap yard right next door to us, the owner wass the son of my old mans mate, we used to get on well, now we have a unit full of dead machines, transport cost is more than scrap weight.

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Yep most every scrap yard around now has a pile of motors and transformers that are wound with aluminum or copper clad aluminum that is not worth the cost to process or haul away.

MrAl said:

What i noticed was that as the line voltage goes low around here as it always does in the summer months, the regular non inverter oven would always cook much much much slower, sometimes barely cook at all. That happens when the voltage goes down like 110, then 100, then 90vac. Once it gets down too low it doesnt cook at all. So i was thinking a variac could be used to force lower power levels in the oven and that would be continuous power too not on/off on/off on/off cycling.

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Your simplest solution would be to use a multi-tap buck/boost transformer to bring your under voltage condition back up to normal line voltage so things can work properly.

Or use a commercial power line stabilizer/conditioner or a CVS (Constant Voltage Source) transformer if you want automatic voltage stabilization.

MrAl said:

So i am pretty sure the power level setting idea would work, but what do you think about the extra inductance?

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I think you're imaging problems where none exist

I wouldn't consider inductance in any way relevant, just as I wouldn't consider the inductance of the MOT in any way either.

As I said though, power level would only work over a limited range, too low and the mag wouldn't fire up.

Theres usually a filter on the mains side of a microwave, it most likely would reduce any inductive kick, and then the o/p of the trans goes to a doubler which would further reduce any spikes reaching the magnetron.

dr pepper said:

Theres usually a filter on the mains side of a microwave, it most likely would reduce any inductive kick, and then the o/p of the trans goes to a doubler which would further reduce any spikes reaching the magnetron.

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It's only a crude high-pass filter on the mains, intended to reduce the amount of RFI fed back out the mains lead - nothing to do with any imagined 'inductive kick'

Incidentally, the filter is why microwaves often have problems passing PAT tests.

Secondary capacitor.
http://www.microtechfactoryservice.com/doubler.html
Max.

Yes I'm sure your correct nige, I made an assumption that the manufacts of microwaves that use contacts in a mechanical timer to modulate the magnetron on and off to regulate power would actually bother to include some sort of contact flash suppresion to make contacts last longer and reduce emi.
I odnt get involved in pat testing anymore, can the settings of newer testers can be changed for a microwaves and other appliances?, I suspect theres a pair of Y rated caps to mains earth causing these issues.

debe said:

Ive found with most SMPS it doesn't take mutch to tip one over the edge with a fault, & its usualy catastrophic.

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Thats due to rubbish design, like anything the designer has to pay attention to detail to make a good product and not cut to many corners, unfortunately in an SMPS there are rather more corners to cut and many more things to not understand properly than in a 50/60hZ power transformer

dr pepper said:

I odnt get involved in pat testing anymore, can the settings of newer testers can be changed for a microwaves and other appliances?, I suspect theres a pair of Y rated caps to mains earth causing these issues.

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Yes, there's a pair of Y capacitors - the PAT tester I use at work (a cheap one ) can be set suitably for them.

Do you have to pat test when doing repairs?, do you do stuff for hotels/public places, or is it used for other purposes.

dr pepper said:

Do you have to pat test when doing repairs?, do you do stuff for hotels/public places, or is it used for other purposes.

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There's no requirement for PAT testing repairs, we had to buy the tester years back as we were a VAX service agent, and they added a condition that their warranty repairs had to be PAT tested, not a bad idea really I suppose as they used water.

We do very little PAT testing these days, mainly for a local antique dealer (for candlesticks he converts to electric lamps), the odd student off to University (many Uni's require any electrical items to be PAT tested), and for people taking laptops in to the local council offices.

If you actually read the H&S website, there's no actual requirement for PAT testing anyway.

Nigel Goodwin said:

It's only a crude high-pass filter on the mains, intended to reduce the amount of RFI fed back out the mains lead - nothing to do with any imagined 'inductive kick'

Incidentally, the filter is why microwaves often have problems passing PAT tests.

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Hi again,

Why do you say it is an 'imagined' inductive kick back?
Inductors give that kind of response when they have current flow and the circuit is suddenly open circuited.

MrAl said:

Why do you say it is an 'imagined' inductive kick back?

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Because it's a transformer, billions of them are in use, and 'imagined' inductive kick back wasn't any design concern on those.