Why Microwave oven transformer is welded?

[polashd]

As I know transformer laminations should be insulated to minimize eddy current.
If so, why microwave oven transformer E I cores are welded together!

 

[kinarfi]

it has minimal effect, it's cheaper than banding(probably the main reason), easier to build, stronger, quieter, takes up less space, OOO
Jeff

 

[MaxHeadRoom78]

To suppress any 50hz/60hz 'buzz' that occurs from the lamination's, especially hi-current types such as a MOT.
Max.

 

[crutschow]

Connecting the edges of the transformer laminations together has no significant effect on the eddy currents since they circulate internally, parallel to the laminations, not across, i.e. there is no significant voltage between laminations to cause additional eddy currents if they are connected together at their edges.
The laminations work by increasing the resistance for the circulating eddy currents. Each lamination has a much higher resistance than a solid-core would have.

 

[schmitt trigger]

You can notice that all of the "E" and "I" laminations are not interleaved like a normal transformer would, but rather all are stacked in the same direction.

There is a lengthy reason why to do so.
But suffice to say, that this creates a loud humming noise. Welding them together, and then resin coating everything, is a way to ameliorate the problem

 

[chemelec]

These Transformers also have a Mid-Set of Laminates between the Primary and Secondary, to create a Current Limit.
(But these are not usually welded)
These Mid-Laminates Prevent Excessive Current Draw on the Primary, Even with the Output Shorted.

 

[MaxHeadRoom78]

The intermediate lamination's are magnetic shunts, usually removed (punched out) if removing the HV winding and rewinding the secondary for other puposes.
Max.

 

[tcmtech]

They weld them because just wrapping them in duct tape looked tacky.

Believe me, MOT's are built as cheaply as they can get away with and if it was allowable to just duct tape them together most manufacturers would.

Unless it's out of an old unit or a higher priced unit the odds are the windings are either aluminum or copper clad aluminum.

 

[chemelec]

I have taken apart Many of these transformers and Never seen one Wound with Aluminum Wire.

Newer Microwaves use High Frequency Switching power supplies and the Transformers are on Ferrite Cores Wound with Litz Wire.

 

[dr pepper]

So they done go zzzzz.
And microwave transformers are designed to be high leakage so they can withstand being loaded, all the E's and I's are aligned making it necessary for some strong way to hold the device togther like welding.

 

[Nigel Goodwin]

I have taken apart Many of these transformers and Never seen one Wound with Aluminum Wire.

Likewise, I've never seen (or even heard of) an MOT using aluminium wire.

Newer Microwaves use High Frequency Switching power supplies and the Transformers are on Ferrite Cores Wound with Litz Wire.

Not really true, not 'newer' ones - but an incredibly small number of ovens used SMPSU's (mostly Panasonic ones), and only very top-end models.

It's not really a good idea, apart from the weight savings - it costs far more, and is far more unreliable - every one I've had to repair has had both the magnetron and SMPSU blown. In each case, if you replace only the PSU it blows again instantly at switch on.

In ovens using conventional transformers, it's EXTREMELY rare for both mag and transformer to blow.

 

[debe]

Same experience here with Panasonic ovens Nigel,. In older style ovens ive replaced plenty of magnetrons but never a transformer.

 

[dr pepper]

The only transformers I've seen with ally windings are in cheapo mig weld sets, not a good place for ally windings as the resistance makes regulation pants, not good for mig as you need constant voltage.

 

[Nigel Goodwin]

In older style ovens ive replaced plenty of magnetrons but never a transformer.

I've replaced very occasional MOT's, but it's pretty rare - and unlike the SMPSU's Panasonic they don't die if the mag fails

 

[schmitt trigger]

I've replaced very occasional MOT's, but it's pretty rare - and unlike the SMPSU's Panasonic they don't die if the mag fails

I also had a Panasonic MO, on which its SMPS had failed. It only lasted about 3 years, whereas the older Quasar which it replaced lasted over 15 (and only had to replace it because it was leaking microwaves).

But rather than make a blanket statement "SMPS-bad, MOT-good", perhaps the Panasonic designs are substandard...... One would guess that as part of the design objectives, you would take into consideration a magnetron failure.

 

[tcmtech]

I have taken apart Many of these transformers and Never seen one Wound with Aluminum Wire.

I've scrapped dozens over the years and I have found that about 1 in 5 of the cheap ones were aluminum or copper clad aluminum wound.

The only transformers I've seen with ally windings are in cheapo mig weld sets,

It's not just common in the cheap no name brand ones. A large part of the big name higher end commercial stuff is aluminum or copper clad aluminum wound as well. Lincoln and Miller, just for two well-known brand names, have been doing it since the 1960's and 70's.

When you do scrapping as part of your livelihood you get to know this stuff pretty well and believe me, aluminum and copper-clad aluminum windings are used everywhere and have been for a long time. Electric motors, lighting ballasts, utility transformers, welders, generators, battery chargers and so on.

If it uses an electromagnetic field you can bet someone somewhere has cut manufacturing costs by putting in aluminum or copper clad aluminum in their products and us scrappers hate it.

 

[debe]

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

 

[Nigel Goodwin]

But rather than make a blanket statement "SMPS-bad, MOT-good", perhaps the Panasonic designs are substandard...... One would guess that as part of the design objectives, you would take into consideration a magnetron failure.

One would guess that they didn't want to price it even further out of the market than it already was

I would imagine, making the SMPSU much more able to stand abuse would cost a great deal more money, and make it not worth doing.

But the blanket statement is pretty well spot on, a complicated and expensive SMPSU is nowhere near as reliable as a crude and nasty mains transformer.

Anyone who does repairs will tell you that, going back to the first domestic SMPSU in the Thorn 3000 series TV's - over the last 4+ decades the majority of service work has been with SMPSU's.

 

[chemelec]

From the Few Microwave SMPS I have seen they are Cheaper and Lighter than conventional transformers.
But I Doubt they are as Reliable.

 

[Nigel Goodwin]

From the Few Microwave SMPS I have seen they are Cheaper and Lighter than conventional transformers.

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.