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Damage phenomena in microwave oven.

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4 months 2 weeks ago - 4 months 2 weeks ago #842 by spog2
spog2 created the topic: Damage phenomena in microwave oven.
See pictures (open each link in new tab)
https://drive.google.com/open?id=0B-RykqIrjzTqV1R4QnBhOXB1dE0
https://drive.google.com/open?id=0B-RykqIrjzTqOWJjVTQxYzc4Rm8


The cracks are caused by burning/sparking. This appliance made about 800 microwave hours without any trouble, and then pretty fast degraded.
The question arises: what is the current distribution around this magnetron hole? Current (heat production) appears to be very strong at the edges.
On this dutch forum we had some discussions about it, but a complete explanation is still missing:

www.circuitsonline.net/forum/view/137527

Can you explain / describe the heat source (currents) distribution around the hole? (In its undamaged state)

Spog2
Last Edit: 4 months 2 weeks ago by spog2. Reason: Improved tekst and links.

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4 months 2 days ago #852 by Microwaves101
Microwaves101 replied the topic: Damage phenomena in microwave oven.
Sorry I don't have a good answer for you. Except some stainless steel is magnetic and that would make it very lossy and subject to heating, see:

www.eham.net/ehamforum/smf/index.php?topic=61068.5;wap2

By word association, "microwave" and "dutch" reminds me that Ampleon was working on a solid-state microwave oven... just wondering if anyone knows what year Santa Claus will have these in stock?

Regards

Steve

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3 months 3 weeks ago - 3 months 2 weeks ago #857 by spog2
spog2 replied the topic: Damage phenomena in microwave oven.
Thanks for your answer.

I just checked if the stainless steel is magnetic.
The answer is: yes!
I never thought of that.

I remember that even with a new oven of same model that a minute microwaving creates certainly more heat in the steel "than I expected".

So the magnetism may help to create fatigue cracks by hot/cold changes after some 50,000 cycles, and then things worsen quickly by sparking.

EDIT:
I studied the Skin Depth formula:
www.microwaves101.com/encyclopedias/skin-depth
It shows how magnetic permeability decreases skin depth, and then the losses are increased.
On the other hand: de manufacturer (Sharp) knows this..., so why use magnetic stainless steel then?

May be magnetic permeability properties are totally different at radio frequencies.
Where can I find rf-magnetic-permeability data for these different Stainless Steel types?
Last Edit: 3 months 2 weeks ago by spog2. Reason: Insights about skin depth

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