microwave radiation in countries where electrical frequency is different

[curwin]

QED did a nice job of explaining the risk of microwave radiation in this column:

http://www.straightdope.com/mailbag/mmicromesh.html

But back in 2001, I posted the following question:

http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=79299

I wrote:

Before I moved from the US to Israel, I participated in a mailing list for people making that move. We would talk about what appliances to buy, whether to buy them in the US or Israel, and if we brought our American appliances, could we use a transformer (Israel uses 220 current).

I had a microwave oven in the states that I wanted to bring to Israel and use it with a transformer. At one appliance store they told me that I had to buy a new microwave (one that ran on 220). Their reason was that not only does the current change between countries but the frequency does as well (from 60 to 50 MHz, I think – the numbers could be off). They said that if the frequency was different, the holes on the screen of the microwave oven would be too big for the microwaves, and they would escape.

In light of the Mailbag article - can anyone give me any more information?

[ftg]

The wavelength of microwaves is determined primarily by the size of the resonator cavities. Different AC Hz shouldn’t affect that aspect. However, certain parts of a microwave operate very close to the limits of the components, e.g., the main capacitor, the step up transformer, and (unfortunately nowadays) the rectifiers. So simple current transformers may not provide a good enough approximation for the AC the microwave is designed for. Problems would generally lead to shortened lifespans of the appliance, not leaking microwaves.

See this for more info.

[sford]

Minor point - Hz means “cycles per second” and MHz means “millions of cycles per second”. The power frequencies are 60 Hz for the US and some other countries, and 50 Hz for most of the rest of the world. As ftg said, the power frequency would not have any effect on the required hole sizes. But other design issues might make it necessary to get a unit designed for 50 Hz. There should be a metal plaque near where the power cord comes out of the unit. If that plaque says “50-60 Hz” then a transformer might do the trick. If it says “60 Hz”, you would be running a high risk running it on 50 Hz.

I’m guessing here, but it’s possible that Israel’s regulatory agency simply requires a greater margin of safety - hence the smaller holes.

P.S. - if an oven’s microwaves run at a lower frequency, the wavelength of the radiation increases, thus making it possible to make the holes larger and still block the same amount radiation. It is higher frequencies that requires smaller holes.

P.P.S. - Microwave ovens tend to require quite a few watts of power. The voltage transformers that one gets for forign travel usually don’t supply that much power. Be sure to check.

[ftg]

I can’t believe I missed this part of Sam W.'s microwave FAQ before. Sorry.

Again, different AC voltage/Hz will not affect the wavelength to be of any concern. The issue is damaging the appliance (or the adapter) and/or reduced efficiency.

[Abel_s_Brother]

He did make a math mistake!

[nitpick] 0.2 mW/cm^2 is not a tenth of 5 mW/cm^2, it’s one 25th [/nitpick]

Either that or he just meant an order of magnitude…

[Ilari]

I’m splitting hairs here, but there was a little point that was missed in the original article: If you put a cellphone inside a microwave oven, it by all accounts shouldn’t work. The frequency the cellphones use is 1.8GHz (or 1.9GHz), which has a longer wavelength than microwave radiation at 2.45GHz. However, put a cellphone inside a microwave and it will ring if you call it (in most microwaves). This is an experiment you can try at home.

The reason for this is that radiation can escape from between the door and the oven itself (the oven is not a real Faraday cage). Radiation at 2.45GHz (oven’s operating frequency) can not escape due to the fact that the door has a cleverly constructed wavetrap surrounding the edges which is accurately optimised for wavelengths that the oven uses. But radiation at other frequencies can still escape since the wavetrap doesn’t affect them.