Why do microwave ovens change the direction of rotation?

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Offline niall

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This seems like kind of a dumb question but...
I've noticed that lots of microwave ovens change their direction of rotation if you open and close the door. I can't think if any reason for this. I know that the microwave field in the oven can have standing waves in it, meaning that there are "hot spots". The food rotates to try and stop these more intense patches heating one bit of the food faster that another bit. But why change the direction of rotation (clockwise vs anticlockwise) after the door is opened. The best I can think of is that it may stop some part wearing out from always rotating the in same direction...but this doesn't seem massively plausible.

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Niall

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Offline lightarrow

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Why do microwave ovens change the direction of rotation?
« Reply #1 on: 01/10/2008 18:27:38 »
This seems like kind of a dumb question but...
I've noticed that lots of microwave ovens change their direction of rotation if you open and close the door. I can't think if any reason for this. I know that the microwave field in the oven can have standing waves in it, meaning that there are "hot spots". The food rotates to try and stop these more intense patches heating one bit of the food faster that another bit. But why change the direction of rotation (clockwise vs anticlockwise) after the door is opened. The best I can think of is that it may stop some part wearing out from always rotating the in same direction...but this doesn't seem massively plausible.

Thanks
Niall

Interesting question. Let's begin with the more simple (silly?) answer: maybe not to make the owen rotate on the table, after some time!?  [:)]
Ok, waiting for the second one.

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Offline Bored chemist

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Why do microwave ovens change the direction of rotation?
« Reply #2 on: 01/10/2008 19:18:41 »
It could just be marketing. It's a way for the maker to say "our product is different". Since most people will not stop to think that it cannot possibly make a difference, they will think it helps somehow and that the "reversing" microwave is somehow better.
Silly possibillity; to avoid this effect
http://xkcd.com/162/
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Offline techmind

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Why do microwave ovens change the direction of rotation?
« Reply #3 on: 01/10/2008 19:59:50 »
Interesting question. Let's begin with the more simple (silly?) answer: maybe not to make the oven rotate on the table, after some time!?  [:)]

Not wholly convinced, but reducing the amount of net "walk" of the oven due to vibration is the most plausible technical reason I can think of...
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"

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Offline Pumblechook

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Why do microwave ovens change the direction of rotation?
« Reply #4 on: 01/10/2008 20:53:56 »
I tried ours and it spins one way or the other when opening and closing the door in a random manner...not alternate. 

I think it is just a function of the type of the simple single phase induction motor which can spin in either direction when it is first powered. 

Other ovens have probably got different types of motor which only spin one way. 
« Last Edit: 01/10/2008 21:05:05 by Pumblechook »

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Offline niall

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Why do microwave ovens change the direction of rotation?
« Reply #5 on: 01/10/2008 22:56:09 »
Quote
I tried ours and it spins one way or the other when opening and closing the door in a random manner...not alternate. I think it is just a function of the type of the simple single phase induction motor which can spin in either direction when it is first powered.

That's not a bad suggestion at all. Unfortunately I don't have my microwave anymore so I can't see whether it was random or not. If it is random I bet you are right.

p.s. gotta love xkcd

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Offline Make it Lady

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Why do microwave ovens change the direction of rotation?
« Reply #6 on: 02/10/2008 22:47:10 »
A lot of foods such as soup ask you to remove them after a set time, take them out and stir them then put them back in. I would think that if the microwave rotates in the opposite direction it would give the best option of reducing the hot spots. I'm do some research and perhaps get back to you.
Give a man a fire and he is warm for a day, set a man on fire and he is warm for the rest of his life.

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Offline Make it Lady

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Why do microwave ovens change the direction of rotation?
« Reply #7 on: 02/10/2008 22:53:34 »
OK I give up, it is the alternating motor after all. It has nothing to do with cooking the food. I'm really sad now as this his demystified a truly magical piece of cooking kit. Can anyone cheer me up again with an amazing microwave fact!
Give a man a fire and he is warm for a day, set a man on fire and he is warm for the rest of his life.

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Offline niall

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Why do microwave ovens change the direction of rotation?
« Reply #8 on: 02/10/2008 22:59:57 »
The microwave rotating in the opposite direction makes no difference to balancing out hotspots - that's why I was so mystified in the first place. Rotating means that the same hotspot isnt exposed to the same bit of food all the time but the bit of food doesn't care if that hot spot appears from one direction or the other...if that makes any sense

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lyner

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Why do microwave ovens change the direction of rotation?
« Reply #9 on: 02/10/2008 23:30:09 »
Good question. I can't think of any good reason. The waves in the cavity don't care and it's not going fast enough to have any effect.
It must make the machine a bit more expensive, which can't be good.

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Offline LeeE

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Why do microwave ovens change the direction of rotation?
« Reply #10 on: 03/10/2008 00:19:23 »
I think it is just a function of the type of the simple single phase induction motor which can spin in either direction when it is first powered.

Most likely reason - cheaper.

If it changed direction during cooking it might make some sense, but to do so only after stopping doesn't.
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