Why does a microwave rotate the food being cooked? And is there ever a time when it shouldn't rotate?

Most microwaves I've used have a plate at the bottom that rotates when the microwave is running. I presume that rotating helps more evenly heat the food.

Interestingly, my microwave rotates by default but has a button you can press to disable the rotation. Under what circumstances or scenarios would you want to stop the microwave from rotating the food?

• If you press stop and start again some modles rotate the opposite direction. Because microwaves cook from the insjde out moving plate helps distrbute the heat – user57430 Apr 28 '17 at 3:19
• Some models restart in a pseudo random direction. – Wayfaring Stranger Jul 4 '18 at 23:50

Microwave ovens have a turntable because the microwaves themselves form what are known as 'standing waves'. This means there are essentially static columns of microwave energy inside the cabinet itself.

You can see this for yourself - spread a tray or plate with grated cheese, take the turntable out (or press that button), and zap it for a couple of minutes. You should see bands of melted cheese interleaved with unmelted cheese. Thus, the food is rotated to ensure even heating.

• By measuring the distance between the melted spots of cheese, you can work out the speed of light! – Skizz Mar 22 '11 at 10:54
• Yes you can. Or you can just remember that it's 299,792,458m/s, using 'We guarantee certainty, clearly referring to this light mnemonic' (the number of letters in the words match the figures) :) This of course the speed of light in a vacuum. Halve it for diamonds! – ElendilTheTall Mar 22 '11 at 12:03
• @Skizz Actually, since the size of a meter is now defined in terms of the speed of light (that is, the speed of light is defined to be precisely 299,792,458m/s, and the length of a second is defined in terms of the speed of a certain excitation change in Cesium, which means the size of a meter is a function of those two definitions), you can really only use this to either check how much slower light travels in air, or alternatively (if you suck all the air out of your microwave before performing the test), you could use it to check the accuracy of your ruler. – Theodore Murdock Feb 19 '13 at 19:06
• ...and obviously, the time when you want the microwave to stop rotating your food, is when you want to measure the speed of light! – SF. Apr 27 '17 at 8:18

You stop the turntable when there's not room for your dishes to rotate - think large rectangular dish, or two plates fitting into the corners. At best, the turntable will strain and do nothing, and at worst, it'll make a mess.

Otherwise, it does help to get your food heated evenly. It can't fix everything, of course. The center point is always in the center, and even if your food is small enough to avoid the center, not all distances from the center will get quite the same heating even as they sweep out a circle. And no matter where your food is, the middle is always going to take a while to heat up. But it's a lot better than fixed hot and cold spots without a turntable.

• About the last paragraph; that's why you put your food as far away from the center of the platter as possible. – Beyond Ramen Jun 8 '15 at 19:40
• [center] Why is you put your food on one or both sides of the turntable. – Joshua Dec 16 '15 at 19:26
• If the turntable is straining, you can often replace the heavy glass with a smaller, lighter plate.I use an 8" Corel, and it runs on the wheels better than the original spinner, which met a dire fate upon hitting the floor at a couple meters per second. The solution was far cheaper than buying a new microwave, or digging up an old plate somewhere. – Wayfaring Stranger Jul 4 '18 at 23:55

There are at least 2 types of units, one with the turntable and one without. The turntable rotates to more evenly distribute the RF waves from the magnitron. The units without a turntable use a rotating reflector just under the magnitron to distribute the RF waves more evenly. Neither are perfect, as there are always hot spots. Microwaves have come down in price a lot since their introduction because they now make a magnitron not used in the radar industry. The magnitron use to be half of the price along with their power supply. Now they are just more throw-away crap like most electronic devices.

• Some models have both the turntable and the rotating reflector. That adds an extra little bit of randomness to the heating process. – Wayfaring Stranger Jul 4 '18 at 23:52
• +1 for mentioning flatbeds. – spacer GIF May 4 '19 at 14:23