Q&A

# Why does AC frequency affect my microwave's RPM?

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I used to live in the United States, and my microwave ovens' turntables turned at six rotations per minute (RPM). I now live in Israel, and my microwave oven's turntable rotates at five RPM. Annoyed at this[1], I mentioned it to someone, and he intimated that this is due to the AC frequency (60 hertz in the States but 50 here). Can someone please explain what the AC frequency has to do with the turntable frequency? What exactly is AC frequency[2], and how does it affect the turntable frequency? I've checked out the Wikipedia articles on utility frequency and alternating current, but they don't explain this point.

1. Why was I annoyed, you ask? Because my microwave oven has a thirty-second button, but the food winds up at the back. ↩︎

2. It's entirely possible that if I really understood what AC was, I'd also understand what AC frequency is. I don't really know what it means to be changing the direction of the electricity flow every hundreth or 120th of a second. ↩︎

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This entirely depends on what motor that sits inside the oven and how it was designed. I wouldn't assume that every microwave that has ever been made is using the same kind of motor. You'll have to come up with some manner of partnumber or preferably a datasheet of the motor, or I don't think this question can be answered. Lundin‭ 4 months ago

Be careful, my dude. People don't take questions about microwaves too well here. Kranulis‭ 4 months ago

As for the question "what is AC frequency", that's a valid question. Alternating currents means you have several phases (simply put: several signals/wires) between which the current alternates. The current in each phase comes as a sine wave and the "AC frequency" is simply the (2*PI) period of the sine wave. So at 50Hz one phase will have 1/50=20ms between sine wave peaks. And this frequency, as well as the AC voltage, are poorly specified around the world. The most common is 230VAC/50Hz. Lundin‭ 4 months ago

@Kranulis - now that you mention it... girlfriend's fine brand microwave just made a viciously loud "snap" noise over the weekend, which I figure is the magnetron. wanted to do a dissection for the turntable // UPDATE - supposedly working again. oh well Pete W‭ 4 months ago

Your only real remedy is to test or research your next microwave purchase. They have various rotation speeds depending on the gear ratio chosen for the synchronous motor. It can vary quite a bit so you may find a suitable speed for your local line frequency when you replace or swap the unit one day. KalleMP‭ 3 months ago

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The turntable is apparently run by a cheap synchronous motor. There is a type of motor that's easy to manufacture, that turns synchronously with the line frequency.

You say the table used to rotate at 6 RPM with 60 Hz power, and now rotates at 5 RPM with 50 Hz power. That is exactly what you'd expect from a synchronous motor.

The turning of the food in a microwave oven is only to even out the effect of hot and cold spots. It's not very critical, and whether one turn takes 10 seconds (6 RPM) or 12 seconds (5 RPM) doesn't matter.

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