It's a synchronous motor that runs equally well in both directions, and
the direction it goes is effectively random. (You could likely get it
to always start in a particular direction by picking the point during
the AC waveform to turn on the relay supplying power to the motor).
Did you ever wonder why, if you select 45 seconds of heating, the
magnetron runs for 45 seconds but the motor continues to run for another
5 seconds after the magnetron is off? Because the turntable motor
always runs for an integer multiple of the turntable rotation period,
usually 10 seconds. So if you put something at the front of the
turntable, near the door, it will be in that position again when the
oven shuts off.
If only. I have junk piled in front of the microwave, and I wish it
would stop in the same position it started.
I've been trying to keep track of how long a rotation takes, but I
forget now. Should I make some tests? Does it matter what brand of
microwave I have? From what you say, it doesn't seem to.
My impression is that it continues to run while it is beeping. I can
tell when the magnetron turns off by running a radio on a fairly weak
station next to the oven. Which I do a lot of the time, because I
listen to two stations 50 miles from here.
Yeah, it would depend on brand. All the ones I've had with turntables
were 10 seconds/turn, but that doesn't mean everybody uses that rotation
rate. And the "always run the motor an integer number of turns" is a
software feature that might not be in some controllers, though I've
noticed it in the ovens I've used.
You can usually tell that the magnetron is on by the humming from the
high-voltage transformer. Set the oven to 50% power, and you should
hear the transformer hum turn on and off with about a 50% duty cycle and
a period of perhaps 15 seconds.
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