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Fan motor runs backwards Bobo 07-13-2007
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Posted by Dave Martindale on July 17, 2007, 2:00 am

>Which brings up a question I've always wondered about: why do most
>microwave oven turntables randomly reverse direction at turn-on?

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.

        Dave

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Posted by mm on July 17, 2007, 7:04 am
On Tue, 17 Jul 2007 06:00:07 +0000 (UTC), davem@cs.ubc.ca (Dave
Martindale) wrote:

>
>>Which brings up a question I've always wondered about: why do most
>>microwave oven turntables randomly reverse direction at turn-on?
>
>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.

>        Dave


Posted by Dave Martindale on July 17, 2007, 9:29 pm

>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.

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.

>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.

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.

        Dave

Posted by Red on July 18, 2007, 4:20 pm
On Jul 17, 1:00 am, da...@cs.ubc.ca (Dave Martindale) wrote:

>
> 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.
>
> Dave

Ok, you got my interest up enough to try it. My Sharp turntable stops
when the timer reaches 0 no matter what it's rotational position.
Yours must be manufacturer specific.

Red


Posted by Ashton Crusher on July 19, 2007, 2:04 am

>> What's up with a third horse whole house fan motor that has begun to
>> occasionally run backwards. This started happening about the same time
>> the switch went bad. I put in an identical switch and didn't mess with
>> the motor wiring. It's AC, 120.
>> Thanks
>
>Which brings up a question I've always wondered about: why do most
>microwave oven turntables randomly reverse direction at turn-on?
>
>Red


I've never seen it to be random. They reverse direction on purpose
because it helps to keep food debris from getting stuck under the
little wheels and making the turntable drag and stutter.

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