[codecouponqw]

my microwave has plate on top of a ring with tiny wheels, when I heat one thing it might turn clockwise, if I heat a second thing it will turn counterclockwise.....why does direction alternate with every item microwaved?
I hope this is the right place to ask a question......I've looked all over the website, and did a search to find questions about microwave ovens.

I've been listening to Dr Karl's podcasts here in New York City, and I love them.....great radio program

[GinaGomesz]

Wow!! That was quick!

[DP5Ups8o]

It's to do with the type of motor used to drive the turntable. Its called an induction motor. Normally they use a bit of cleverness in the way the coils are wired up to get it to spin in a particular direction but with a simple job like driving a microwave turntable it doesn't matter so they dispense with a few bits of copper and save themselves some money.
More info here.http://www.phys.unsw.edu.au/hsc/hsc/...c_motors1.html

Its quite an educational read and entertaining as well. This is from his list of things to be careful of with motors.
1. Before disassembling an electric motor, make sure you have owner's permission. This is really important – especially if you don't succeed in getting it back together again before they find out.

4. In many permanent magnet motors, simple disassembly and reassembly, even if you do it 100% right, will result in the magnetic field being weaker than before. Paradoxically, the motor will now spin faster than it did previously. This could lead to alarming results if, for example, the motor is part of your grandmother's electric wheel chair.

[Dkavtbek]

I had always assumed it was on purpose. You learn something new every day!

[Zavdpacq]

You'll need to fix your link Wataday or until you do http://www.phys.unsw.edu.au/hsc/hsc/...c_motors1.html

[Mabeavyledlib]

I had always assumed it was on purpose.

As did I - didn't really give it much thought: "It happens so it is probably meant to do it".

[paypaltoegold1]

The motor in a microwave is a Synchronous motor.

http://en.wikipedia.org/wiki/Synchronous_motor

Similar to an induction motor however.

Synchronous motors will run equally well in both directions and the direction it starts running in is random.

So why does the turntable consistently run in a different direction each time?

Because the motor is connected via a gear train to the mass of the turntable. This gear train has significant backlash.

Hence when the motor attempts to start it will meet resistance if it tries to go the direction it did on its last run as the gear backlash is taken up.

It will stall on that first cycle of curent then attempt to go the other way where there is slack to be taken up in the gears. Here it meets no resistance and can build up to its synch speed before the load of the gears takes up.

[OwdBKKHO]

Those motors driving the reduction gears go both directions, and the direction they start depends where it is stopped when it is started. Probably has the [unintended] benefit of making the wear on the drive gears equal on both sides.

AC single phase motor and if you connected the power up backward it would operate exactly the same.

[Grainiary]

Those motors driving the reduction gears go both directions, and the direction they start depends where it is stopped when it is started.

As well as the part of the ac cycle when you hit the start button.

The motor will however run in the opposite direction each time due to the backlash effect I outlined.

[Emalodoulouts]

>>>As well as the part of the ac cycle when you hit the start button.

Yes, critically so, TGTOF, you get the first gold star for the morning.

*hands him a coffee*

Relays and switches have contact bounce also, and other aspects of the load like and reactance being switched may be involved.

[hygtfrdes]

Well my microwave cacked itself yesterday so it was time to get the screwdriver out and confirm things.

Yes its a synchronous motor. I pulled it to bits too and the difference is the rotor is a permanent magnet rather than a squirrel cage.
No obvious relays or switches with contacts to have contact bounce. the switching appears to be done in a chip.

Thanks taste of fish, I always thought that a synchronous motor was just a different type of an induction motor and included under that umbrella..

[AndyPharmc]

Of course, this is a SCIENCE forum, so someone far more intelligent and skilled and better-smelling than I am could actually do an experiment to see whether the phemonenon as described happens.


They would, of course, discover that the direction of motion does /not/ alternate consistently!


Note too that I don't claim to have done the experiment -- only that I suggest that someone far more intelligent and skilled and better-smelling than I am could actually do the experiment.

The thread nevertheless provides for good SCIENCE, and I approve.


/* I hope this is the right place to ask a question......I've looked all over the website, and did a search to find questions about microwave ovens. */

Seems reasonable. A better SCIENCE forum (in its time, which is no longer) was
http://www2b.abc.net.au/science/k2/stn/
, which does have answers relating to what 'u seek:
http://www2b.abc.net.au/science/k2/s...pic541294.shtm
http://www2b.abc.net.au/science/k2/s...ic1301199.shtm
http://www2b.abc.net.au/science/k2/s...ic3827577.shtm
. I can't give any reliable indication of how long these contributions will be retained.

[Alina20100]

As noted in those previous threads, my experience has been that it doesn't always reverse direction every time it is started - it seems to be more of a random thing when I have opened the door to tend to the cooking/heating substance and then restarted it.

[HaroTaure]

OK. I've done the experiment.
As mentioned earlier my microwave attempted to cook itself yesterday, so I have a turntable motor I just pulled out of the ex microwave. Oh I also found the fault while wrecking the thing, the relay that I said earlier it didn't have had its contacts welded together meaning the magnetron was energised constantly until the thermal cutout cut out.
Results
Where c=clockwise a=anticlockwise.
CCAACACCCAACAAACCCCCCCCACCAAAC

a small experiment but it looks fairly random, that was just the motor on the bench no platter.

Edit to add:
motor with platter results
AAAAAAACCCAACCACCCCAACCCA

[mikajuise]

>>>>No obvious relays or switches with contacts to have contact bounce. the switching appears to be done in a chip.

No door switches either I suppose.

[Enjoymms]

Here is an experiement for someone.
1. Put something to heat up in the microwave.
2. Turn it on.
3. Note the turntable's direction.
4. Open door.
5. Attempt to move the turntable in one direction.
6. Restart microwave.

Expected result. The turntable will rotate in the opposite direction to what you attempted to turn it. It does not matter which direction the turntable was going originally.

The above should be true if what you (The Great Taste of Fish and others) were saying above is true.

[spineeupsenry]

>>>>No obvious relays or switches with contacts to have contact bounce. the switching appears to be done in a chip.

No door switches either I suppose.

Not anymore.
yeah there were three, and as I said below I found the relay that drives the magnetron and lots of clicky switches. Not visible until I pulled the timer/ control board out.

[Andrius]

Only marginally related, one of my best 2 dollar purchases. A little plastic dome with baffles that sits over food in the microwave. Stops splatters. I hate cleaning microwaves so it it brillig. Put thick kitchen paper underneath on the turn table and no cleaning ever.

[Misespimb]

Only marginally related, one of my best 2 dollar purchases. A little plastic dome with baffles that sits over food in the microwave. Stops splatters. I hate cleaning microwaves so it it brillig. Put thick kitchen paper underneath on the turn table and no cleaning ever.

I just use a lid, or glad wrap, which are usually already on the dish.

[StoyaFanst]

Second experiment using rjh01s parameters.

same setup motor on bench with platter
push clockwise momentarily leave settle then switch on

CACCCACCACACAAA

push anticlockwise momentarily leave settle then switch on

CCCAACCCCACACCA

EDIT:
I tried this again with a much older motor and it became a lot less random, possibly more backlash in the gears.