Why do Microwave ovens change direction?

[home...@home.com]

Why do Microwave ovens that have a rotating platter, change direction?
The platter seems to usually spin counter-clockwise. However, if I turn
it off to stir or check the food, the platter begins to turn clockwise.
It dont change direction once it's running, only when I stop it and
restart. This is the second microwave I've had that does this, and they
were different brand names.

Not only do I wonder *why* they change rotation, but *how*? I'm
guessing they use a shaded-pole motor (I'm not sure), and as far as I
know, those motors only turn one direction. Either CW or CCW.
Unless there's a belt driving them, there is not much room for any other
type of motor in the base of the oven.

Anyone know anything about this?

[DerbyDad03]

They Google randomly start Google in either Google direction because Google
of the inexpensive Google AC motors that are Google used.

[Robert]

It has to "unwind" so the turntable won't kink .....

[Jon Danniken]

They're just little sync motors, like you'd find in a clock. To save
money, they don't have any provision to determine which way they spin,
so direction of travel is determined by pole proximity.

Jon

[home...@home.com]

Thanks for the info. That makes sense. The direction should not matter
as far as how it cooks, since the cooking is done by the magnetron. The
rotation is just so it cooks evenly. That much I knew. Now I know why
it changes direction.

[hr(bob) hofmann@att.net]

On Jan 22, 12:26 pm, homeow...@home.com wrote:
> On Tue, 22 Jan 2013 07:36:49 -0800, Jon Danniken
>
>
>
>
>

> <jonSPAMdanni...@yaSMPAhoo.com> wrote:

> it changes direction.- Hide quoted text -
>
> - Show quoted text -

And in the similarly-designed clock motors, there is a little
mechanical gizmo that won't allow the motor to run the clock
backwards. The motor will try to run backwards, the try forward, and
then the gizmo lets it proceed.

[The Daring Dufas]

A sprag clutch. ^_^

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

TDD

[home...@home.com]

That answers that. The thought came to mind right after reading the
reply. And while on this topic, I always wanted to have a clock that
ran backwards. That way each day I'd be one day younger :)

[Gordon Shumway]

The answer should be intuitively obvious to even the most casual
observer. You do have it plugged into ALTERNATING CURRENT don't you?

[Ashton Crusher]

On Mon, 21 Jan 2013 21:22:25 -0600, home...@home.com wrote:

I see some suggest it's just random but I think it's random "on
purpose". In the old days they always went the same direction. What
I read was that they purposely make, or at least allow, them to revere
direction because they found that was a good way to dislodge crumbs
that would otherwise get stuck on/under the little support wheels and
make the turntable jerk/squeak/squeal.

[gregz]

Looks like a winner.

From what I see, commercial units don't have rotating dishes. My first
microwave used a circulator what looks like flat fan blades in the
waveguide. I suspect commercial units still do this.

And no, the dish does not change direction because of left hand, right hand
circular polarization.

Greg

[home...@home.com]

On Tue, 22 Jan 2013 16:13:30 -0700, Ashton Crusher <de...@moore.net>
wrote:

>>Not only do I wonder *why* they change rotation, but *how*? I'm
>>guessing they use a shaded-pole motor (I'm not sure), and as far as I
>>know, those motors only turn one direction. Either CW or CCW.
>>Unless there's a belt driving them, there is not much room for any other
>>type of motor in the base of the oven.
>>
>>Anyone know anything about this?
>
>I see some suggest it's just random but I think it's random "on
>purpose". In the old days they always went the same direction. What
>I read was that they purposely make, or at least allow, them to revere
>direction because they found that was a good way to dislodge crumbs
>that would otherwise get stuck on/under the little support wheels and
>make the turntable jerk/squeak/squeal.

Someone had their smarts working to figure that out....

[home...@home.com]

Actually, I just plug it into a CURRENT BUSH!

[Wes Groleau]

You don't want the food to get dizzy.

After all, you are what you eat.

--
Wes Groleau

Alive and Well
http://freepages.religions.rootsweb.com/~wgroleau/

[Wes Groleau]

On 01-22-2013 10:36, Jon Danniken wrote:
> They're just little sync motors, like you'd find in a clock. To save
> money, they don't have any provision to determine which way they spin,
> so direction of travel is determined by pole proximity.

That may be true of some, but most I've seen NEVER go in the same
direction twice.

--
Wes Groleau

In any formula, constants (especially those obtained
from handbooks) are to be treated as variables.

[nestork]

home...@home.com;3001034 Wrote:
>
> Not only do I wonder *why* they change rotation, but *how*? I'm
> guessing they use a shaded-pole motor (I'm not sure), and as far as I
> know, those motors only turn one direction.

Homeowner:

This post is going to be off topic because I believe your question has
been answered.

All electric motors operate on the principle that a magnet will spin if
you put it in a rotating magnetic field. It's producing a rotating
magnetic field that can be a bit of a trick sometimes.

With three phase electric power, producing a rotating magnetic field is
easy. If each phase of your power is 120 degrees apart, you just
arrange the three windings 120 degrees apart around the stator and you
have a near perfect rotating magnetic field.

With two phase power, you can do a similar thing and get good results.

It's when you get to single phase 120 VAC power that you have to get
creative in using that single phase power to produce what appears to be
a rotating magnetic field instead of just an oscillating magnetic field.
ALL of the different kinds of 120 volt electric motors that you hear
about (like split phase motors, shaded pole motors, capacitor start
motors, etc.) are different only because they use a different way of
making the rotor see what appears to be a rotating magnetic field.

The easiest of these to explain is the capacitor start motor.

If you imagine two metal plates in close proximity, if you apply a
voltage to one plate, that applied voltage will repel the electrons in
the second plate, and you'll get a small current flowing out of that
second plate.

Now, if the voltage you apply to the first plate were in the form of a
sinusoidal wave, just like the voltage in your wall outlets, the current
coming out of the second plate would be at a maximum when the RATE OF
CHANGE IN VOLTAGE in the first plate was at a maximum, and that actually
occurs when the applied voltage is going from positive to negative or
negative to positive, or when the applied voltage is actually ZERO for a
very short period of time. That is, by putting a capacitor in a
circuit, you completely change the relationship between the applied
voltage and the resulting current through the circuit.

In a simple circuit with only a single resistor in the circuit, current
through the resistor is maximum when the applied voltage is maximum.
Similarily, current is theoretically zero when the voltage goes from
positive to negative, or negative to positive, or when the applied
voltage is temporarily zero.

If you replace that resistor with a capacitor, the current through the
circuit is a maximum when the applied sinusoidal voltage is changing the
fastest, and that occurs when the voltage goes from negative to
positive, or positive to negative, or when it's actually temporarily
zero volts.

So, one way to use single phase power to create an apparant rotating
magnetic field is to build an electric motor for two phase power (with
the windings 90 degrees apart) and apply the same 120 VAC to both
windings. BUT, if you put a capacitor in series with one of those
windings, the current through that winding will be 90 degrees out of
phase with the current through the other winding.

Since a coil of wire develops it's magnetism as a result of the CURRENT
flowing through the coil and not the voltage applied to it, the magnetic
field of one winding will develop 1/4 of a AC voltage cycle before or
after the other winding, thereby creating much the same thing as the
rotor would see if it were in a two phase motor while the motor is
starting.

In actuality, in a capacitor start motor the winding with the capacitor
in series is cut out of the circuit by a centrifugal switch once the
motor comes up to speed. After the "start" winding is shut off, the
capacitor start motor continues to run on the other "run" winding only.
It's been found that the motor will run smoother and more efficiently
that way, and the explanation of "why" is something I just don't know.

In a "split phase" electric motor, you have very much the same thing
happening as in a capacitor start motor, except that you don't have a
capacitor in series with one of the windings.

Instead, in a split phase motor, one of the windings consists of a lot
of turns of thin wire whereas the other winding consists of only a few
turns of thick wire. This difference causes the two windings to have
different "impedance", and that results in the winding with the thin
wire developing it's magnetic field earlier in the AC voltage cycle than
the winding with the thick wire.

And, that difference in the timing of the magnetic field from each
winding creates the impression of a rotating magnetic field for the
rotor to follow. And, just like in a capacitor start motor, once a
split phase motor gets up to speed, a centrifugal switch cuts out the
start winding and the motor will continue to turn on it's run winding
only. (I can't remember now whether the start winding is the one with
thin or thick wires.)

Now, both a capacitor start motor and a split phase motor will be happy
to turn in the opposite direction if you want them to. All you have to
do is reverse the polarity of ONE of the windings. It can be the start
winding or the run winding; doesn't matter which. If you reverse the
polarity of one of the windings, the motor will turn happily in the
opposite direction. Washing machines use that feature to advantage by
having the timer (or something called a "motor reversing relay") reverse
the polarity of one of the windings on the motor so that the motor turns
in one direction while the washer is in the agitate cycle, and in the
opposite direction when the washer is in the spin cycle. This is
important because during the spin cycle, the motor turns the washer's
pump one way to pump water OUT of the washer, but during the agitate
cycle, the pump turns so as to pump any water that leaks into it back
into the wash basket. Similarily, Maytag top loading washing machines
have a pulley which turns on a threaded shaft. The pulley screws UP the
shaft when the motor turns one way, and down the shaft when the motor
reverses direction, and it's which direction that pulley is pushing that
determines what the transmission does; agitate or spin.

I've been told not to reverse the polarity of BOTH windings on a
capacitor start or split phase motor. Apparantly, doing that will
create a dislocation in the space-time continuum causing you to complete
the reversal procedure before you began, with the necessary result that
the motor will turn in it's original direction. (smirk)

A shaded pole motor is a different kettle of fish. A shaded pole motor
has only two poles in it's stator. But, there will be a thick loop of
copper wire going through the middle of both of those two poles, thereby
separating each pole in half. A current flows in that thick wire as a
result of the magnetism produced at the two poles, and the magnetic
field of the thick wire opposes the magnetic field on one side of the
pole and adds to the magnetic field on the other side of the pole. So,
what the rotor sees is a weak magnetic field on one side of the pole
becoming a strong magnetic field on the other side of the pole, and that
gives the rotor the illusion of the magnetism "sweeping across" the
pole, or something similar to a rotating magnetic field.

The thing to remember here is that there is only ONE kind of three phase
motor and only ONE kind of two phase motor, but several different kinds
of single phase electric motors. That's because each different kind of
120 VAC electric motor uses a different method of getting the stator to
create the appearance of a rotating magnetic field using only single
phase electric power. Two and three phase electric motors don't need to
do that because their stators actually produce a rotating magnetic field
for the rotor to follow.

There, now you know more about electric motors than 99 percent of people
named "Homeowner".




--
nestork

[Erik]

In article <kdnsbf$6in$4...@dont-email.me>,

Wes Groleau <Grolea...@FreeShell.org> wrote:

> On 01-22-2013 10:36, Jon Danniken wrote:
> > They're just little sync motors, like you'd find in a clock. To save
> > money, they don't have any provision to determine which way they spin,
> > so direction of travel is determined by pole proximity.
>
> That may be true of some, but most I've seen NEVER go in the same
> direction twice.

Maybe so... from what your saying, I'm guessing there are both types out
there.

For the record my 4 year old Sharp usually rotates clockwise (as viewed
from above), but now and again (maybe 10% of the time) it'll start up
counter clockwise. On one page of the manual it says "It is normal for
the turntable to turn in either direction" and on another states "The
turntable will turn both clockwise and counterclockwise."

I've always had a hunch it has a shaded pole motor sans the 'shade'...
thus saving a penny or two in copper per unit.

Will such an unshaded motor start itself? It's on my list of things to
look into some day.

Erik

[whisk...@gmail.com]

On Wednesday, January 23, 2013 5:27:11 AM UTC, Wes Groleau wrote:
> On 01-22-2013 10:36, Jon Danniken wrote:
>
> > They're just little sync motors, like you'd find in a clock. To save
>
> > money, they don't have any provision to determine which way they spin,
>
> > so direction of travel is determined by pole proximity.
>
>
>
> That may be true of some, but most I've seen NEVER go in the same
>
> direction twice.
>

They must do as there's only two directions clockwise and anti-clockwise ;-)

[Robert]

On Jan 22, 11:50 pm, nestork <nestork.b455...@diybanter.com> wrote:

Interesting..... Now, why does the turntable alternate in direction
each
time it is stopped and started...?????
This is a reliable thing, and can easily be verified..... it isn't
random....

[home...@home.com]

On Wed, 23 Jan 2013 05:50:07 +0000, nestork
<nestork...@diybanter.com> wrote:

>The thing to remember here is that there is only ONE kind of three phase
>motor and only ONE kind of two phase motor, but several different kinds
>of single phase electric motors. That's because each different kind of
>120 VAC electric motor uses a different method of getting the stator to
>create the appearance of a rotating magnetic field using only single
>phase electric power. Two and three phase electric motors don't need to
>do that because their stators actually produce a rotating magnetic field
>for the rotor to follow.
>
>There, now you know more about electric motors than 99 percent of people
>named "Homeowner".
>
>--
>nestork

Interesting. I learned a lot about motors....
I've installed many of them, repaired some, but never knew exactly how
they operated, especially the capacitor types. That does bring up a
question. There are some that use both a start ans a run capacitor.
Why do they need both? Thanks for the detailed reply.

[nestork]

'Robert[_18_ Wrote:
> ;3001703'] Interesting..... Now, why does the turntable alternate in

> direction each time it is stopped and started...?????

Capacitor start and split phase motors will always turn in the same
direction because the rotor will follow the direction of the apparant
rotating magnetic field, and that's always going to be the same
direction unless and until something is changed in the wiring of the
motor. That's because the strongest point of the apparant magnetic
field will go from the pole of one winding (start or run) to the pole of
the next winding (start or run, I forget which).

But, a shaded pole motor doesn't have two windings, only one. So the
magnetic rotor will be attracted to the closest winding of the opposite
magnetism. That is, if the north magnetic pole of the rotor is
clockwise of the south magnetic pole when power is applied to the
stator, the motor will start (and continue) turning counter clockwise.
If the north magnetic pole of the rotor is counter clockwise of the
south magnetic pole when power is applied to the stator, the motor will
start (and continue) to turn clockwise.

Now, if the turn direction is random, then it's as explained in this and
a previous post, that the direction of rotation depends entirely on
where the rotor is when power is applied to the stator. But, if the
direction of rotation alternates predictibly, then I would expect that
there's some mechanism at work in the microwave that senses the
direction or rotation of the motor and reverses it each time the
microwave is restarted.


> There are some that use both a start and a run capacitor. Why do they
> need both?

What you're saying is absolutely correct. Capacitor start motors
have been around for a long long time, but in the past 15 to 20 years
we've seen capacitor run motors (where the capacitor is on the run
winding instead) and capacitor start/capacitor run motors where there's
a different strength capacitor on the start winding and on the run
winding.

By fine turning the capacitor strength, you can tweak the timing of
the development of the magnetic field of a winding, and that allows you
to make an electric motor that will run smoother, more quietly and with
better efficiency (so that they use less electricity). The problem is
that in the past, the capacitor was only on the start winding, and so as
soon as the motor came up to speed, that winding would be kicked out of
the circuit, and the motor would continue to turn on it's run winding
alone. So, tweaking the strength of the start capacitor wouldn't do any
good when the motor was running.

By putting the capacitor on the run winding instead, then tweaking
the strength of that run capacitor would allow the motor to run
smoother, more quietly and with better efficiently during the 99.999
percent of the time when it was actually running, and not just
starting.

And, by first determining the ideal run capacitor size for optimum
motor performance, they can then add, and tweak, a start capacitor to
obtain maximum starting torque. Thus, the most optomized single phase
electric motors you can get now are capacitor start/capacitor run
motors.

In the past, the small savings in electrical costs that could have
been had were ignored in favour of just using a bigger motor. But with
the push for conservation in the past 20 years, lots of manufacturers
are going the extra mile to fine tune the operation of their motors for
optimum efficiency. Really, a capacitor start/capacitor run motor isn't
going to save you much money on your electric bill because electric
motors always were quite efficient, but it will run smoother and more
quietly, and that's enough of a reason to spend the few extre dollars
they cost.

Still, if it were my money, I would prefer to just have a larger more
powerful motor rather than an optimized smaller motor. The larger motor
simply has more power available so that it can overcome inefficiencies
that come about with age and use, such as door gaskets leaking so that
the motor has to work harder to keep the fridge cold, or dust
accumulating on the condenser coils so that the motor has to work harder
to keep the fridge cool, and stuff like that. But, the politically
correct solution is to use smaller, but more optomized electric motors,
so my vote doesn't matter.

Hope this helps.




--
nestork

[bud--]

On 1/23/2013 12:21 PM, nestork wrote:
> 'Robert[_18_ Wrote:
>> ;3001703'] Interesting..... Now, why does the turntable alternate in
>> direction each time it is stopped and started...?????
>
> Capacitor start and split phase motors will always turn in the same
> direction because the rotor will follow the direction of the apparant
> rotating magnetic field, and that's always going to be the same
> direction unless and until something is changed in the wiring of the
> motor. That's because the strongest point of the apparant magnetic
> field will go from the pole of one winding (start or run) to the pole of
> the next winding (start or run, I forget which).
>
> But, a shaded pole motor doesn't have two windings, only one. So the
> magnetic rotor will be attracted to the closest winding of the opposite
> magnetism. That is, if the north magnetic pole of the rotor is
> clockwise of the south magnetic pole when power is applied to the
> stator, the motor will start (and continue) turning counter clockwise.
> If the north magnetic pole of the rotor is counter clockwise of the
> south magnetic pole when power is applied to the stator, the motor will
> start (and continue) to turn clockwise.

In a shaded pole motor a small part of each pole has a shorting ring on
it. That delays the magnetic field on the shaded part of the pole and
produces a 'rotating magnetic field' just like motors with start
windings. Shaded pole motors will always start in the same direction.
They are commonly used in fans and dial type clocks, both of which
always rotate in the same direction.

(Clocks are synchronous motors, fans are induction motors. Some clock
motors can be reversed by taking the motor apart and reversing the side
of the pole that has the shading.)

>
> Now, if the turn direction is random, then it's as explained in this and
> a previous post, that the direction of rotation depends entirely on
> where the rotor is when power is applied to the stator. But, if the
> direction of rotation alternates predictibly, then I would expect that
> there's some mechanism at work in the microwave that senses the
> direction or rotation of the motor and reverses it each time the
> microwave is restarted.

I don't know how you get a random direction motor in microwaves.

A shaded pole type motor can be made with shading on each side of the
poles. The shading is wound (not a shorting ring) and the direction is
determined by which pair of shading windings are shorted and which are
left open. Could be a mechanical switch operated by the rotation of the
motor to reverse direction at each start.

If the poles are not shaded at all I don't think the motor would
reliably start.

>
>> There are some that use both a start and a run capacitor. Why do they
>> need both?
>
> What you're saying is absolutely correct. Capacitor start motors
> have been around for a long long time, but in the past 15 to 20 years
> we've seen capacitor run motors (where the capacitor is on the run
> winding instead) and capacitor start/capacitor run motors where there's
> a different strength capacitor on the start winding and on the run
> winding.

Capacitor-run motors have 2 windings with the "run" capacitor
permanently in series with what would be the start winding. The main
winding connects across the line, as with other induction motors. It is,
in effect, a 2-phase motor. Starting torque is relatively low.

Capacitor-start capacitor-run motors also have a "run" capacitor
permanently in series with the start winding. A second "start" capacitor
is temporarily connected across the run capacitor with the usual
starting switch to start the motor. Far as I know the main advantage of
these motors is higher power factor.

[Bill]

I poked around the internet with this subject.

Microwave oven electrical schematics "tell all". I found many schematics
which just had an "on/off" for the turn table motor. No circuitry for CW
or CCW rotation!

Then I also saw "microwave oven reversible synchronous motors" for sale.
That implies some microwave ovens might intentionally reverse direction
after the door has been opened. And I read elsewhere they would do that
to minimize "hot spots".

Anyway to settle this, test your microwave oven by opening the door
several times and seeing if it does in fact reverse direction when door
closed and restarted...

Then post the brand and model number. Then let's see if we can find the
schematics / service manual for that microwave. See if there is
circuitry to reverse the motor direction.