> ;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.
> 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
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
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
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
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.
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.)
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
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.
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.
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.
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