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.
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.
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.
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 reliably start.
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.
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