Every microwave oven I’ve seen, when set to something other than “full power,” achieves a time-averaged partial power by dithering between full power output and zero power output.
Is there something about a magnetron that means it only runs well at full load, or is this some sort of cost-saving measure that allows manufacturers to leave a thyristor out of the design?
AIUI, the power output of a magnetron is determined by its physical properties (size, shape, number and size of cavities). There might be some sort of control circuitry that allows the user to simulate different properties, but compared to the circuitry that manipulates the duty cycle, that would probably be both too complex and too costly to put into a household appliance.
This. ^
Actually, some recent models use fast pulse-width modulation to produce an output that is closer to constant partial power than the usual on-off-on-off (basically, making the on-off cycle so fast that the microwave emitter never really quits or reaches full blast).
Note that the main use of a lower power setting on a microwave is to better match the rate at which heat is delivered to the object (usually to its outer parts) and the rate at which thermal conductivity transmits this heat to the center of the object. One could match these by adjusting the actual rate at which power is delivered and leaving the power on continuously; or one could just cycle the power on and off. Since the choice of method won’t significantly affect cooking time, manufacturers are free to pick the one that saves them the most money.
As an analogy, imagine filling a jar with honey by using a funnel with a narrow neck. If you pour the honey in too fast, the honey will start to accumulate in the funnel and overflow. To keep this from happening, you have to do one of two things: dump in a goodly amount of honey at a time and then let the funnel drain, or carefully adjust the rate at which you’re pouring the honey so that the rate at which the honey flows into the bottle is roughly the same at which it’s being added to the funnel. The second method is perhaps more elegant, but requires more concentration on your part, and in the end you don’t get a full jar of honey any faster.
Thank you, Sir, for that most delightful analogy.
And that most certainly would be less efficient.
I just bought a new microwave and was surprised that it still regulates the power by pulsing.
The old one, that gave good service for 15 years, had various settings shown as percentages on a dial. 25% obviously allowed the power on for 25% of the time and this was easy to hear as it switched on and off.
The new one is ‘digital’ and the power is four buttons marked in watts. 900, 600, 300 and 100. I had (naively) assumed that this meant that the power was proportional, but it is clear from listening to it operate, that it is just the same as the old one.
The answer
^ Exactly. My room mate cannot defrost food because she hits the “Defrost” button. I don’t know what power level that is but defrosting (Dethawing as she calls it) should always be done on the lowest power setting to give time for the heat to distribute. And even for reheating, if you think 2 minutes at full power is required, 10 minutes at 20% will bring superior results.
Notwithstanding that pulsing is used to simulate partial power (mine does that, too) I’m pretty sure that the “defrost” setting has its own very low power emission. When mine is on “defrost” it cycles roughly about 50% on, 50% off, but it’s clearly much lower power than a 50% power setting.
Huh? How is 50% on, 50% off, clearly much lower power than a 50% power setting.
So if it’s possible, one way or another, for a microwave oven to defrost frozen dinner bricks, then why doesn’t the built-in defrost function know how to do it? I’ve noticed that too, with my current μwave and my previous one (by a different manufacturer), that the built-in defrost function works terribly badly.
I just tried mine, which is called “Auto Defrost”. After hitting the Auto Defrost button you then have to press 1 - 9. Different numbers give different total time but they do seem to be about 50%. That’s WAY too high.
As to “why doesn’t the built-in defrost function know how to do it” it’s because that’s how it’s programmed.
Let me rephrase that. The programMERS think partially frozen/partially cooked is acceptable for a thaw function. Save yourself the curse of trying to make hamburger patties from partially cooked thawed ground beef. Just manually put it to 10% and start on low time and rinse/repeat as necessary.
Panasonic Inverter Microwaves are marketed as using a more advanced power control system. Don’t know if it is simply PWM or something else.
He means that the defrost mode operates at 50% on, 50% off measured by time, but ALSO is operating at a lower output level in *energy * while it is on. Contrast this with using the “50% power level” setting which he believes is cycling 50% on and 50% off by *time *but operating at maximum energy level when it is on.