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Microwave oven PWM frequency

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rea5245:
Anyone want to jump into the middle of a domestic dispute? (*)

My wife, who uses the microwave oven a lot more than I do, says that when she puts something in the microwave oven at low power, she can tell that the microwaves are turning on and off with a cycle time of a few seconds. E.g. she can see water boil for a few seconds, then stop boiling for a few seconds, then start again.

I, while candidly admitting that I didn't know for sure, expressed surprise that it would work that way. I had assumed that the magnetron would be pulsed on and off at at least 1Hz, if not faster.

Yet she does seem to have compelling evidence. Why would the magnetron be cycled at such a low rate? Is it incapable of being turned on and off faster? Or did the designers not think it would matter?

Thanks,
   Bob

(*) Rest assured that no matter who is right, my marriage will not break apart.

Fred27:
Your wife is right. You can even hear it turning on and off every few seconds - maybe even slower than that.

Cliff Matthews:
FWIW, listen for fan rpm variations. I've had several since the 80's and don't recall any rate faster than about 0.5hz..

tom66:
Your wife is correct.

The average magnetron takes about 1-1.5 seconds to warm up from cold because it's a vacuum tube and the heater needs to start boiling off electrons.  You can often hear the tone of a microwave change after the first second of operation.

So any PWM needs to be quite slow.  Also,  the inrush current on the transformer on each cycle would be likely to blow fuses if it happened so rapidly. 

Perhaps keeping the tube warm using a separate supply would be one solution, but magnetron heaters are quite power hungry.  One example I saw was 3.5Vac at 25 amps.  So the additional supply for this is probably not worth it for such a mild improvement.  It is just derived off the main transformer.

Inverter microwaves, marketed by Panasonic and possibly others, still take time to warm up but the inverter directly controls the power to the magnetron.  So the low power setting does not require PWM and the power delivery is constant.  The lowest settings (defrost and warm on my Panasonic oven) still use PWM because magnetrons are not good at producing small amounts of power.  The PWM has a longer period and higher frequency though (about 0.2-0.3Hz).

/rant:
What does annoy me is despite knowing that a magnetron takes a second to warm up - and despite being able to directly monitor magnetron current on the inverter units - no microwave seems to delay the timer if the magnetron is not generating microwaves.   If you want to nuke something for ten seconds, if the magnetron is completely cold it can take 3-4 seconds to start delivering power.  This makes timing inconsistent at the lower end of the scale. 

Zero999:
She is right. I remember dismantling an old mechanically controlled microwave, when I was about 12 years old and looked at the power control. It consisted of a synchronous motor driving a gear system, which turned on and off a microswitch. Adjusting the power control altered the position of a cam, so the duty cycle was adjusted. The switching frequency was very low, much under 1Hz.

Now there are some inverter microwaves, which don't use a large iron cored transformer, but a switched mode power supply. I believe these either use a higher frequency PWM, or just alter the current through the magnetron, in a linear fashion.

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