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Electric Galley - Microwave
#1
When building the Battery Charger / Inverter I have used a transformer from a Microwave Oven.
I have dismantled a few of them, and have been thinking about how to make them more efficient for use on an electric boat.  

Microwave Ovens are generally very efficient, in the sense of using more energy to heat food than other methods.  Even inductive cooktops heat the metal container which then heats the food, whereas a microwave heats the food directly.  
However the Magnetron that generates the microwaves is not 100% efficient, I've seen figures of 65% efficiency, so 35% of the electricity is generating heat in the Magnetron and not heat in the food.
The good news though is that heat can be put into the oven chamber, so it that sense it is not wasted.
In any case, we have no real way of increasing the efficiency of the Magnetron itself, although there are ways to do it indirectly.

One of the major improvements is to simply use a less powerful microwave.
It will take longer to cook things, so the total energy will be approximately the same, but less power means consuming less watts from the battery.  This creates much less stress on the battery, or in some cases allows the oven to work at all.  For example 1000 watts is 80 amps at 12 volts, but 500 watts would draw only 40 amps.
The problem here is that the only low power microwaves I have seen are a very small size, which I find unappealing.  I want a full-size unit that is low power.  So it looks like this might need to be custom built.
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#2
To make a better microwave we must first understand how it works. They are surprising very simple.
The basic circuitry is just a few components. They key is the transformer, which has a secondary winding that produces 2000 volts from the line input. This 2000 volt AC power is then converted into 4000 volts DC using a capacitor and a diode. This works by using the diode to stop flow into the Magnetron on the positive wave, and instead the current charges the capacitor. The negative wave then has the transformer output and capacitor in series to double the voltage and send it into the Magnetron. The Magnetron has the property of high DC resistance until the input is 4,000 volts, so once the voltage exceeds that, the power flows and it generates the 2.45Ghz microwaves into the cooking chamber. So the Magnetron input is a pulsed DC that is OFF one-half the 60hz cycle, and ON the other half. The transformer limits the current by using magnetic shunts, but primarily simply by its physical size. So a high power microwave simply has a larger transformer than a lower power microwave.

Therefore, to convert a high power microwave into a low power microwave, we can simply swap out its big transformer with a small one. Smile
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#3
After all the free ones, I went out and actually bought a microwave, only a year old, made in 2012, for $15.
It is basically the same as the one built in 1985, and the small one we show here built in 1990.
It looks nice, isn't too big, and most importantly it is big enough to hold the Microwave 'Pressure' Cooker.

   

The smaller low-power one is just too small to fit this cooker, which is an important part of the all-electric galley setup, and we will look at it again, but for now we look at these two microwaves.
The small one doesn't have a 'rating', but it consumes around 7 amps, or 800 watts when operating.

   
   

The larger one is rated as a 900 watt oven, but it actually consume 1600 watts when operating.
So we are going to swap the transformer from the old small one into the larger one, and convert it into a full-size low-power microwave.  

   
   

But before that I tested how long it took to heat up a coffee cup full of water.
Rather than do a calibrated test, I just used the human digit test, put my finger in the water to see if it was too hot to leave it there.   The 900watt unit took 1 minute, the small unit took 1.5 minutes.  I'd expect 2 minutes, but was good at only 1.5.  I also used it to do a microwave popcorn, and set it for 5 minutes, as my big one takes 3.5 minutes, but it was done at 4 minutes.  So half the power doesn't mean twice the time!  So perhaps a scientific calibrated test is in order to confirm this non-intuitive result, maybe later. Smile

I wanted to make note about the wires in the microwaves. The small unit did not work when I got it (free) because the filament wire coming from the transformer was broken where it connected to the capacitor. It is a solid wire. When used in a home, it probably doesn't see too much shock and vibration, but on a boat, a solid wire like this is going to break sooner or later. So for boat use, these solid wires must be replaced by stranded wire.
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#4
So I pulled out the transformers from each microwave, and you can see how the higher power oven has
a bigger transformer.  I also noticed the capacitor is larger, .95uf vs .63uf in the low power.

   

I installed the smaller transformer and capacitor into the larger unit, and now we have a mid-sized low-power microwave.  Smile

   

Can we make more improvements?   I think so.  I have never understood why they have a light on inside the microwave while it is running.  So we can remove the bulb and call it good.  In one unit I disassembled, it had two 45 watt bulbs!   There is also the cooling fan, it is another 15 watts.  It could be replaced by a lower power more efficient fan, if one wants a fan at all.  It does put the heat of the Magnetron into the chamber and move the air around inside, so probably worth having.  Then there is the turntable that spins the food.  I'm not so sure these are needed.  With something heavy it might consume a good amount of power.  Older microwaves used a 'microwave mixer' that deflected the microwaves around, which is more efficient than moving the food around.  But these don't seem to be used in newer microwaves.
Some testing would be needed.  I did find a paper on redesigning the chamber from the square to a hex or rounded one, which distributes the waves more effectively.  But we might be going for the last few percentage improvements here.  Might just be happy to let the food rotate.  Wink

What might be next?  Looks to me that running directly on 12v and not requiring 120vac power might be a significant improvement.  A transformer at 60hz must be quite large.  Going with the smaller 800 watt transformer is itself a big weight reduction.  If you can operate at 16,000hz, the transformer size and weight can shrink significantly, and weight is always an issue on a multihull.  Smile
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