microwave and invertors

[chrisarvor]

i know you experts get fed up with this.
i want to put a microwave on the boat not ot cook full meals but just to use to re-heat food. so couple of 3 minute bursts a day.
i have room to put a 12v battery on its own just for this and to put it on charge back at the marina at night.
what size battery?
what size invertor ?
the microwave wil only be the small 650w model.
thanks
is this set up ok

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[Johnboy2004]

i was just thinking of this too.... what is the smallest microwave available?

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[pvb]

In theory...

In theory, it's just about feasible, but the life of your battery might be short. The quoted power ratings of microwave ovens represent the cooking power - the actual power consumption is much higher. A 650watt microwave would need something like a 1200 or 1500watt inverter to operate properly. When the microwave is on, the inverter could be drawing around 100amps, so you'll need a battery capable of supplying that current, which really means a cranking battery rather than a deep cycle battery. If you do only use the microwave for 6 minutes a day, you'd remove around 10Ah, so providing the battery is fully charged to start with, you'd probably be OK. In terms of battery size - at least 110Ah or, if there's room, the biggest you can fit.

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[steve_cronin]

Caravan shops sell low consumption ones.

Choose new rather than old though. I am looking around for one myself.

Today the best I could find was a 700watt cooking power unit with a turret timer - some inverters don't like microprocessors (or V.V.) which only consumed 1100 watts at 240V. In comparison, my stepmother has just thrown out an old monster of a machine which only cooked with 600watts but consumed 1400watts.
AS to choice of inverters, my new 1500 watt continuously rated unit will output 2200 watts for startup. It runs off of 2 X 140 Ah batteries but I may increase this capacity sometime during the season if the M/wave can't deliver hot baked beans in a rammekin dish after 60 secs at full power.

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[davidwf]

What about buying a 12V one Samsung do them but they do draw around 65 amps when running see

<A target="_blank" HREF=http://www.ripples.co.uk/12v.html>http://www.ripples.co.uk/12v.html</A>

they are about £199


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[petery]

I understand that if you don't use a true sine wave inverter, that a microwave will not work at full efficiency. Lot's of differing views on this - though.

True sine wave inverters are still expensive - so I think the Samsung 12v microwave sounds the best deal unless you want to run other mains equipemnt on the boat as well.

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[Gypsy]

I have a Samsung 12v onboard. It is only 550w but for the quick warm-up of soup and vegies etc it is great. I use a 95Ah "marine" grade battery with charge from a diode splitter from the alternator and also a separate output from my shorepower battery charger.

Be sure to mount the battery as close as possible and use thick cables. I think Samsung supply 25mm2 cables for up to 1m run but any longer and you need to go to 40-50mm2 to avoid the voltage drop with 65A flowing. It doesn't take much to drop 1v at those currents.

I went through the same questions about 220v with inverter from the battery. The big question is the inrush/startup current which I would imagine varies for each uW model as will the capacity of the inverter to deal with such currents. In the end I figured the low power and internal inverter of the Samsung was a better bet and at the time was still cheaper than the 220v uW and 1800-2400w inverter. Another alternative is to ask the inverter supplier if they will let you test a typical uW on their proposed inverter - and not just with a fully charged battery!

I'd love to hear from anyone who has actually done these tests with uW and inverters so that we could all scope the problem better than just guesses based on fixed operating power specs.

It's a "jungle" out there in DC land!


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[William_H]

Looking at the Samsung 12 volt microwave I reckon that would be the go. A domestic microwave oven takes 240 volt AC into a transformer up to 1000 volts AC rectifies it to DC to feed the magnetron. This process in itself may only be 85% efficient. To feed this with an inverter itself about 85 % efficient leaves you wasting a lot of power. A 12 volt microwave would convert DC directly to 1000 volt DC at a typical efficiency around 90% So on efficiency alone you must be way ahead. Then modern domestic microwaves use an AC iduction motor for cooling another for platter rotation and either electronic clock or another induction motor for timer switch drive. All these motors though low power can do strange things on any but sinewave AC and certainly will waste you hard gained 250 volt AC from your inverter. Go for the Samsung on thory at least. regards will.

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[charles_reed]

Your proposal is

probably not feasible as it stands, due to the loss in efficiency: however there are a number of 12v microwaves now on the market, which might be a route to your requirement as well as being far cheaper than buying an invertor and microwave.

Have a look at one of them and find out the current draw, my guess is it will be about 50 amps.
This means you'll need a fairly large battery (about 65 ah) and very large cabling to carry the current.

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[steve_cronin]

Re: Your proposal is

Can't agree on costs charles.

Just returned from our local caravan shop where they have a microwave for £59.99 which consumes 900watts to give a cooking power of 650watts. (Maker JLS I think)

Now if you buy one of these combination units you have a dirty great lump to bolt in and accomodate and they are running at over £350. OK there is the Roadmate at around £220 but that is a very low power unit intended for re-heating pasties in trucks. Whereas a suitable invertor might only set you back £200 or less. Also when the combi packs up - and it'll be the magnetron - you have to replace the whole thing whereas with separate units you only need a new microwave. Then of course the inverter can be used for lots of other purposes like SWMBO's hairdryer!

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[andyball]

Seems OK setup, assuming microwave behaves with cheapish inverter. Battery type depends on space available + tolerable weight imo. If you've space & can stand the weight a b-i-g truck/leisure battery will do nicely; smaller space/weight needs more upmarket battery, I'd say.

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[charles_reed]

Thanks for your confirmation

£60 against £200 for the invertor alone?

230v magnetrons need clean power - you need a genuine sine-wave invertor which you might pick up for as little as you indicate. Moreover, unlike a radar magnetron which warms up gradually, the microwave magnetrons have a start-up power surge of probably double the rated power - that means an even bigger and more expensive invertor.

My experience is that even small invertors only work properly when you've got the alternator running to keep volts up, the squeal for low-volts is quite deafening.

Finally the word was "not feasible" not "impossible" - any engineering problem has an answer if you throw enough money at it

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