More on invertors, advice please.

[alanporter]

I am electrically and electronically challenged, I don't even understand the "Idiots Guide To .....", so I would very much value some expert advice.

I have bought a 1000 watt invertor just to operate a microwave oven while out at sea. The oven's specs are, power consumption 0.86 kW (8 amps, I believe). Output 450 watts. My house batteries are two series 27 deep cycle, 105 ah each. I intend to only use the oven with the engine running, 25 hp driving a 51 amp alternator.

Will this work or not ? Is it safe or dangerous ? I have connected the ground terminal on the invertor to the common ground point on the engine, and put a 100 amp fuse in the hot line from the batteries.

Do I need to do anything else, apart from attending evening classes on marine electrics ?

Fair winds to all.

 

[alanporter]

Oops, forgot to mention that my ac power is 115 volts on this side of the Pond, and that my batteries are 12 volt.

 

[bluedragon]

Alan, I'm sure lots of electrical engineers will answer you, but as I see it 0.86 kW is going to pull over 70 amp from your battery even assuming 100% conversion efficiency. You'll need big cables all the way and use it sparingly.

 

[sailorman]

i have kept mine totally seperate from the other boat circuits & installed 2 isolator switches ( one to each cable ) .
i would not wire-up to the engine in any way at all, if a cable goes down , will the engine still start / run ???.
i originally had a 1500 watt modified sine-wave to start with but the microwave wouldnt run now have a 1200w Pure sine-wave & its very good, yet to be used in anger but may run engine when its running any higher loads.
found that 80% power setting gives the system a little rest between loads comming on ( ie power on 80% of the time )
i have 330 Ah domestic batteries

 

[Anonymous]

Firstly we need to consider the type of inverter. A square wave inverter almost certainly won't work but a modified sine wave might. A pure sine wave inverter will work. The reason is that most microwaves use a voltage doubler to get the required voltage to drive the magnetron and, as you will probably remember, the peak value of a sine wave is 1.414 times the rms value. The peak value of a square wave is equal to the rms value, so a 230Vrms (or 120Vrms if you are over the pond) square wave isn't enough to generate the volts needed by the magnetron whereas a 230Vrms (120Vrms) sine wave is enough, being 325Vpeak (170Vpeak).

As for the questions about wiring it in and safety, I will pass - there are too many diverse opinions about how to wire (and how not to wire) an electric circuit in this forum and I don't want to generate an interminable thread and fall out with everyone /forums/images/graemlins/wink.gif Sorry, maybe someone else is braver than I. I will say that a 100A fuse might not be enough for surges - if you could get a thermal/magnetic breaker you would be better off and you could re-set it when it trips. Slow blow 100A fuses and fuse holders are quite expensive, anyway! It would be a sound investment.

Finally, make sure you only use it when the engine is running and don't use it for too long or you will not only run your battery down, you will harm it. Ten minutes at a time would be about right, or twenty if you are running it on half power - if the engine is running, of course!

 

[VicMallows]

A friend has been speccing a boat destined for charter purposes. He is adamant that it must have a microwave to keep the punters happy .... but definately not an invertor. Makes good sense to me!

Vic

 

[Anonymous]

I once hired a Broads cruiser that had a 230V ac supply run from a main engine-driven alternator. There was a company that specialised in selling them in kit form, so the hire company mechanic told me. You set the RPM to some specified figure - 1800 or so - and could run a couple of kW. It worked very well for the microwave, toaster and electric kettle (not all at the same time).

 

[alanporter]

Thank you for your answers everyone. I will have completed the installation by next weekend, so I'll let you know whether it works or not.
The problem one of you mentioned, that I didn't know about, was the type of invertor. It is a modified sine-wave, sooo I guess we'll wait and see.

 

[sailorman]

modified cost up tp £300
pure sine 2 > 3 times that

 

[TrueBlue]

More info:

I'm here, so I've got to reply.

Firstly, Lemain, I like your simple explanation; never though of it that way. Foolish me; I thought there was a "proper" power supply in them, but being a consumer item a centre tapped transformer with two fw diode bridges is a whole lot cheaper.

The reason for this post is that I had heard microwaves work OK, it's just you have to run them for longer - which accounts for Lemains post.

The second reason is that the latest Panasonic micros: have switch mode power supplies in them (Panny call them "Inverter" models) which reduces the weight - that's why I bought one for my home installation. I wanted to avoid getting a hernia every time I moved my 1975 Tappan unit - 60lbs: plus.

Anyway switch mode p/s are less susceptible to either input voltage or waveform fluctuations, so may well run OK on Quasi-sine wave inverters.

SWMBO won't have one on the boat so I'll never know.

 

[Shanta]

Re: More info:

How about a 12volt/24volt Microwave ??? Available from all good truck stops / shops. Saves all that mucking about with invertors and BIG cables... /forums/images/graemlins/cool.gif

 

[VicMallows]

Re: More info:

Sounds like for once SWMBO knows best. /forums/images/graemlins/smile.gif

Vic

 

[TrueBlue]

Re: More info:

Ah yes, but you don't get Owt for Nowt, you need about 800W for a micro to be any use, and what power is an internal inverter Big cables.....

 

[TrueBlue]

Re: More info:

Yes, inclined to agree (reluctantly), and She's a jolly good cook with conventional equipment....

 

[VicMallows]

He! He!. Did it come with a frequency meter as well. (always take mine with me, never know).

My first introduction to sailing was on a Broads boat with a dynastart, and young girls who insisted on their TV. The two don't mix!

Vic