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martinsmit
martinsmit

Posted: Fri Jul 17, 2009 8:10am

Post Subject: inverter

hi tony i,ve just installed a sterling 1800 inverter for the 230v microwave and anything else we use at 230, the microwave is rated at 800 watt. when we have the engine running and come to use the microwave the engine revs drop down and the microwave seems to sound differant and dosent appear to heat things up as it should, so we have to heat the food again until hot. is this just due to the power being used or is there something else wrong. to counter the engine revs dropping i just rev the engine a bit more before turning the microwave on. thanks again for any help martin

Tony-B
Tony-B

Posted: Fri Jul 17, 2009 1:32pm

Post Subject: inverter

Dear Martin, First of all your 800 watts is the cooking power not the power required to work the microwave. I suspect it requires far closer to 1600 watts. The next thing is that microwaves can be touchy about the waveform they receive. Have you saved a few quid by purchasing a "modified sine wave" inverter? If so I expect thats the problem if it works fine on a shoreline or at home. I am not surprised the engine note drops. 1600 watts equates to about 160 amp draw so the alternator will go to maximum output. You also need to ensure the cables between battery and inverter are large enough. They need to be capable of carrying (say) 200 amps with less than 0.5 volt of voltdrop. Tony Brooks

martinsmit
martinsmit

Posted: Fri Jul 17, 2009 9:57pm

Post Subject: inverter

hi tony thanks for the info. the inverter is a quasi cine wave and as for the cables these were already fitted to the inverter but i have fitted one of the leads through a standard battery isolation switch and then through a 300 amp fuse which was recommend by the chandlers then the cable size from the fuse to the battery terminals is the same size as the battery leads, which happens to be bigger than the inverter leads. the other side of the inverter is then connect to a 25 amp rcd which in turn has a 10 amp mcb for the 230v ring main well its not actual a ring main it just goes straight out through four sockets. could you confirm if everything is ok thanks again, martin

Tony-B
Tony-B

Posted: Sat Jul 18, 2009 8:00am

Post Subject: inverter

Dear Martin, I can not confirm that the 12v part of the system is OK because I have no cable run lengths or the conductor area of each cable. The voltage lost in the cable is proportional to the cable length but from what you say it sounds as if its OK. I also will not confirm that you 240v AC is OK because I am not qualified to, but again I can see little wrong with it. I think you have one major problem and one one potential one. First check the plate on the back of the microwave to find its actual demand. If its more than about 7 amps or 1.8 Kw the inverter is too small. However modified sine wave inverters often have a significant overload capacity for several minutes so I suspect the whole thing is down to the modified sine wave output. I fear you either need to forget the microwave or buy a pure sine inverter. Personally I would get a larger one than 1800 watts because generally they have less overload capacity for less time. Tony Brooks

martinsmit
martinsmit

Posted: Mon Aug 10, 2009 6:20pm

Post Subject: inverter

hi tony with regards to the power consumption of the microwave, you asked about. i checked this out and on the rear of the microwave it says 1100 watts what i would like to confirm is that when we use the microwave and the alternator goes to max output, hence the engine note dropping, would this be causing any damage to the alternator? thanks again martin

Tony-B
Tony-B

Posted: Mon Aug 10, 2009 9:57pm

Post Subject: inverter

Dear Martin, Just to recap, I think the lack of heating is down to the modified sine wave. The engine changing note and perhaps dropping the revs is down the alternator going to full output but I would have expected the engine governor to pull the revs back up, however simpler ones may not. The question about is it damaging the alternator is harder to answer. If I just stuck to theory its easy - no it will not do any damage. This is because a well designed alternator with the proper cooling fan and supplied with adequate cool air is self protecting and will never deliver more output than it's designed to. However in boats the cool air may not be all it could be and I know some alternators are not as mechanically robust as they should be. Some mariniser impose warranty conditions that shout "we fit poor alternators" so although it should be fine there will always be that little bit of doubt. I assume the load is only for a few minutes so it should be OK. Tony Brooks

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