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Old 19-07-2013, 09:35   #1
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Moving towards an AC boat

Just curious, why do you reckon with the advent of economical inverters also backup alternators / generators, that Alternating Current (AC) has not replaced most circuits on a boat, you can have domestic equipment such as refrigerators, AC motors to drive your Water maker, pumps, fans, Autopilot, Anchor windlass. Think about your Power losses (amp*amp*Volt) and saving in cable sizes.
You still need your House Bank, and for redundancy get another inverter.
Of course there are losses in inverter but there is a gain in more efficient AC machines and circuits. As regard safety and unwanted circulating currents we solved that problem years ago shore side. And best of all AC motors are more reliable, less maintenance, longer lived, cheaper, smaller in size & light weight.
I've not seen it but there is no reason why a inverter can't turn itself on, on a “as needs” basis, maybe the start circuit could switch its or group inverter “on” at the same time to save standby power. It'll be nice if we could have 3 ph inverter if a circuit needed it for high start torque, maybe multispeed (Pole changing), slow for hauling an anchor in fast for letting out, no brushes to worry about.

This is meant as a discussion only, I think some of you have already gone part way there I’ve seen a Water maker with both AC and DC drive. Suggestions of what is possible/limitations encouraged.

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Old 19-07-2013, 10:06   #2
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Re: Moving towards a AC boat.

too many efficiency losses

My inverter in standby mode doing nothing uses as much energy per day as my refrigerator
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Old 19-07-2013, 10:32   #3
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Re: Moving towards a AC boat.

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Originally Posted by Don L View Post
too many efficiency losses

My inverter in standby mode doing nothing uses as much energy per day as my refrigerator
Well gee Don. Either you have a really great refrigerator or a really crappy inverter.

Seriously, my Victron inverter has a search mode that is pretty efficient. It draws just enough power to run a smart circuit that senses a load and then turns on the inverter. Uses about 250 mA or 6 AH/day. Bet your refrigerator can't run on that.
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Old 19-07-2013, 10:55   #4
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Re: Moving towards a AC boat.

And back to the OP's question about AC vs DC gear on a boat. A lot of AC equipment looks pretty good on power consumption until you start doing the math.

For example, say you have a refrigerator that draws an average of 1 Amp at 125 VAC. if you are running the boat off 12V batteries to get that 1 Amp AC from a 12 VDS battery, ignoring all conversion losses, power factors, etc you will need just over 10 Amps from the battery. Figure in all the losses and maybe 12-13 Amps. All of a sudden AC doesn't look as good.

Not to say that it can't be done and with careful choice of AC equipment and the inverters it can be practical (ask sailorchic34 about her AC refer) but the use of AC gear isn't a slam dunk, no brainer change.
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Old 19-07-2013, 11:04   #5
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Re: Moving towards a AC boat.

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Well gee Don. Either you have a really great refrigerator or a really crappy inverter.
Must be the inverter because the frig is only using around 60 AH/day.

But you are just being jealious because my frig makes cold beer and yours is doing .......................... what except holding up the counter top sitting in the woods?
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Old 19-07-2013, 11:09   #6
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Re: Moving towards a AC boat.

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Originally Posted by Don L View Post
Must be the inverter because the frig is only using around 60 AH/day.

But you are just being jealious because my frig makes cold beer and yours is doing .......................... what except holding up the counter top sitting in the woods?
Well since I haven't finished redoing the counter top my refer isn't even doing that.

But I am getting yeoman service out of my inverter to run power tools.

AND, I will be placing the inaugural six-pack in the box this weekend. Come on down and I'll give you a cold one.
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Old 19-07-2013, 17:55   #7
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Quote:
Originally Posted by Oceanride007 View Post
Just curious, why do you reckon with the advent of economical inverters also backup alternators / generators, that Alternating Current (AC) has not replaced most circuits on a boat, you can have domestic equipment such as refrigerators, AC motors to drive your Water maker, pumps, fans, Autopilot, Anchor windlass. Think about your Power losses (amp*amp*Volt) and saving in cable sizes.
You still need your House Bank, and for redundancy get another inverter.
Of course there are losses in inverter but there is a gain in more efficient AC machines and circuits. As regard safety and unwanted circulating currents we solved that problem years ago shore side. And best of all AC motors are more reliable, less maintenance, longer lived, cheaper, smaller in size & light weight.
I've not seen it but there is no reason why a inverter can't turn itself on, on a “as needs” basis, maybe the start circuit could switch its or group inverter “on” at the same time to save standby power. It'll be nice if we could have 3 ph inverter if a circuit needed it for high start torque, maybe multispeed (Pole changing), slow for hauling an anchor in fast for letting out, no brushes to worry about.

This is meant as a discussion only, I think some of you have already gone part way there I’ve seen a Water maker with both AC and DC drive. Suggestions of what is possible/limitations encouraged.

Oceanrider
I'm boatless and I like DC.
Firstly DC permanent magnet motors require very simple control equipment , whereas many AC motors especially where reversing and speed control are needed require quite complex systems.
secondly AC efficiencies tend to occur at higher voltages , which have safety connotations aboard.
Savings in cable sizes only get generated at higher operating voltages. The same benefit accrues to higher DC voltage operation too.

AC is used extensively on large ships where the power transmission requirements make sense.

For a small boat the situation makes no sense.


By the way power losses are I squared R ! The reduction in losses at higher voltage is due to lower currents for the same power.

Dave
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Old 20-07-2013, 00:45   #8
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Re: Moving towards a AC boat.

Have had one 240V, 3000W inverter burst into flames in the dark of night. Didnt enjoy the experience much - so personally unless on a very large vessel which feels more like an appartment block than a boat, I far prefer the idea of 12v over 240V with water around.
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Old 20-07-2013, 03:29   #9
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Re: Moving towards a AC boat.

To goboatingnow, sorry made a mistake with my formula Power loss = I*I*R, its news to me that permanent magnet motors are all that common on boats, so are you saying that big ones like Anchor, powered self tailing winch and Autopilot have these kinds of PM motors. That'll be great, some of my dislike for DC comes from brush / commutator problems.

Re your comment about switch gear I believe your comment does not refer to 1ph there is nothing complex about starting a fan or refridgerator at home.

I wouldn't be tempted to go high voltage DC, 12V is enough for storage, 24V for windlass maybe, but high voltage that news, but unlikely to get anywhere with me. With a efficient inverter, I'd reckon the losses from DC will exceed that of AC and therefore I think that some redundant circuits could be run in AC now, for example no reason why a refrigerator could not have two cooling circuits one from DC, one from AC. Who knows may be the AC circuit would be preferred at sea.

I don't know cost well enough other than a 24Volt motor I'm contemplating for Autopilot costs a couple of thousand and the control gear for it another 1.5 Thou, I bet the equivalent AC motor is a couple hundred. Its only I feel that the Marine Market has this momentum towards DC so there is no AC unit to compare, DC is unique and pricey.

nmit5903 sorry about your tribulation but I don't trust Switch mode power supplies or transformers at home either. DC can make a bit of a mess, next door boat had a catastrophic fire from 12V and they didn't do anything silly, but they lost all power, AMSA got a call, (On the epirb) it was the only thing still working, that from a Million dollar boat.

I believe all important circuits should have redundancy, and you don’t jump in with both feet, there is a possibility here that some AC circuits could prove more bullet proof than DC. Really I don’t know small vessels that well, but I do come from big ships Icebreakers, Superyachts and more humble stuff. Really a lot can be done to improve redundancy and life on a small boat and make it more economical and I am encouraging those who have had the opportunity to suggest where AC has worked then please lets hear it.

Oceanrider
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Old 20-07-2013, 06:06   #10
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Re: Moving towards a AC boat.

There are many boats using 240AC household equipment & many with electric motors for propulsion. The higher voltage will result in lighter wiring but you need to cost it all out. 3000w inverter normally refers to peak & not continious. 800w is more like what is usable. Standby current sensing is normal in good inverters. (can recommend one made locally).This is why you have to do your homework. If you are doing a new boat, then good. A little of both.

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Old 20-07-2013, 07:05   #11
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Re: Moving towards a AC boat.

I'm slowly growing accustomed to having continuous AC (from a 2000W inverter) on board. I decided not to replace the apartment-size fridge with a proper marine AC/DC unit (at $1500+) just yet. I got a lot of work done one day at anchor, running power tools and vacuuming up the sawdust. I really like not having to reset the clock on the microwave when we reconnect shore power. And just being able to use the microwave on a whim without having to start the generator is great.

That said, I'm a little reluctant to get TOO dependent on AC. If the genset dies, I'd have to run the mains a lot to keep the house bank charged. Plus the inverter becomes a single point of failure, which seems pretty likely reading this forum. I'll probably bring my Honda 2000i on longer trips, and have even thought of getting a backup inverter. All issues I wouldn't have to deal with if I went with 12VDC for everything.
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Old 20-07-2013, 07:30   #12
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Re: Moving towards a AC boat.

Because with DC you can have more than one source of DC feeding into a common DC system. With AC if you want to do this you must match the phase. Also, storage of electrical energy is most conveniently done with batteries, which of course are DC.
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Old 20-07-2013, 07:35   #13
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Re: Moving towards a AC boat.

Traditionally AC appliances have been inefficient. This is not so so much because of any inherent ineffiencey in AC but rather because there was little point for manufacturers to produce efficient appliances when the cost of electricity was so cheap and the consumer was indifferent to the running costs.

Recently that has changed with domestic electricity charges rising sharply in many countries and a consumer push for more environmentally friendly products.
We are now starting to see efficient AC appliances like fridges that rival the DC equivalent.
These are worth considering for boat use.

The old adage of trying to run everything possible directly from DC still is not a bad rule of thumb, however.

The only appliances that need AC on my boat are rarely run such as printer, a few tools and stick mixer for the kitchen. The inverter is only used a couple of times each month.
The DC alternative is more efficient, more reliable and safer.
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Old 20-07-2013, 07:42   #14
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Re: Moving towards a AC boat.

Quote:
Originally Posted by David M View Post
Because with DC you can have more than one source of DC feeding into a common DC system.
Which is well developed technology and technically not too difficult. You can get your DC from: alternator, battery charger (of course needing AC to run it), solar, wind but all running to the common point of the batteries. But redundancy there as well if you have a multi battery bank. One dies just take it out of the loop and run on the rest until you can replace it.


Quote:
Originally Posted by David M View Post
With AC if you want to do this you must match the phase.
Which is pretty easy these days (I said easy but not cheap ). My Victron inverter/charger transparently does the phase matching and works just like adding another battery to a DC system. Wire it so all the AC goes through the Victron. Set the Victron at the max amps of the AC supply (shore power or genset) and if the demand onboard exceeds that limit the Victron automatically kicks in to supply the extra. The only thing I notice is the click of a relay. I love it.
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Old 20-07-2013, 08:23   #15
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Re: Moving towards a AC boat.

By AC, we're meaning common distributed AC (100 to 240 v, 50 or 60 Hz). These parameters were established to provide a reasonably economical distributed power system. On land. But there's nothing particularly useful about those voltages or frequencies for use on a recreational boat.

Part of the attraction of AC is... there's tons of low-cost devices and appliances available that run on AC. Like refrigerators, microwaves, and that other "AC" - air-conditioning. The cost-saving is attractive, but don't forget that most AC appliances aren't built for the rigours and corrosion from life at sea.

Just about all electronic systems run on DC. So in every piece of AC-powered electronics is a power-supply converting the AC to DC. So there's really no argument for powering marine electronics from AC, when DC is readily available.

AC and DC electrical machinery bored me at school, so I can't argue the relative merits with high confidence, but I don't believe that AC motors offer much improvement and cost-reduction for intermittent, variable-speed, high variable load applications like windlasses, winches, thrusters. I also know that the most efficient AC drive applications rely on precise control of voltage, current AND frequency, so a fixed-voltage, fixed-frequency AC supply isn't going to be an improvement over DC supply.

The lowly DC computer fan is a good example - each fan contains a complex control circuit which supplies multi-phase AC to a brushless PM motor, with speed feedback from a Hall effect sensor.

Lighting - there's zero net benefit to AC supply for lighting, especially when efficient LED devices are used.

So... I don't believe there's a strong case for preferring terrestrial AC standards over DC for general use in a recreational boat. There may be niches and a convenience factor to having AC (and we all have it to one level or another), but in the majority of boat applications terrestrial AC offers no advantages over DC, especially when it puts extra reliance on inverters. Where an electrical machine can be lower-cost and more reliable with AC drive, the best efficiency comes from a dedicated variable frequency drive paired with the motor, and itself powered by DC.

Also... haven't yet seen an AC battery.
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