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Old 03-06-2017, 02:27 PM   #8
twistedtree
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City: Vermont
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Charging batteries, both a DC generator and a conventional AC generator incur the same losses. KChace mentioned much of this.

With a DC generator, it's actually (typically) a 3 phase alternator being driven to generate 3-phase AC power. The AC is then rectified to create DC, the voltage is regulated, and it charges your batteries or powers DC loads. Any AC loads get powered through an inverter at approximately 90% efficiency, so additional 10% loss.

With an AC generator, you get single or split phase AC directly from the generator end with no rectifiers. That power can be used for AC loads with no loss penalty. To charge your batteries or power DC loads, the AC gets rectified and regulate just like in a DC generator, just by an external charger. But the losses are comparable.

So the bottom line is that when you look at conversion losses, generating DC power is the same from a DC or AC generator. Yet generating AC power incurs less loss when generated from an AC generator vs a DC generator.

But there is another factor at play which is the loading on the motor that is driving the generator. If lightly loaded - say 25-30% or less, the efficiency pretty much goes to hell because the diesel is operating inefficiently. But I would presume that you would equip a boat with roughly the same capacity generator whether AC or DC, so also presume both diesels would be similarly loaded.

You might find this article interesting

Adventures of Tanglewood: Engine Alternator or Generator: Which is More Efficient?

It's looking at a similar, but slightly different question. Regardless, many of the same principals and considerations apply equally to what you are considering.

There are also some equipment considerations. With an AC generator, you need a battery charger to get DC for battery charging. But you arguably need that same charger anyway for charging off shore power. So that's a wash.

With a DC generator you probably need an inverter for whatever AC appliances you want to run. I would personally have an inverter under any circumstance, but if you have an AC generator you could possibly get away without having an inverter.

So assuming you are going to have a battery charger and an inverter no matter what, the cost difference comes down to the difference in cost between say a 5kw DC gen vs the same size AC gen. If you are comparing different capacity machines then the savings comes from a smaller machine, not from DC vs AC. And beware of some of the low cost generators. Lots work fine, but if you read around you will also find a lot of nightmares. And consider serviceability, especially if you will be in remote areas.
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