How common are DC Generators?

Bowedtoothdoc · #1 (**permalink**) 10-01-2011

I came across a boat on yachtworld that I am very interested in. After closely looking at the specs I discovered that the boat has a DC generator and I am pretty sure it came that way from the factory. Being in the south I am not sure what the uses of a DC generator would be other than charging the battery's. The boat also has a 2500 watt inverter that I would imagine could run allot of AC appliances short of AC. How common are DC generators and why? Also the boat does not have AC, can an 5000 watt inverter run AC or would I need to get a new genset?

Zanshin · #2 (**permalink**) 10-01-2011

DC generators are not yet common. The main advantage of the DC generator is that the diesel engine can run at more efficient and different speeds than an AC generator. The AC models need to run with an RPM that is divisible by 60 or 50 in order to generate 60Hz or 50Hz for alternating current (typically 1500/3000RPM or 1600/3200RPM for slow or fast gensets and for 50Hz and 60Hz. With DC the generator doesen't need to run at any specific speed and thus can run more efficiently.
With progress in battery technology, specifically Lithium, the weight of batteries has gone down while the capacity and in particular the charging amperages have gone up and soon DC generators might come into their own; Fischer Panda has a hybrid ship which uses the DC genset for propulsion as well as for supplying the onboard electrical system.
Several months ago when I was deciding which sailboat to get, I mapped out a similar hybrid electrical propulsion using a DC generator ans would have opted for such a system had lithium batteries not had a price tag per KwH that was 10x higher than for lead-acid.

How many Kilowatts does your DC genset put out? It should be sufficient to run an air conditioning unit through the battery bank (used as a storage buffer) and a large inverter. Remember that the manufacturer ratings are optimistic at best and that inverter performance degrades quickly with higher temperatures and a 5000W inverter might lose 15%-20% and thus only deliver just over 4000W.

erps · #3 (**permalink**) 10-01-2011

Quote:

I discovered that the boat has a DC generator and I am pretty sure it came that way from the factory.

Are you talking about a separate stand alone generator, or the belt driven generator on the ship's engine? Old cars and tractors had DC generators on them and they were replaced with alternators, which got more power at lower RPM.s

Quote:

Also the boat does not have AC, can an 5000 watt inverter run AC or would I need to get a new genset?

I can't imagine a battery bank large enough to be able to power a 5K inverter that would run AC. 5000 watts requires around 400 amps of battery power.

mitiempo · #4 (**permalink**) 10-01-2011

Bowed

Can you be more specific? Do you have a link?

Bowedtoothdoc · #5 (**permalink**) 10-01-2011

It's the Freedom 45 listed here: http://www.yachtworld.com/core/listi...g_id=1712&url=
Apparently has a separate 12V Balmar generator? I think they meant a 12 volt Balmar Alternator.

mitiempo · #6 (**permalink**) 10-01-2011

It may be the Ample Power Genie shown here Genie Diesel Alternators

Basically a small diesel with a large alternator as it is more efficient than running the larger main engine at a low load to charge batteries.

VetMike · #7 (**permalink**) 10-01-2011

Several months ago I was thinking about building a diesel generator using an old diesel engine and a generator but couldn't find a small diesel motor. I then found an article in either This Old Boat or a similar magazine with an article about building a diesel genset. I'll try to find the article and post it.

Zanshin · #8 (**permalink**) 10-02-2011

Quote:

Originally Posted by erps

...I can't imagine a battery bank large enough to be able to power a 5K inverter that would run AC. 5000 watts requires around 400 amps of battery power.

Well, I have 1200Ah @ 12V (600Ah @ 24V) aboard my new boat and a Victron 5000W inverter which could run AC, were I of a mind to do so. But the original poster didn't want to run the AC off the stored energy in the batteries, just charge the batteries from the DC genset while running the AC, in which case the capacity of the battery bank isn't that important.

Bowedtoothdoc · #9 (**permalink**) 10-02-2011

I think the listing has a misprint, and they meant that the yacht has an additional 12v Balmar alternator.

mitiempo · #10 (**permalink**) 10-02-2011

May be a misprint but DC generators are quite common and the most efficient way to charge the batteries. Beats running the main engine or having a big generator for AC. A DC generator and an inverter, which you will probably have anyway, is a lighter, lower maintenance solution. On a larger boat, say above 40', I would want one.

I have seen several, one on a 35' catamaran and one on a Saga 43 for example.