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Old 08-08-2011, 19:14   #31
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Re: DC Generator - The Smart Way to Go

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Originally Posted by sailorchic34 View Post
One small note about the difference between dc generators and alternators. Back before the 1960's most cars had DC generators for charging. The problem with DC generators is they present the same load to the engine weather they produce max current or minimum. Going to an alternator allowed the alternator to unload or control shaft loading with the field current. So when the batteries are full the alternator goes to no load and reduces engine fuel consumption. Plus it was easier to get more amps from a smaller case size with an alternator...Just FYI
When I was 16, my 53 ford had a generator on it, but it DID have a regulator which changed the voltage... I know because that particular car kept eating generators and voltage regulators! Wouldnt that change the HP needed...?
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Old 08-08-2011, 22:19   #32
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Re: DC Generator - The Smart Way to Go

Well sort of. The old DC generators used three relay regulator to control voltage delivery to the battery. The field was always on while the engine was running but could be increased by one step, say 50% ish. So you always had a pretty good field load even if the battery was charged. Under higher load one relay would close to increase the field current which increased the output current. But it was a one step, pretty much clicking on and off all the time to control the current.


Where as on the alternator the field current was lower even at max amp delivery about 2 amps maybe and reduces to almost nothing when the system is charged.
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Old 09-08-2011, 10:56   #33
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Re: DC Generator - The Smart Way to Go

Ahh... I see. Yeah, I remember those 3 relays in there. The gens in this car kept going out.. inside the case you could see solder thrown around the perimeter. Never did figure out why. It was originally a 6 cyl but had a 312 T Bird engine when I got it. Maybe the 6V gen couldnt handle the rpm!
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Old 11-08-2011, 07:05   #34
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Re: DC Generator - The Smart Way to Go

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All this will give me 255 amps of charging power and 40 gallons of water with one hour of running.

I can't really understand why you would have an AC generator on a small sailboat. As small, I mean anything under 75 feet , bigger then that and you just leave the ac generator on all the time. But us, without deep pockets, have to conserve our energy and our diesel. So that means storing your energy in batteries .

So I ask, why use an AC generator to make power then change it to dc to charge your batteries, its nuts! And to just fire up a big genset to make toast? Thats nuts too. So that means you should have a very good inverter on board something like a Victron or a Mastervolt . Leave that on all the time for your AC power , good ones use nothing at idle, and draw all your power from your batteries. You should have at least 1000 ah of batteries so they can handle the high current charging. Get that power back in there as fast as you can so you don't have to run the engine for very long.
You obviously don't have air conditioning, or at least anything near the size it would take for a 75' boat. At most, you could generate sustained 2kva of AC power? (at a much higher up front cost than a normal AC genset)

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Hey I dont want to start and stop my generator for a piece of toast then in a hour for the hair dryer. Inconvenient. Hard on the engine, if you start it, run up to temperature and under a proper load.
That's what 1500w inverters are for, and they can be run off 500ah 12v battery systems.

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No your main engine has no " governor" You set the throttle and it stays there, add more load and it slows down , the governor is like an autopilot for your engine, keeps rpm the same regardless of load. Most generators have two settings hi and low , or 1500 and
1800 according to the load.
No, my main engine(s) governor works the same as my genset, spinning weights and springs. I agree if you are talking about gasoline engines.


BTW, be sure to carry spare brushes for your alternator (those disappeared years ago on AC gensets).
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Old 11-08-2011, 10:10   #35
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Re: DC Generator - The Smart Way to Go

I believe most gen sets run at 3600 rpm. Some are 1800. They have to to keep the voltage/cycles correct. That's my main complaint with powerboaters, I like powerboats.... just dont want to listen to that 3600 rpm gen set whining in a nice anchorage about all day and half the night....
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Old 12-08-2011, 18:03   #36
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Re: DC Generator - The Smart Way to Go

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I believe most gen sets run at 3600 rpm. Some are 1800. They have to to keep the voltage/cycles correct. That's my main complaint with powerboaters, I like powerboats.... just dont want to listen to that 3600 rpm gen set whining in a nice anchorage about all day and half the night....
Not quite - most diesel marine gensets (5kW and up) run at 1800 (60Hz) or 1500 (50Hz) rpm to save fuel and wear. Northern Lights, Westerbeke, Kohler, Beta Marine, Norpor - all run at 1500/1800 rpm, not 3000/3600. See http://soundmarinediesel.com/generators.html for some specs.
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Old 13-08-2011, 23:24   #37
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Polar Power makes them:
Polar Power Inc.- DC Generators, Battery Chargers, Solar Refrigeration, Photovoltaic Systems, Fans & Blowers, Metal fabrication
If you are in the scavenging mode, many big dc motors can be rewired into a dc generators.
Hi Newt,
I am newbie to DC motors, can you elaborate on the use of a DC motor as generator?
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Old 14-08-2011, 08:50   #38
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Re: DC Generator - The Smart Way to Go

Now I am no expert, but from what I understand most DC motors can be over drove and generate the same voltage as they are used to power with. Maybe someone else who is better than I am can elaborate. I have smaller brushless motor on my bike which go up the hill in 24 volts and brake down the other side, recharging my batteries. I find those only charge about 30-40% of what they used going up the hill, most of it is lost to friction, heat etc...
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Old 14-08-2011, 09:08   #39
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Re: DC Generator - The Smart Way to Go

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... DC motors can be over drove and generate the same voltage as they are used to power with ...
... I have smaller brushless motor on my bike which go up the hill in 24 volts and brake down the other side, recharging my batteries ...
Electric motors and electric generators (such as an alternator) are essentially two sides of the same technology. Both use magnetic fields and coiled wires, but in different configurations. Regenerative braking systems, like newtís bike, take advantage of this duality. Whenever a DC electric motor begins to reverse direction, it becomes an electric generator or dynamo.

Electric motors and generators
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Old 14-08-2011, 09:12   #40
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Re: DC Generator - The Smart Way to Go

Gord... dunno if “reverse direction” is what you meant... perhaps reverse “effort” would be more accurate?
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Old 01-09-2011, 08:52   #41
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Re: DC Generator - The Smart Way to Go

Just about to build the box around the unit, any suggestions on the best sound insulation ?

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Old 01-09-2011, 09:37   #42
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Re: DC Generator - The Smart Way to Go

It really boils down to what are the larger loads on your boat, AC or DC. With either generator you will have to convert a portion of it over to AC or DC, depending on what you buy. It is electrically simpler to convert AC to DC than the opposite.

I would not have a DC generator on a larger vessel since almost all of the larger loads are AC. DC is also less efficient over distance. It also requires larger gauge wire for the amount of power carried.
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Old 01-09-2011, 09:54   #43
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Re: DC Generator - The Smart Way to Go

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It really boils down to what are the larger loads on your boat? AC or DC? With either generator you will have to convert a portion of it over to AC or DC, depending on what you buy. It is electrically simpler to convert AC to DC than the opposite.

I would not have a DC generator on a larger vessel since almost all of the larger loads are AC. DC is also less efficient over distance requiring larger gauge wire for the amount of power carried.
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Old 01-09-2011, 10:11   #44
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Re: DC Generator - The Smart Way to Go

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Not quite - most diesel marine gensets (5kW and up) run at 1800 (60Hz) or 1500 (50Hz) rpm to save fuel and wear. Northern Lights, Westerbeke, Kohler, Beta Marine, Norpor - all run at 1500/1800 rpm, not 3000/3600. See Kubota Diesel Generators ,kubota diesel generator,isuzu diesel generator, yanamr diesel gensets for some specs.
Genset operating speed is determined by the electrical head being either 2-pole or 4-pole. A 2-pole head needs to be operated at higher rpm to provide the correct frequency. Many of them use pulley ratios to bring the engine rpm lower than 3000/3600. 2-pole heads are lighter and somewhat smaller, but produce dirtier power. They are found mostly in generators up to 6-8kw. Larger generators use 4-pole electrical heads and run at the slower engine speeds, but are physically larger and heavier.

For comparison:

Northern Lights 6kw - 1800rpm, 362 lbs, 27"x17"x20"
NextGen 5.5kw - 2800rpm, 230 lbs, 23"x17"x20"

Northern Lights 8kw - 1800rpm, 529 lbs, 33"x19"x23"
NextGen 8kw - 2800rpm, 350 lbs, 23"x19"x20"

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Old 01-09-2011, 10:17   #45
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Re: DC Generator - The Smart Way to Go

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It really boils down to what are the larger loads on your boat, AC or DC. With either generator you will have to convert a portion of it over to AC or DC, depending on what you buy. It is electrically simpler to convert AC to DC than the opposite.

I would not have a DC generator on a larger vessel since almost all of the larger loads are AC. DC is also less efficient over distance. It also requires larger gauge wire for the amount of power carried.
A friend of mine put in a DC generator run by a tiny Kubota diesel, and you couldn't hear it beyond the walls of the engine room. All he wanted to be able to do was charge his batteries at anchor. Running a large-gauge wire from the generator to the house bank was a non-issue, because it was only a run of a few feet.

It seems that the dividing line is whether you want to run air conditioning. If you don't, a DC generator makes a lot of sense.
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