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Generator question


trebby91's Avatar
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09-22-12, 03:33 AM   #1  
Generator question

Hi, All.
New in here!
I live on a boat and need emergency battery charging.
I have seen a new small diesel engine for sale 3HP would this drive a standard car alternator rated at 80 amps or higher?
The engine revs at 3000 would I be able to go 1 to 1 with the pulleys or does it need a reduction?
My battery bank is 550 AH.
Hope somebody can help.
Regards. Brian.

 
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09-22-12, 04:13 AM   #2  
Hello Brian,


Technically a alternator is designed to maintain battery voltage, Not re-charge low/dead batteries, They will do it but it's harder on them and takes a while.

Automotive crank pulleys can be anywhere from 5" to 8" diameter therefore spinning the alternator much faster at 3000 RPM, I would suggest starting with a crank pulley at-least twice the diameter of the alternator pulley. Then adjust engine RPM to get a alternator output of 14.3 to 14.7 volts.

If this voltage range can't be reached at the 3000 RPM of the engine, A larger crank pulley will be needed.


Good Luck

 
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09-22-12, 04:33 AM   #3  
Yes I agree, a modern vehicle alternator can be damaged by charging a battery when it is nearly depleted.
What about purchasing a 2000 watt generator and use a 15 to 20 amp battery charger?


GregH.........HVAC/R Tech

 
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09-22-12, 05:07 AM   #4  
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Hi,
I have solar panels with an output of 1000 watt or around 50 amps (on a good day) so the batteries never fall below 12 volts.
At present I have to run the engine to top up the battery bank the total output of both engine alternators (normal car alternators) is 197 amps.
a 20000 Watt is far too heavy to carry on-board.
The diesel engine I have been offered only weighs 18 kilos and with an alternator attached should reach about 25 kilos, a standard 2000 watt diesel generator weighs around 50 kilos and is very noisy, no contest!
All I really wanted to know is would a 3 HP. diesel engine run an alternator, what speed the alternator needs to run and what is the highest amp alternator would the engine run.
The idea is to run the generator instead of the main engine to top up the batteries and save on main engine hours when not cruising.
Regards, Brian.

 
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09-22-12, 10:09 AM   #5  
Sounds like it would work okay, but there is no set formula to know what size alternator a 3hp diesel will run. There are far too many unknown variables. The load on the alternator also is unknown. A 3hp engine could run the highest amp alternator if the battery bank needed little to no charge, but if they were low enough to consume some amperage, the engine could stall. You should be fine with a 3hp engine and a standard self-exciting delco 85amp alternator. I don't know if you specifically want the diesel, but you could probably just buy a cheap push mower and use that engine.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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09-22-12, 12:43 PM   #6  
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Hi,
Thanks for the information.
Would it be ok to use a 1 to 1 pully set up?
Regards, Brian.

 
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09-22-12, 12:46 PM   #7  
forgot.

I want a diesel as I do not like petrol aboard,
Regards Brian

 
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09-22-12, 01:17 PM   #8  
Brian,

You can try a 1 to 1 pulley, But it's likely not going to work and likely burn the alternator up. Figure your truck/car with a 6" crank pulley and your alternator pulley at 2.5"......Travel down the highway at 3000 RPM (approximately 65 MPH) and think how fast your alternator is spinning.

1 to 1 at 3000 will be way low to produce enough power out-put, Especially for trying to charge/top off a "bank" of batteries.

I agree with Mr Cheese on a one wire Delco alternator but you have to get it wound up......

Good Luck

 
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09-22-12, 01:32 PM   #9  
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Hi,
Sorry, call me thick!
If the generator is running at 3000 RPM what should the alternator be running at if I wanted to use the full output of 85 amps?
Would a 4HP engine be better?
Regards, Brian.

 
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09-22-12, 03:07 PM   #10  
Well Brian,

The first part of my now 35 years were in the automotive business, But I never had a reason to find alternator RPM's at any given engine RPM. As much as I hate admitting, I'm better at spinning wrenches than numbers.

As long as you stay close to automotive gearing between the engine and alternator it should put out just like it does on a automobile, Just because a alternator is rated at 85 amps doesn't mean that's what it puts out continuously. Alternator out-put is dependent upon how much load is applied to it via the voltage regulator, More load the more amps. It all depends on how low a battery is or how much draw is being taking out of the battery.

You do have some what of a advantage here wanting to run a diesel engine, Most have a longer stroke and more torque than a gas engine.


What kind of batteries are these and how many are in this bank ?


Again, Gearing between the engine and alternator is what's going to make this work if it's going to......

 
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09-22-12, 04:51 PM   #11  
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Battery bank, 6 traction batteries (stacker truck) giving 550 AH.

Usage
TV 6 amps per hour over 4 hours.

Over 10 hours.
Fridge approx. 3 amps per hour
Freezer approx. 3 amps per hour
Lighting (L.E.D.) approx. 3 amps per hour
Miscellaneous about 5 amps per hour

Think I have found all the answers here:- Building your own generator.
Regards Brian.

 
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09-22-12, 05:13 PM   #12  
Post up some pictures when you get it all put together.....


Good Luck

 
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