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Old 08-06-2017, 07:35 AM   #1
City: Tasmania
Country: Australia
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AC 8kw Alternator......on a Gardner 6L3B

Hi Everyone,

maybe you can help me out..

I have the chance to fit a 8kw AC alternator thats in good shape, to
my 6L3B Gardner

We have a 65' retired charter boat with a huge flat roof deck, thus plenty of solar power and a 3000ah battery bank to handle our modest lighting, washer, microwave etc needs, but occasionally the
big water 2kw pump needs to be started, and I also, would like the option to run a couple of portable ac units whilst at sea.

I don't really want to buy a new diesel generator for this kind
of sporadic use.

I don't want to spend $6k on a super powerful inverter.

What I would appreciate is your opinions on ( Ive changed my approach, and now inform people that "I don't give advice, but I can offer an opinion", its just easier on all concerned.....)
are the following:
-Is it worth the trouble? The unit is costing $600 with a warranty.
-Loss of power ?
-Are there any issues to watch out for?
-What are the best ways to disconnect the unit ( elec clutch etc) when
not required.
-I have plenty of room around the Gardner, is there a preferred
method to fit it, thats better, ie: direct drive, 1,2,or3 belts, ?

The unit was previously fitted to another Gardner 6LX



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Old 08-06-2017, 08:13 AM   #2
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"The unit was previously fitted to another Gardner 6LX"

Get ALL the brackets , and the drive pulleys , swop yours for theirs if needed.

" direct drive," off a PTO is the best , but not easy to do,some have clutches.

IF it puts out DC the load will almost never be noticed.

If it produces AC , you may need to operate at one set engine speed .

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Old 08-06-2017, 08:21 AM   #3
City: Tasmania
Country: Australia
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Thanks FF, good stuff, I hadn't considered the engine speed, guess I can optimise through the pulleys
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Old 08-06-2017, 09:48 AM   #4
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How much horsepower does an alternator pull ? You can mathematically determine the horsepower cost at any given load. (Remember, alternators respond to load. If there is no load present the alternator is basically freewheeling.)

Amps x Volts = Watts
Watts / 745.7 (one HP) = Electrical HP Produced by the Alternator
HP x 15% Efficiency Loss = HP Loss
HP + HP Loss = Total HP Used

57A x 14.9V = 849.3 Watts
849.3 Watts / 745.7 = 1.14 HP
1.14 HP x 15% = 0.17 HP
1.14 + 0.17 = 1.31 HP Total
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Old 08-06-2017, 11:48 AM   #5
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3000ah of battery and presumably < 2KW inverter seems a little out of balance for the systems I have seen.. If you've got a couple of KW of solar and say 150A+ alternator capacity I would not rule out a bigger inverter.
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Old 08-06-2017, 07:16 PM   #6
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Send a PM to Tidahapah if he doesn't chime in soon. He has an AC alternator on his 6LXB, not sure of capacity, but he can advise (sorry, offer opinions) on set-up and frequency control. Your 6L3B will operate at lower RPM, but as you say that is readily managed with pulley selection.
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Old 08-08-2017, 06:59 AM   #7
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Thanks to all....
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Old 08-08-2017, 08:58 AM   #8
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Likely a synchronous AC machine that will need to run at a specific rpm to make the correct frequency output. So you can select pulley size but then that will restrict the main engine to a very narrow range of rpm if you want to use the output.

What rpm is the alternator? If 60hz it will be 3600, 1800, 1200, etc depending on the number of poles.

If 3600, that will require a large pulley on engine and a small pulley on the machine, not a real favorable arrangement for high load.

Can be done, though. Bracketry needs to be well engineered and made.
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Old 08-08-2017, 02:47 PM   #9
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You will not always be running the engine when you want a/c etc. Do you want to run the main at anchor? I doubt it, especially if you need a steady output and rpm for the AC. If your battery bank is in good shape I would go entirely solar - I only have 600 watts but I have not run the genset at all this year and the batteries are charged by noon. The controller is smart enough to cut off the drain from batteries when they are pulled down by the inverter so no worries about hurting the battery bank. You could also add another inverter dedicated to particular loads but they are more money than that generator.

If the AC generator needs to be run at a specific rpm for cycles etc., what sort of controller does it need? That might be the expensive bit.

Don't believe everything that you think.
What are we offended about today?
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