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Old 03-13-2020, 07:39 PM   #1
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Running an alternator in reverse?

My predicament: I have a 25' trailer steamboat. There is no electrical system, just 2 batteries to run the lights, bilge pump, VHF, and most importantly the boiler blower, as when that stops the boat stops. My range is limited to 5 hours of operation before the batteries need a charge. (that's 2-1/2 hours out and 2-1/2 hours back to the dock). To increase my range I would like to install a small alternator.

A steam engine has no transmission, it's direct drive. When you want to reverse, the engine turns backwards for a few minutes. When you stop the boat, the engine is not turning.

My question to you is, Can I run an alternator in reverse? What happens when an alternator runs in reverse? Do I need to wire it up differently than what is considered normal?

Thanks in advance for any replies.
Ryan
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Old 03-13-2020, 07:53 PM   #2
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The alternator itself should not care at all. It will work either rotation.

However the fan may not work well in the reverse rotation. If the blades are slanted then the fan is designed to work best one way, not the other.

There usually are universal fans whose blades do not have any slant. Those should work fine either direction. Talk to an alternator shop.
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Old 03-13-2020, 08:11 PM   #3
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I doubt he'll be running in reverse long enough for the fan direction to matter, I may be wrong but.....
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Old 03-13-2020, 08:46 PM   #4
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I can't imagine a steam engine turning fast enough for an alternator to work.
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Old 03-13-2020, 08:55 PM   #5
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Maybe another avenue would be add another battery or 2 to your bank and add a couple of 100w solar panels to offset the discharge. 2 100w panels should be good for 6-8 amps of charge in good conditions. Plus it'll be easier than trying to engineer a steam powered charging system, IMO.
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Old 03-13-2020, 09:15 PM   #6
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Quote:
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I can't imagine a steam engine turning fast enough for an alternator to work.
It's all about gearing. Since the alternator addition is a 'clean sheet' design, the OP
just needs to select a driving pulley that's big enough to get the alternator into its
working rpm range.
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Old 03-13-2020, 09:27 PM   #7
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It's all about gearing. Since the alternator addition is a 'clean sheet' design, the OP
just needs to select a driving pulley that's big enough to get the alternator into its
working rpm range.
You think that would be the best way? Idk. That would be some serious gearing. I don't charge all that great until I'm above 12-1400RPM and that's my engine RPM. The alternator turns faster than the engine!
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Old 03-13-2020, 10:00 PM   #8
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There is a thing as generators run by steam, leastwise with railroads in the past and in most current electrical plants.
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Old 03-13-2020, 10:02 PM   #9
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Why not add a couple more batteries it would be cheaper. Solar would help to.
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Old 03-13-2020, 11:09 PM   #10
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The short intervals and low rpms when reversing there will not be an issue with the alternator regardless which type of fan is used but a straight blade fan will be preferred.
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Old 03-14-2020, 02:50 AM   #11
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A second battery would double his cruise time or halve his depth of discharge. Both good things. Perhaps a bank of golf cart batteries.
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Old 03-14-2020, 06:44 AM   #12
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Yes, you can run an alternator in reverse and the straight bladed fan will work in both directions.

As previously mentioned, "how much time will you spend in reverse "? The last part is RPM. Alternators have a minimum RPM before they start producing any amount of power (and generate heat). If most of the reverse RPM will be at or just above idle speed, I wouldn't worry about changing the fan.

Regarding RPM and output, alternators and generator ends are wound to match engine RPM. There are lots of slow speed direct drive generators turning 1,800 RPM and below. What you will want to look for is an alternator for ambulance, EMS, or fire trucks. The alternators are designed for continuous high output at low RPM while working above idle at an accident scene. Leece Neville offers these units in a large frame lower RPM models in a variety of amperages. Most include an output graph (amps versus RPM).

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Old 03-14-2020, 07:25 AM   #13
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Agree that rotation direction and fan won't be an issue.

But alternator rpm could be an issue. On my Cummins 450C, the alternator runs about 3x engine rpm due to pulley sizes. At idle of 500rpm that has the alternator running about 1500rpm and it does not put out much. It is rated at 100 amps and at idle it might put out 15A. That used to be a problem but once I switched to all LED lights, it is tolerable. There have been times at night with fog and searchlight and fridge inverter that alt could not keep up. Ended up having to fire up the gennie.

On the steam engine, it will need a very large diameter pulley/flywheel to get the alt to work. And being direct drive, it probably does not need a flywheel, the prop takes care of that.

And some alternators need a good bit of rpm to get the exciter circuit to "wake up". Don't want one of those. I use an Autolite alt with external regulator that powers up as long as oil pressure switch is closed, you will need something similar.

Another option is a permanent magnet alternator like used on some boat gensets. Like the Onan/Kubota MDKD. Those don't make many amps, but do make over 12vdc at a pretty low rpm. Worth looking into.

And get your running lights converted to LED. That would completely solve the issue!!
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Old 03-14-2020, 09:02 AM   #14
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What you are describing is a prop shaft generator. Google that and you will find a variety of solutions for sail boats looking to generate power off of a freewheeling shaft while under sail. Some are packaged products, and some are home brewed. But they have all addressed the gearing issue in one form or another.
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Old 03-14-2020, 10:47 AM   #15
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A possible option, although a bit pricey, if you don't have the room for additional batteries, replace the existing batteries with lithium.

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Old 03-14-2020, 12:08 PM   #16
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Thumbs up

Lower RPM 12 volt alternators are available.
Don't bother with automotive alternators these are typically 12,000 maximum rpm range.
Class 8 trucks, dozers, industrial diesels use large frame alternators in the 6000 maximum rpm range.
For example http://www.prestolite.com/literature...ors_FL1211.pdf
Assuming your using a tripile expansion engine turns 200 rpm
Chain or belt drive it off prop shaft at 200 rpm using 7:1 ratio and get 1400 alternator rpm @ 50 amps.
Either buy it new or go to used truck or equipment parts and look for similar used alternator.
Also look for 12 volt automotive generators from 50's to 63, they topped out about 6000 rpm. Most were about 40 amps
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Old 03-14-2020, 02:18 PM   #17
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Leece Neville is part of Prestolite.

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Old 03-14-2020, 02:48 PM   #18
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https://www.dbelectrical.com/product...2305-2320.html

dbelectrical can answer all your questions - good guys!

This little alternator puts up 20A at 12 vdc - @ 6000 rpm - 35 hp tractor turning 1500 most of the time. You'll need the matching regulator - total < $150 new.

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Old 03-14-2020, 02:48 PM   #19
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C Lectric, O C Diver, Ski in NC, all good suggestions and thanks! I think twistedtree nailed it -- I'd never thought of a prop shaft generator. That would be ideal! The other way to go might be the suggestion by roddy.
My original thoughts were to use a serpentine toothed belt and toothed pulleys off the prop shaft and step up the RPM that way. At hull speed my shaft turns at 333 RPM and with steam I have enormous power. But I will investigate prop shaft generators first.

As for solar, being in the PNW we are cloudy a lot of the time, and I have hardly any flat space left to mount panels. I would if I could.

Thanks all!
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Old 03-14-2020, 02:57 PM   #20
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Quote:
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https://www.dbelectrical.com/product...2305-2320.html

dbelectrical can answer all your questions - good guys!

This little alternator puts up 20A at 12 vdc - @ 6000 rpm - 35 hp tractor turning 1500 most of the time. You'll need the matching regulator - total < $150 new.


Olebird, that is a great suggestion and I'll sure look into it. My boiler blower needs 15 amps and will quit if it gets less than 11A.
I have a John Deere mower yet never even thought of that.

Thanks!
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