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Old 03-10-2009, 21:41   #1
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Propshaft Alternator

OK you technical types, I need some help.

I had a prop shaft alternator that died some time ago, the builder of the old one is no longer around.

To recreate what I used to have I have been given a "special winding" that is reported to allow full voltage out put at low rpm, I have the casing that the winding is go into, I have the large pulley for the shaft, I have the small pulley for the alternator, I have the bracket and the pulley. So the system is ready to be put back into service.

The difference in the new/old systems as best I can tell is the alternator is a Delco which is internally regulated, and the old one was an Ingram with an external regulator.

On my last system, there was a switch that normally was kept in the off position when at rest or when the engine was driving the prop. With the switch in the off position, the DField did not have power, so even though the alternator shaft was being driven the shaft alternator was not putting out power. When you switched the switch on when sailing, the dfield would recieve voltage, the alternator would then start producing output voltage so long as the boat speed was sufficient to turn over the prop, usually at about 3 knots. Full out put, about 20 amps, was achieved at 6 knots.

Now I have been told that such a system is likely to burn up the diodes unless you have a "special" alternator. The argument goes that when the switch is off, with the shaft turning but the df ield not recieving power, the diodes can still be fried. My non technical mind thinks that the diodes can only be fried if there is power output, so if the dfield is not powered, there is no output power, therefore the diodes can not be blown.

Yes or no??? Or is this one of those, I do not know it depends on....!! I sincerely hope you do not ask me too many more technical questions about this because I have just about exhausted my technical knowledge in decribing the system!!!.

Regards

Tom
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Old 03-10-2009, 22:06   #2
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You can switch the field current what you like without damaging the diodes. The only way to damage the diodes is either overheat them, or disconnect the load they are feeding into, like by switching the engine-start battery switch which will probably be in the circuit. This is why you can get those switches with a small extra pole to switch the field off (the small one disconnects before the big one).

cheers,
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Old 03-10-2009, 22:14   #3
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The previous arrangement I had and which I would like to use is just a simple seperate switch to the df post (usually a spade type fitting) on the alternator. The battery isolation switch is not involved in the setup at all.

The guy who said this would not work said it had something todo with the exciting the fields, but I did not really understand all of what he was saying.
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Old 04-10-2009, 07:34   #4
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Nick is correct...you cannot damage the alternator by switching off the field (excitation) voltage. You will damage the output diodes if the load that the alternator is serving is turned off/removed. He used the alternator disconnect battery switch as an example of the application of this concept.

Hope this helps,
Charlie
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Old 04-10-2009, 07:44   #5
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Ok thanks guys for the help, you are basically agreeing with my non technical thoughts were on this, so I will just go ahead and install the unit and hope I am back in business!

Tom
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Old 04-10-2009, 08:25   #6
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The guy you talked to was probably worried about using a switch instead of a regulator: you had the alternator at full output without regulating which can lead to overcharging and even frying the battery. I assume you always kept an eye at the voltage and switched it off in time.

Now with your new alternator you have an internal regulator? You can relax looking at the voltage a bit with that.

When you connect the B+ (big red wire) from the alternator to your engine-starter circuit, you would have the battery switch in the circuit. I think you might have connected this alternator to you house batteries instead. But there will be a switch between those batteries and the alternator, right? Remember not to switch that to "off" with the prop-shaft alternator charging as that would blow the diodes.

ciao!
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Old 04-10-2009, 08:50   #7
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Thanks for the comments Nick

The old system, and the way I wish to keep the new one is wired to engine selection switch, I can use either battery bank or both for engine start, the same switch decides which battery bank (or both) will recieve output from the engine alternator and the shaft alternator.

I leave this switch on 1, the house bank, almost all the time unless I need the start battery for starting the engine. The only time the switch is ever in the off position is if I am doing maintenance on systems or leaving the boat for an extened period of time so there unles I screw up, there is very little likelyhood of removing the load from either the shaft alternator or the engine driven one.

The previous shaft alternator setup had an external regulator, but it was not a smart regulator inthe typical sense of the word, it was just designed to work with the Ingram alternator and there were no boost or special charging cycles. The new alternator has an interal regulator. My thinking is that a smart regulator is not needed for this alternator since in operation the alternator is working more like a regular car alternator. I am not trying to quickly boost batteries and reduce the run time on this alternator, just keep the power up on the batteries. I am hoping to get the same performance I got before, if I was sailing at 4.5 knots or better, the only reason to start the engine was to make sure it worked, I did not need to boost the batteries at all.

Just as you note, the only reason I want the df switch is so I can take this alternator off line when I am motoring. It is not intended to be a means to regulate the output, although if I was concerned about boosting the output, I could always put a reostat in line to control the DF, then I would need to monitor the batteries! Too much hassle for me though! I wll go with the regulated output.
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Old 04-10-2009, 09:37   #8
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Alternator type?

I have a shaft alternator set up but have never used it as the PO said they could never get it to work. So when painting the bilge I removed it and never tried it.

Most things I have read say that the problem is in getting enough RPM

My shaft pulley is 14 inch diameter so what should I expect in alternator RPM?

Also I wonder if PMA type is suitable?

Nick….Is this something you would recommend for the shaft alternator?

Permanent Magnet Alternators PMA wind power wind turbines wind mills PMG Permanent Magnet generators PMA wind power turbine hydro power wind turbine
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Old 04-10-2009, 19:44   #9
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It would take some serious pulley sizes to go from prop shaft rpm to the 10,000 rpm +/- a few thousand rpm's that an alternator needs. Looking at an on line pulley size vs rpm converter shows 14" pulley to 2.5" at 2000 rpm shaft gives 11200 rpm at alternator. That should work fine - - but -- what about prop shaft flex? To get sufficient v-belt tension I would think you would be inducing a large side load on the prop shaft causing stuffing box wear or even cutlass bearing wear. Maybe why some folks mention chain drive or maybe toothed belts.
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Old 04-10-2009, 21:04   #10
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Hi Osirissail

Would a prop shaft get as high as 2,000 Rpm under sail? I really have no clue from persoal experience as to how fast mine turns, but I had heard somewhere that 800 or so was about tops. We are talking about free wheeling,not under power. Is there an easy way to find out the RPMs?

You are right about the side load issues, most of the designs I have heard about suggest some sort of bearing brace near the pulley, I do not have such a thing and the cutlass wears faster than on my previous boat.

Tom
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Old 04-10-2009, 21:27   #11
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Quote:
Originally Posted by osirissail View Post
It would take some serious pulley sizes to go from prop shaft rpm to the 10,000 rpm +/- a few thousand rpm's that an alternator needs. Looking at an on line pulley size vs rpm converter shows 14" pulley to 2.5" at 2000 rpm shaft gives 11200 rpm at alternator. That should work fine - - but -- what about prop shaft flex? To get sufficient v-belt tension I would think you would be inducing a large side load on the prop shaft causing stuffing box wear or even cutlass bearing wear. Maybe why some folks mention chain drive or maybe toothed belts.
If I remember correctly my reduction gear is 2:1 so at 1400 engine RPM the shaft is rotating at 700 rpm

Prop rotation while sailing would also be slower due to slippage so at about 7 knots of speed I am guessing about only 500 RPM shaft revolutions.

Applying the 14” pulley advantage of 5.6 to 500 shaft RPM means the alternator would only spin at 2,800 RPM

That is why I am curious about PMA type for this application.

Anyone ever used them?

Tom Side Brace is important…hard to see in my photo but in the foreground is a heavy triangle bearing brace just aft of the coupler
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Old 04-10-2009, 22:47   #12
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- - Yeah I got for 14" & 2" with 500 rpm => 3600 rpm at the alternator which would not put out anything for a normal alternator - and the cooling would be inadequate due to low rpm's. A portable strobe tach could be used to see exactly what the "free-wheeling" rpm actually is. The size of the prop would determine how many rpm's are lost to slippage.
- - How about a wind generator alternator? They obviously do not turn at 10,000 rpm's so they might be a better solution to a normal alternator. I see broken and abandoned units sitting around boat junkyards. I wonder what rpm's they work at?
- - The only way to support the prop shaft would be to install a hull mounted thrust bearing which are commercially available. This would solve a lot of other problems such as engine mount wear; transmission thrust bearing wear; and shaft log alignment. I was going to install one on my boat but ran out of money before I could do it.
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Old 04-10-2009, 23:43   #13
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Quote:
Originally Posted by osirissail View Post
- -
- - How about a wind generator alternator? They obviously do not turn at 10,000 rpm's so they might be a better solution to a normal alternator. I see broken and abandoned units sitting around boat junkyards. I wonder what rpm's they work at?
That is what got me thinking about PMA alternators because that is what the wind gens use.

Take a look at the power curves in my link above and you will see what I mean

The Super Core 24 puts out about 46 amps @ 40Volts with only 1400 RPM.

Need to better understand how these wold work in shaft gen application
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Old 05-10-2009, 07:43   #14
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- - The first link -post #8 - starts with 16,000 and 18,000 rpm's. That's a long way from 3,000 rpm or less. The second post shows units with outputs starting in the low rpm range. But the power graphs badly show rpm vs volts vs amps in open circuit use. RPM's seem to be in the 1500 range to get a charging voltage for a 12VDC system.
- - I have worked with wind generators. They start producing voltage as they start turning, but you need to get some significant wind to get that voltage up to about 13VDC before any battery charging occurs. Amp output seems to be "rider" on the rpm's. That is voltage and amps increase with rpm's. That would mean using a PWM -diverter charge controller between alternator and batteries. Also the typical alternator used in wind gen's like KISS and AirMarine series put out AC voltage not DC voltage. That must be converted to DC by a separate diode system. It is internal in an AirMarine series unit and external in the KISS unit.
- - That is why using a "standard" car alternator is attractive as all that stuff has already been worked out. Finding and buying (Ebay?) an old AirMarine would be the easiest as everything is already inside the unit. All you need is the pulley and an external diverter regulator to protect the batteries.
- - Otherwise, you will have to do the math to convert the AC output to usuable DC volts/amps. - and re-invent the system.
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Old 05-10-2009, 08:10   #15
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Am I hearing you guys right, that there is no reason not to use an alternator with internal regulator? I have stumbled on to a Bosh 150amp alternator that needs a rebuild for free. Came out of a Sprinter Van. The old noodle went click and ah sez to myself, "Self this might work"

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