Choosing a small retrofit alternator

[bluedragon]

This is a follow-on from my ealier post regarding electrical power generation on a small yacht. My elderly 8HP Sabb diesel runs beautifully has been very well looked after for 37 years. The Dynastart and voltage regulator are new and work as well as they ever can...but that means a maximum of about 12-15A charging current. I'm looking into wind generators, solar panels and other means of filling the gap between supply and demand, but fitting an alternator is obviously another option. I can't replace the Dynastart as it's used to start the engine (and it does give me a useful current)...so it's an add-on alternator. Now there isn't much room, and with only 6-8HP I can't afford to use much driving a large unit. It would also be better to have an external voltage regulator I guess so I can optimise charging voltage. With so many alternators on the market (marine & auto) where should I start looking and what for? Many thanks.

 

[FullCircle]

Car breakers for a small car unit like say a Daewoo Matiz, or your little Sabb will be overcome. About 50 amps. Consider running it off the prop shaft maybe or if you can get another pulley on the engine somewhere then run it off the front end.

I have a 120amp unit that I will be attaching to the front of my 29hp Yanmar, just getting round to it is the problem.

 

[KRG]

[ QUOTE ]
I have a 120amp unit that I will be attaching to the front of my 29hp Yanmar, just getting round to it is the problem.

[/ QUOTE ]

Crikey, that's some unit, are you gonna fit a turbo as well /forums/images/graemlins/laugh.gif

 

[Tisme]

Sorry if you are a grandmother and used to sucking eggs but.......

Make sure that whatever alternator you get that it rotates the right way for your engine. Some need to go clockwise, some anti-clockwise and IIRC some work both ways.

If you go to a Lucas depot they have very useful catalogues on different manufacturers products.

 

[charles_reed]

Alternator sizes

I've got a 100 amp alternator on an 18hp diesel. I prefer fat batteries to moving fast under power.

Fully charged batteries 6-7 knots, part-charged 4-5 knots.

 

[KRG]

Re: Alternator sizes

I don't want to thread drift on bluedragon's post but it may be relevant, I've got a 50 amp alternator with a high output regulator and 18hp engine, if my batteries, 220ah, are a bit low it takes a while before there is much power available for the prop.

 

[pvb]

Horsepower and kilowatts...

Your 50A alternator will (at 12v) produce 600watts max. Let's assume it uses 1000watts to produce that output. 1000watts is about 1.3horsepower. 1.3hp isn't a lot to lose out of 18hp, is it?

 

[KRG]

Re: Horsepower and kilowatts...

No, you're right the engine copes well in neutral but with a single lever control I have to drop the engine to tick-over to engage gear and with a 3 blade prop I can push the lever as far as i want but the engine wont rev very high until the initial high load of the alternator has dropped. Probably to do with the power curve of the engine. I've heard some of the more clever regulators apply the load progressively to get over the problem.

 

[bluedragon]

Re: Horsepower and kilowatts...

But it is when you've only got 6-8 to start with...that's one of my worries. Do I understand that the HP required by any given alternator is proportional to the current it is producing (ie. battery state of charge)?

 

[KRG]

Re: Horsepower and kilowatts...

I think with a small output alternator, say 35-40 amp with a standard car type regulator you would be ok, the problem I descibe is bought about because I've fitted an aftermarket regulator to extract as much power in as little time as possible.

 

[DaveS]

Re: Horsepower and kilowatts...

In short, yes. The output in watts will be about 14.8V X the current in amps. (This is at high output; the voltage will be controlled to a lower value when less current is required.) An alternator delivering 100A is therefore delivering about 1500W. Small rotating machines are not brilliantly efficient however, say 80% or so, so to get 1500W out will require about 1850W or 2.5 hp of mechanical input.

Incidentally, having a small diesel fitted with a relatively big alternator could have advantages. If a 100A alternator as above was being driven flat out by your 8 hp engine, the engine would be loaded to about 30% of its rated output. Giving it worthwhile work to do is probably much better for it (see previous posts about bore glazing) than the more normal situation. Mine is fairly typical: 18 hp engine, 50A alternator, engine loaded to 7% of rated output at most if used only for battery charging.

 

[andy_wilson]

Re: Horsepower and kilowatts...

I bank on about 2hp being absorbed by my 60A alternator (VP 18hp - 16years ago, who knows what now!) which is a huge % of power available at the relevant revs.

Full charging kicks in at 1300 to 1400 rpm but of course this ramps up from v. little charge at tickover. Due to the power requirement of the alternator climbing faster than the output curve of the engine, there is a point at which revs will not rise if the batteries are low enough. This is compounded when the engine is cold and therefore producing even less hp.

This of course could be a massive problem in say an emergency start to clear an untenable anchorage after a long evening of entertaining (heating/lights/music etc.).

I consider this reason enough to maintain manual comand of battery supply and demand, with 1/2/Both/Off switch.

For someone contemplating an automatic system with high output controller and VSR or Diodes, I would strongly recommend a model with charge ramp-up. The really savvy would arrange wiring with a disable switch that could return the alternator to standard mode in the event that full engine power may be required. Charles illustates the issue perfectly.

Meanwhile, for the questioner, such an arrangement - say a 40Amp alternator and advanced controller - would be just the job if running the engine just for charging, but I would suggest would require a scheme to reduce the load of the auxilliary charging to zero if required.

 

[bluedragon]

Re: Horsepower and kilowatts...

With what I've learned from these responses I'd definitely go for a system where the alternator can be disconnected from the engine, either mechanically or electrically and used as an auxilliary charger only when needed. Thinking it through, it is of course now self-evident that as the field current fluctuates, the "electromagnetic drag" on the engine does the same. Would a switch or relay in the field coil circuit be a simple and safe way of disabling the unit? I've come across a reconditioned Lucas ACR18 auto alternator putting out 35-45A I think. It seems these might have been used for marine engines also in the past. Would this be a good choice? Another issue...alternator speed. My engine has a max output at around 1800-2000 rpm. It's very happy around 1000-1500 rpm. Fortunately the Sabb drive belt runs around the exterior diameter of the flywheel, so I have a multiplication factor of about 4x depending on the alternator pulley size. This would give a machine speed of about 4000-6000 rpm. Is this enought to get close to rated output? Sorry for all the questions!!

 

[Norman_E]

I agree with finding a small alternator, but do not even think of driving it with a pulley off the prop shaft. Alternators should be run in one direction only. Go for low power, something like 50 amps as suggested. 100 amps at 14 volts output equals 1400 watts, or very nearly 2 horse power output. Input requirement would be greater due to mechanical and electrical efficiency factors, probably 2.5 to 3 horse power being absorbed to drive a 100 amp alternator.

 

[KRG]

Re: Horsepower and kilowatts...

I've got an alternator handbook in PDF form which I sourced from the internet, if you want a copy PM me with your email address and I'll send it to you. It's 6MB.

 

[bluedragon]

Re: Horsepower and kilowatts...

Thanks, will do...