Look at any alternator output curve and you will see that amps increase as
RPM increases up to the point where they approach full output and then level
out. That point is usually between 2500 and 3500 shaft RPM. There are a
very few very expensive alternators, like the Eco-Tech, that are built to
produce close to full output at less than 2000 RPM that have a very steep
curve but most alternators require a minimum 2500 RPM to reach any
significant output.

Look at the LN curve I've posted - it is not as stated above. Full
output is at 5000 alt rpm and it then levels out.

That said, the OP's choice of alternators may not be the best if he wants to
charge at dead idle. The output curve has a particularly slow slope. He
should also consider that it takes a certain amount of power to produce 85
amps. Besides the output, the other item on that chart he should consider
is the Torque curve. When you use a pulley ratio that allows close to full
output at 800 engine RPM the torque increases to the point that invites belt
failure and excessive side loads on bearings.

My engine can deliver up to 20kW at engine idle according to the makers
engine manual - more than enough for the 2 - 3 kW taken by the
alternator. Torque will also not be an issue with dual belts.

Look at the LN curve I've posted - it is not as stated above. Full
output is at 5000 alt rpm and it then levels out.

That is what I meant about "OP's choice of alternators may not be the best
". That particular alternator has a relatively slow slope up to full power.

My engine can deliver up to 20kW at engine idle according to the makers
engine manual - more than enough for the 2 - 3 kW taken by the
alternator. Torque will also not be an issue with dual belts.

Off which end? My Yanmar can handle a lot of torque off the back end but
the front crankshaft bearings are not designed to handle a lot of side load.
Double belts will stop the slippage but a double pulley extends the center
of load further from the bearing increasing the leverage of the side load.

There is also the principle that diesel engines are much happier running at
1800 RPM than they are at 800. For many reasons running at idle under light
loads for long periods is hard on diesels. I am not saying don't do it.
Just that there are better ways to tackle the problem.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

Fundamental AC - the higher the freq (= more rpm) the greater the power
that can be transmitted. Which is why aircraft AC systems are often at
400Hz - smaller kit but higher power.

I was explaining why aircraft sometimes use 400Hz AC -- because they can carry
smaller transformers.

I won't disagree that you can generate (not transmit) more power by spinning the
alternator faster (up to a limit).

"CS" wrote in message ups.com...


Higher AC frequencies are good for transformers, but don't have an effect on the
transmission of power reasonable distances and a boat is way smaller than that.
But since your AC is being converted to DC, the transformer argument is
irrelevant.

I'm lost - I have not mentioned transformers???

And your AC frequency will be pretty high start with -- there's more than one
cycle per rev.

Yes, a 10 pole alt at 8000 rpm is generating AC at around 1332Hz. More
A can be produced at 1332Hz than at 50Hz.

Glenn Ashmore wrote:

Look at the LN curve I've posted - it is not as stated above. Full
output is at 5000 alt rpm and it then levels out.

That is what I meant about "OP's choice of alternators may not be the best
". That particular alternator has a relatively slow slope up to full power.

My engine can deliver up to 20kW at engine idle according to the makers
engine manual - more than enough for the 2 - 3 kW taken by the
alternator. Torque will also not be an issue with dual belts.

Off which end? My Yanmar can handle a lot of torque off the back end but
the front crankshaft bearings are not designed to handle a lot of side load.
Double belts will stop the slippage but a double pulley extends the center
of load further from the bearing increasing the leverage of the side load.

Off the front crankshaft and or a PTO shaft. The engine is an 8 litre
6 cyl natural aspirated 160 Hp engine which weights 1000kg and is a
proper industrial marine engine as used by fishing vessels - not a
bleeding micky mouse Yanmar engine designed for pleasure yachts. I
have done a certain amount of research and I am amazed by the complete
rubbish being spouted about alternators.

Colin,

Don't sweat the overspeed.

Unfortunately, this engine is too new for my catalogues, but I and quite
sure that it is using a Hitachi copy of a Bosch TE injection pump. This
pump includes a controlling governor that not only controls the engine
to produce the ordered crankshaft speed but also limits the speed to a
safe maximum. The only way to exceed that speed is by introducing
additional fuel (often that is injested lube oil) and it that occurs
there is often little the operator can do without getting too close to
the engine.

I was glad to see that you are running double belts because that is just
about and maybe a little bit over the capability of a single belt.

Matt Colie
Lifelong Waterman, Licensed Mariner (Diesel Chief...)



CS wrote:
Engine is a Daewoo L136 and the original alternator, 50A 5000 rpm max
continuous, was replaced by a Leece Neville unit, 110A 8000 rpm max
cont . To get more amps from the alternator, I reduced the pulley so
8000 alt rpm is now achieved at just over 2300 engine rpm - engien
normal max is 2200 rpm. The engine is beneath my feet in a luxemotor
barge - so I think the oversped is somewhat superfluous.

Colin,
I'm really confused. If you already know so much about alternators, why ask
your question to the group in the first place? Glenn has expressed very
valid concerns when equiping an engine with an alternator. It appears that
you have out engineered yourself. If you need to spin your alternator 8k rpm
to get it to perform, you have clearly chosen the wrong alternator. Go out
and buy a proper large magnet Leese-Neville industrial grade alternator and
spin the thing acording to OEM instructions. Problem solved.
Steve

"CS" wrote in message
ups.com...

Glenn Ashmore wrote:

Look at the LN curve I've posted - it is not as stated above. Full
output is at 5000 alt rpm and it then levels out.

That is what I meant about "OP's choice of alternators may not be the
best
". That particular alternator has a relatively slow slope up to full
power.

My engine can deliver up to 20kW at engine idle according to the makers
engine manual - more than enough for the 2 - 3 kW taken by the
alternator. Torque will also not be an issue with dual belts.

Off which end? My Yanmar can handle a lot of torque off the back end but
the front crankshaft bearings are not designed to handle a lot of side
load.
Double belts will stop the slippage but a double pulley extends the
center
of load further from the bearing increasing the leverage of the side
load.

Off the front crankshaft and or a PTO shaft. The engine is an 8 litre
6 cyl natural aspirated 160 Hp engine which weights 1000kg and is a
proper industrial marine engine as used by fishing vessels - not a
bleeding micky mouse Yanmar engine designed for pleasure yachts. I
have done a certain amount of research and I am amazed by the complete
rubbish being spouted about alternators.

On Sat, 14 Oct 2006 05:52:24 +0200, "Steve Lusardi"
wrote:

Colin,
I'm really confused. If you already know so much about alternators, why ask
your question to the group in the first place? Glenn has expressed very
valid concerns when equiping an engine with an alternator. It appears that
you have out engineered yourself. If you need to spin your alternator 8k rpm
to get it to perform, you have clearly chosen the wrong alternator. Go out
and buy a proper large magnet Leese-Neville industrial grade alternator and
spin the thing acording to OEM instructions. Problem solved.
Steve
Colin did not originally ask about specifying an alternator,he was
discussing engine overspeed alarm requirements.
The alternator he indicated is a Leece-Neville!

I cannot see the problem with his setup!

Richard

Nb "Pound Eater" Parkend G+S

Richard,
Colin's overspeed problem started when he changed the speed of the
alternator. He needs to use an alternator that generates the required power
at the original alternator speed. He may have a Leece-Neville alternator,
but it is not the large magnet variety, if he has to spin it at 8K rpm.
Steve

"Electricky Dicky" wrote in message
...
On Sat, 14 Oct 2006 05:52:24 +0200, "Steve Lusardi"
wrote:

Colin,
I'm really confused. If you already know so much about alternators, why
ask
your question to the group in the first place? Glenn has expressed very
valid concerns when equiping an engine with an alternator. It appears that
you have out engineered yourself. If you need to spin your alternator 8k
rpm
to get it to perform, you have clearly chosen the wrong alternator. Go out
and buy a proper large magnet Leese-Neville industrial grade alternator
and
spin the thing acording to OEM instructions. Problem solved.
Steve
Colin did not originally ask about specifying an alternator,he was
discussing engine overspeed alarm requirements.
The alternator he indicated is a Leece-Neville!

I cannot see the problem with his setup!

Richard

Nb "Pound Eater" Parkend G+S

Steve Lusardi wrote:
Richard,
Colin's overspeed problem started when he changed the speed of the
alternator. He needs to use an alternator that generates the required power
at the original alternator speed. He may have a Leece-Neville alternator,
but it is not the large magnet variety, if he has to spin it at 8K rpm.
Steve

OK - I've just looked at the military alternators and indeed some do
generate full power at a lowly 2500/3000 rpm. But they are pretty vast
bits of kit and will not readily mount on my engine. The work involved
to fit would immense. A small pulley for £10 and disconnect a wire is
a simple cost effective option. BTW my 240v AC alternator on the same
engine spins up to 14000 rpm quite happily.

CS,
They are not just Military, they are industrial as well. They can be found
on all big trucks, gererators, water pumps and other stationary diesel
applications. I assume you are in England. If so go to www.valkenpower.com
Call or send an email to Jack. They are in Holland and the prices are good.
Tell him Steve sent you.
Steve


"CS" wrote in message
ups.com...

Steve Lusardi wrote:
Richard,
Colin's overspeed problem started when he changed the speed of the
alternator. He needs to use an alternator that generates the required
power
at the original alternator speed. He may have a Leece-Neville alternator,
but it is not the large magnet variety, if he has to spin it at 8K rpm.
Steve

OK - I've just looked at the military alternators and indeed some do
generate full power at a lowly 2500/3000 rpm. But they are pretty vast
bits of kit and will not readily mount on my engine. The work involved
to fit would immense. A small pulley for £10 and disconnect a wire is
a simple cost effective option. BTW my 240v AC alternator on the same
engine spins up to 14000 rpm quite happily.