Put another way, is a large alternator operating at 60% of capacity more
or less efficient than a smaller one operating at close to maximum output?

--
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

Larry wrote:
Saw a 380A 24V, gear-driven alternator for a big bus at the electric
shop while buying my Chevy stepvan a new
starter..........DROOL....(c;

It had oil passages the big diesel pumped engine oil through the core
of it to cool it!

That'll recharge that big Heart Inverter battery.....quick!

To answer your question, alternators are very efficient. The only
problem I'd see with a huge alternator is the weight of the unit
making the engine lean to one side and mis-aligning the shaft, which
would be BAD.....and the flywheel effect of a larger armature
starting and stopping and changing engine speeds. I'd be afraid of
the weight/alignment issue in a small sailboat engine bending the
mountings over to one side. I suppose you COULD mount the alternator
off the motor but then the pull on the belt a big alternator would
pull would pull the engine to that side all the time, probably
ruining the engine mounts. My friend Geoff had that trouble with his
Yanmar 3GM and a huge car air conditioner compressor that used R-12
to run the fridge's big cold plate. You could SEE the engine pull
towards the externally-mounted compressor.....That's got to hurt
shaft bearings.

I am avoiding side load all together. Welded up a frame in heavy 6x2
channel that bolts tothe stringers in front of the engine with a short
1.5" diameter shaft on double 1.5" pillow blocks. The Niehoff and Cat
watermaker pump mount on the bracket with all the V-belts and pulleys on
the shaft. It is connected to the damper plate with an Aquadrive.
(Basicaly a very short CV joint) The engine can wobble all it wants
without any side load.

I am still a little worried about the total horsepower coming off the
front though. I am figuring I will need about 9HP with the alternator
and the pump running flat out. The Yanmar installation manual does not
give limits for this kind of mount. It is more concerned with side load
on directly attatched drive pulleys.

--
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

What worries me on off-engine powered equipment is the belt tugging
away on the engine to that one side. On Geoff's old boat, you could
see the engine vibrating sideways as the freon pump went over TDC each
cylinder, then releasing as the piston went down slackening the load.
It's GOT to pull the engine mounts and wear them that way. This is
not an issue where the load is mounted directly on the engine and not
pulling against the engine mounts. Engine mounted, however, needs a
little weight and balancing, too. I see too many engines, especially
in outfitted sailboats with limited space, that have too many addons
bolted to ONE SIDE of the engine, unbalancing the weight distribution
on the engine mounts, which again must be bending with that load.

Either way, I'd think it would be hard on shaft alignment, worse after
time, wearing out those shaft bearings and cutlass bearing.



Larry W4CSC

"No, NO, Mr Spock! I said beam me down a WRENCH,
not a WENCH! KIRK OUT!"

Dual belts, if you got the space, are a good idea, anyways....no
matter what the ratings are.



On Thu, 31 Jul 2003 23:23:11 -0400, Joe Wood
wrote:

Rule of thumb that I heard was that anything over 90 Amps should have
dual belts.

Joe Wood

Glenn Ashmore wrote:
Put another way, is a large alternator operating at 60% of capacity more
or less efficient than a smaller one operating at close to maximum output?




Larry W4CSC

"No, NO, Mr Spock! I said beam me down a WRENCH,
not a WENCH! KIRK OUT!"

Larry wrote:
What worries me on off-engine powered equipment is the belt tugging
away on the engine to that one side. On Geoff's old boat, you could
see the engine vibrating sideways as the freon pump went over TDC each
cylinder, then releasing as the piston went down slackening the load.
It's GOT to pull the engine mounts and wear them that way. This is
not an issue where the load is mounted directly on the engine and not
pulling against the engine mounts. Engine mounted, however, needs a
little weight and balancing, too. I see too many engines, especially
in outfitted sailboats with limited space, that have too many addons
bolted to ONE SIDE of the engine, unbalancing the weight distribution
on the engine mounts, which again must be bending with that load.

Either way, I'd think it would be hard on shaft alignment, worse after
time, wearing out those shaft bearings and cutlass bearing.

What I am doing is kind of hard to visualize. The only places I have
seen is it on an Arodyne 46 and a USCG MLB. The heavy auxillaries are
mounted off the engine. Nothing but torque reaches the engine. It can
put stress on the engine mounts but compared to the propulsion torque it
is minor.

It will be about 6 months before I can set the engine and the
auxillaries but then I will post some pictures and you can see what I mean.

--
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

It is not a rule of thumb. It is an engineering fact. All the belt
drive makers have web sites and they all have pages about sizing belts.
I used the Martin sheave formulas and double checked on the Gates Belt
site. "A" and "AX" belts which are normally used on boat engines have a
maximum of about 4 HP each at the speeds and pully sizes we use. (6-7"
drive at 1500-3000 RPM and 2.5" to 3" driven at 3200-7000 RPM)

There IS a rule of thumb involved though. Counting efficiency losses
and the ever present safety factor, the rule is that an alternator
requires an average of 1 HP per 25 amps of output at 13.8V. So when you
get past about 90-100 amps a single AX belt just can't handle the power.

Joe Wood wrote:
Rule of thumb that I heard was that anything over 90 Amps should have
dual belts.

Joe Wood

Glenn Ashmore wrote:

Put another way, is a large alternator operating at 60% of capacity
more or less efficient than a smaller one operating at close to
maximum output?




--
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

Getting back to your original question.....

I don't think a bigger alternator is less efficent and it might be
more efficient, but I freely admit I'm speculating. Consider:

-- The bigger alternator will have bigger windings and therefore less
I^2 R loss inside. This is a small factor, as by far the biggest heat
producer in an alternator is the voltage drop across the output diodes
and that won't be affected. Nonetheless, it should be a plus for the
bigger unit.

-- The bigger alternator may have a bigger case and should therefore
run cooler, always a good thing.

I must add that I'm not quite sure why you ask the question, Glenn --
your various posts make it clear that you have an excellent
understanding of DC systems..... What do you think?

Jim Woodward
www.mvfintry.com


Glenn Ashmore wrote in message ...
It is not a rule of thumb. It is an engineering fact. All the belt
drive makers have web sites and they all have pages about sizing belts.
I used the Martin sheave formulas and double checked on the Gates Belt
site. "A" and "AX" belts which are normally used on boat engines have a
maximum of about 4 HP each at the speeds and pully sizes we use. (6-7"
drive at 1500-3000 RPM and 2.5" to 3" driven at 3200-7000 RPM)

There IS a rule of thumb involved though. Counting efficiency losses
and the ever present safety factor, the rule is that an alternator
requires an average of 1 HP per 25 amps of output at 13.8V. So when you
get past about 90-100 amps a single AX belt just can't handle the power.

Joe Wood wrote:
Rule of thumb that I heard was that anything over 90 Amps should have
dual belts.

Joe Wood

Glenn Ashmore wrote:

Put another way, is a large alternator operating at 60% of capacity
more or less efficient than a smaller one operating at close to
maximum output?

Jim Woodward wrote:
Getting back to your original question.....

I don't think a bigger alternator is less efficent and it might be
more efficient, but I freely admit I'm speculating. Consider:

-- The bigger alternator will have bigger windings and therefore less
I^2 R loss inside. This is a small factor, as by far the biggest heat
producer in an alternator is the voltage drop across the output diodes
and that won't be affected. Nonetheless, it should be a plus for the
bigger unit.

-- The bigger alternator may have a bigger case and should therefore
run cooler, always a good thing.

I must add that I'm not quite sure why you ask the question, Glenn --
your various posts make it clear that you have an excellent
understanding of DC systems..... What do you think?

Jim Woodward
www.mvfintry.com

It is more a matter of balance. The immediate problem is that I
suddenly have a surplus of alternators. I was planning to use the big
270 amp hot rated Niehoff that I picked up off a fire engine that got
crushed by a falling wall. It needs about 10 HP at full output. It is
to be mounted off engine through a CV joint arangement to eliminate side
loads but when I add the Cat pump for the watermaker it runs the maximum
HP off the front of the engine up to about 12HP max. That is getting a
bit much even with the CV joint so when I got offered a 200 amp (cold
rated) Leece Neville fot $140 I bought it.

Now I am wondering which should be the primary and which the spare. For
all practical purposes the 750 amp bank is only going to take 175 amps
max and more like no more than 150 amps most of the time so the question
was will I be wasting enengy by swinging the heavier alternator? I
believe I agree with you that the answer is probably not. I think I
will mount the Niehoff and keep the LN in reserve.

--
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

BOEING377 wrote:
The wind generator crowd swear that in the case of single wire alternators, low
speed output is better on a low power alt (eg one capable of 30 amps max) than
a high power one (eg 100 amps). Is this true? If so why?

Probably because of the limited power available. The torque required to
turn the alternator is governed by the current in the field coil. The
regulator sees only the vlotage going to the battery and has no idea how
much power is avilable to turn the alternator. When the regulator sees
power is required it loads up the field coil to increase output. The
bigger the alternator, the more torque is required so the wind can't
spin the larger one as fast.

The power that an alternator puts out relative to RPM is a curve that
starts off steeply and levels out. Optimum RPM is about at the point
where the curve starts to level out. Even with specially designed low
speed alternators that point is more than 2,000 RPM. The wind generator
designers want to get the RPM as far up the steep part of the curve as
fast as possible with the available power. It is much better to get 60%
of capacity out of a 30 amp alternator than 10% out of a 100 amp
alternator.

--
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