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From: Arild Jensen (no email)
Date: Thu Oct 02 2003 - 12:14:36 EDT
D. Hess wrote:
."The dealer claims about one gallon per hour at this speed and frankly,
this is where the boat will spend the bulk of it's running time."
Know Your Objective,
REPLY
While this may be the speed at which a boat may spend most of its running
time, this may not be the speed most often used when charging such as when
anchored out. This is where alternator selection and installation often
falls short of the mark.
I know of one boat where they burned up three 200 amp alternators before
calling me in to diagnose the problem.
They burned up another alternator afterwards because the owner disbelieved my
recommendations.
The total cost of that ill considered choice, amounted to about $15 K before
the matter was finally resolved fully.
The basic problem stemmed from the fact the owner had chosen this particular
model based on the max electrical output curve.
The builder to their credit, installed the alternator with due regard for
all parameters and selected a pulley accordingly.
What is not mentioned in the spec sheets is the fact alternators have a
maximum RPM limit beyond which they must not be spun.
Consequently the builder selected a pulley which limited the rotor RPM to
less than the max speed when the engines were running at full throttle l.
The owner was attempting to pull maximum current out of the alternator at
near the minimum RPM with the boat doing 7.5 knots.
This put the alternator into an overloaded condition at all times and due to
the slower RPM the shaft fan was not delivering sufficient cooling air.
The installation was further complicated by several additional design and
wiring errors not to mention a problem created by the engine builder who
provided a fan belt shroud which effectively blocked any air flow from the
pulley mounted fan blades.
You should be aware that many large frame alternators should not be spun at
more than about 5000 RPM rotor speed.
I realize that the curves mr D. Hess supplied shows a curve that advances
well beyond this point, so it occurred that people might assume this was as
typical situation.
As an example, Balmar does offer one model of alternator which is guaranteed
to run at 10,000 RPM
This alternator was specifically designed to be a drop in replacement on
gasoline fuelled Mercruiser engines that run a t 4000 RPM and the pulley
ratio is typically about 2.5 to 1
The alternator is limited to 100 amps output. When used on slower turning
engines, a larger ratio is required to give suitable outputs when doing a fast
idle such as when charging at anchor.
conversely you may have to settle for less than maximum output when charging
at anchor and only idling the engine.
Cheers
Arild
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