We have a 55A Hitachi alternator on our 2 cyl 13HP Yanmar. It is charging 2
banks of batteries - 200AH in house and 100AH in Starting.

Today, I had an alarm sound when I started the engine (turned out to be low
oil level!), but it caused me to check the engine belts etc.

The alternator belt was loose, so I adjusted it - it had been slipping. I
noticed that despite only having run for about 5-10 minutes at low rpm
(~1200), the alternator was quite hot to the touch - I would guess in the
50-60 deg C range (120-140F), so I would keep my hand in contact with it.
The engine block was not nearly as hot - I could put my hand on it
anywhere - just warm.

There is a combiner, so both banks MAY have been connected. Refrigeration
was running at time. Batteries were not fully charged - maybe just below
12v. I don't have an ammeter, so don't know what amps are being put out.

Should the alternator run that hot?

Would low speed caused by slipping belt cause overheating?

If not, what could cause be? Or is it normal?

GBM

GBM wrote:
We have a 55A Hitachi alternator on our 2 cyl 13HP Yanmar. It is charging 2
banks of batteries - 200AH in house and 100AH in Starting.

Today, I had an alarm sound when I started the engine (turned out to be low
oil level!), but it caused me to check the engine belts etc.

The alternator belt was loose, so I adjusted it - it had been slipping. I
noticed that despite only having run for about 5-10 minutes at low rpm
(~1200), the alternator was quite hot to the touch - I would guess in the
50-60 deg C range (120-140F), so I would keep my hand in contact with it.
The engine block was not nearly as hot - I could put my hand on it
anywhere - just warm.

There is a combiner, so both banks MAY have been connected. Refrigeration
was running at time. Batteries were not fully charged - maybe just below
12v. I don't have an ammeter, so don't know what amps are being put out.

Should the alternator run that hot?

Would low speed caused by slipping belt cause overheating?

If not, what could cause be? Or is it normal?

GBM

They get hot - easily over 120-140F. How hot is "normal" for your
setup you'll have to figure out. My guess is that the slipping itself
adds to the heat. Certainly I usually notice slipping on mine when I
smell the burnt rubber.

BTW,your alternator and batteries are not too large, but still they
can drain 2-3 HP from from your engine, so if your batteries are very
low, don't be surprised if you engine overheats if pushed hard. My 18
HP 2GM with a 100 Amp alternator will run 15 degrees hotter and even
refuse to rev up fully when charging. Fortunately, I can turn down
the output to 30 Amps if that suits my pans better.

GBM wrote:

We have a 55A Hitachi alternator on our 2 cyl 13HP Yanmar. It is
charging 2
banks of batteries - 200AH in house and 100AH in Starting.

Today, I had an alarm sound when I started the engine (turned out to
be low
oil level!), but it caused me to check the engine belts etc.

The alternator belt was loose, so I adjusted it - it had been
slipping. I
noticed that despite only having run for about 5-10 minutes at low rpm
(~1200), the alternator was quite hot to the touch - I would guess
in the
50-60 deg C range (120-140F), so I would keep my hand in contact
with it.

Couple of things.

Yes, your alternator getting hot is quite normal.

You want to idle the engine around 1,400 RPM to prevent "wet stacking"
which happens when the engine is not hot enough provide combustion of
all the fuel in the chamber and that leads to VERY expensive repair
bills.

The sheave ratio between engine an alternator is about 2:1, so at
1,400 engine idle, alternator is about 2,800 RPM and probably putting
out maybe 25-30 amps, if you are lucky.

The 1,400 RPM will also help minimize belt slippage.

My guess is the 55 amp output will require an alternator RPM of
between 5,000-6,000.

It's just the nature of the beast.

Have fun.

Lew

On Fri, 14 Jul 2006 21:02:47 -0400, "GBM"
wrote:

We have a 55A Hitachi alternator on our 2 cyl 13HP Yanmar. It is charging 2
banks of batteries - 200AH in house and 100AH in Starting.

Today, I had an alarm sound when I started the engine (turned out to be low
oil level!), but it caused me to check the engine belts etc.

The alternator belt was loose, so I adjusted it - it had been slipping. I
noticed that despite only having run for about 5-10 minutes at low rpm
(~1200), the alternator was quite hot to the touch - I would guess in the
50-60 deg C range (120-140F), so I would keep my hand in contact with it.
The engine block was not nearly as hot - I could put my hand on it
anywhere - just warm.

There is a combiner, so both banks MAY have been connected. Refrigeration
was running at time. Batteries were not fully charged - maybe just below
12v. I don't have an ammeter, so don't know what amps are being put out.

Should the alternator run that hot?

Would low speed caused by slipping belt cause overheating?

If not, what could cause be? Or is it normal?

GBM


Any electronic equipment you can place your hand on for 30 plus
seconds should have a good service life. The electronics in question
would be the rectifier banks, if built in. A slipping belt
provides heating to the belt and pulleys.
An alternator reaches charging voltage sooner than an old time
generator, but self heating is roughly proportional to charging
current, which is lower at low speed.

This situation you describe does not seem specially iffy, but if you
are this anxious, get a spare alternator and a spare belt and sleep
sound

Brian Whatcott Altus OK

"GBM" wrote in news:caXtg.46517$Uy1.96
@read1.cgocable.net:

Should the alternator run that hot?

Sure. They're not 100% efficient. The core heats up from the magnetic
loading, the windings with 100 amps going through them heat up because
they are wound so tight. It'll smoke if it gets too hot and the windings
will turn black. The rectifier diodes mounted to the case to cool them
also make the case quite hot. The diodes can withstand 10 times what
your hand will fry, too.

Would low speed caused by slipping belt cause overheating?

A belt slipping over a steel pulley gets REALLY hot. Lots of heat is
transferred from the pulley to the rotor bearings on the pulley ends,
heating the case of the alternator, I presume.

If not, what could cause be? Or is it normal?


If it's not smoking or smelling like it's cooking insulation, it's
normal.

Test this in your car. Start your car and turn everything on, especially
headlights and air conditioning fans wide open. Drive to the store.
When you get there, shut down the engine, open the hood and see if the
alternator is hot. Don't get burned and blame me, ok?


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