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How Hot should an alternator get?



 
 
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  #1  
Old July 15th 06, 02:02 AM posted to rec.boats.cruising
GBM
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Posts: 48
Default How Hot should an alternator get?

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


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  #2  
Old July 15th 06, 02:18 AM posted to rec.boats.cruising
Jeff
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Posts: 1,301
Default How Hot should an alternator get?

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.
  #3  
Old July 15th 06, 03:30 AM posted to rec.boats.cruising
Lew Hodgett
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Posts: 348
Default How Hot should an alternator get?

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
  #4  
Old July 15th 06, 03:25 PM posted to rec.boats.cruising
Brian Whatcott
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Posts: 812
Default How Hot should an alternator get?

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
  #5  
Old July 15th 06, 10:06 PM posted to rec.boats.cruising
Larry
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Posts: 5,275
Default How Hot should an alternator get?

"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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  #6  
Old July 16th 06, 03:05 AM posted to rec.boats.cruising
GBM
external usenet poster
 
Posts: 48
Default How Hot should an alternator get?


"You" wrote

Should the alternator run that hot?


that's why they put pullys with fans built-in on alternators,
to blow some cool air thru the alternator windings.......

Would low speed caused by slipping belt cause overheating?


Not having enough air blowing thru the alternator, due to
slow speeds will certainly cause the alternator to get Hot.....


Possibly - Perhaps the heat output of the alternator and the cooling
produced by the fan are not matched. But, would running at a higher speed in
fact result in a cooler alternator?

Heat produced is proportional to square of current, so heat produced should
increase dramatically as speed increases. As the current (speed) increases,
then cooling capacity also needs to increase.

It would be interesting to see if typical alternator fan air flow increases
in proportion to the heat produced. I somehow doubt it - they are more
likely sized for the maximum and should therefore provide more than enough
cooling at idle speeds.

Thanks for all the input - I won't worry about my hot alternator and I do
carry the old 35A unit as a spare along with several belts!

GBM


  #7  
Old July 19th 06, 12:26 AM posted to rec.boats.cruising
Lew Hodgett
external usenet poster
 
Posts: 348
Default How Hot should an alternator get?

Larry wrote:

This only applies if you're going to leave your engine idle for

HOURS, not
minutes.....sheez. Go to any rest area and walk around the sleeping
truckers. See how many of them have the engine idling at 1400 RPM to
prevent "wet stacking" with the heat or air conditioning running

all night
while they sleep it off. What? None of them?! Hmm......


Sounds like you need to get your ears recalibrated.

In Alaska, it's so cold in winter they never shut them

off.....well, maybe
will now that fuel oil is headed for $10/gallon....


My father never shut his truck off except for oil changes from
November to April, and he was a long way from Alaska.

Lew
  #8  
Old July 19th 06, 03:34 AM posted to rec.boats.cruising
Larry
external usenet poster
 
Posts: 5,275
Default How Hot should an alternator get?

Lew Hodgett wrote in news:z6evg.1737$157.1166
@newsread3.news.pas.earthlink.net:

My father never shut his truck off except for oil changes from
November to April, and he was a long way from Alaska.

Lew



Did he run it over 1400 RPM all the time to prevent "Wet Stacking"?

  #9  
Old July 19th 06, 04:01 AM posted to rec.boats.cruising
Lew Hodgett
external usenet poster
 
Posts: 348
Default How Hot should an alternator get?

Larry wrote:


Did he run it over 1400 RPM all the time to prevent "Wet Stacking"?



Didn't have to worry about it, back in those days his trucks were
gasoline powered.

Lew

  #10  
Old July 20th 06, 02:04 AM posted to rec.boats.cruising
Ruskie
external usenet poster
 
Posts: 7
Default How Hot should an alternator get?

On Sat, 15 Jul 2006 17:11:08 -0400, Larry wrote:

Way too many boat diesel books are being sold.....



Speaking of diesel engine books. I read that cruising sailbooats with
diesel engines should have a heavy-duty blower installed in the engine
compartment, with proper venting, to extend longevity of all related
components.

Sounds like a good idea. Yet, as far as I know, not even Baby-Blake
endowed Swans bother with this.
 




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