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Old 17-10-2015, 04:07   #1
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Derating An Alternator

Is there any way to de-rate a 200.00 Leece Neville/Prestolite 8MR series 105A alternator?
The goal is not to run it at full chat in the tropics so as to kill it prematurely
I have a 675 AHr Trojan flooded bank so about 70A from the alternator would seem about right

I do not have a Balmer regulator with belt/current limiting, and not keen on buy a new regulator unless there is no other way.
I am also not keen on spending 5-1000.00 on an alternator that can run flat out for long periods.

My application is a sailboat so while the engine is an auxiliary there are times when I do motor all day, or the alternator will be called upon for full output to charge up a depleted battery bank in tropical temperatures.
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Old 17-10-2015, 10:30   #2
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Re: derating an alternator

There was a recent thread on this topic where people were using rheostats (variable resistors) to vary the field current to the alternator. I don't know if that works since I have never done it nor I have I seen it in the field. But they claim it works.

The problem with that is you forgetting to do it, and, where do you set it.

As a battery gets full the alternator becomes self-regulating as it will not continue to pump out full amps if the batteries are resisting the current. But with a large 675Ah bank, if they are down, it will take a 105A alternator quite a while to get them up very far. You may have to get a regulator with a temperature sensor to the alternator. And it might not hurt to have one to the batteries too.
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Old 17-10-2015, 10:41   #3
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Re: derating an alternator

Used the rheostat own work boats back in the 60's and the system works. It requires constant watching or you will have to buy a new set of batteries. A regulator would probably be cheaper in the long run.


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Old 17-10-2015, 10:45   #4
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Re: derating an alternator

I would think that buying a $250-$300 regulator would be the easiest and most reliable way to go. this also keeps you from having to remember to do something, it's automatic.

https://www.google.com/shopping/prod...bSLoXYeJ-mm9AJ
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Old 17-10-2015, 11:04   #5
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Re: derating an alternator

Alternators with a regulator never run at full output for very long. As soon as the battery voltage starts to rise, the regulator cuts the output back. I ran for several years without a regulator, but as has been previously stated, it took constant attention. Buying the correct regulator will make your cruising life much easier, and much less chance of cooking your alternator from forgetting to check things. Using rheostats is normally used to get MORE OUTPUT (faster charge) and causes more heat in your alternator. Just making sure your engine compartment is properly ventilated will go a long way in making all components last longer. ______Grant.
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Old 17-10-2015, 11:17   #6
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Re: derating an alternator

You need to know what it IS putting in , and not what it is rated at.
It might be only putting in 80 A initially and then fall off fast. I agree with your rate of charge for a long life of the bank. I also have 675Ah, (at 24v) and I always aim for 12% of bank size as max charge rate.
If it is still too much, the easiest derate will be a slightly larger pulley.
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Old 17-10-2015, 11:53   #7
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Re: derating an alternator

How Alternators & Regulators Work PLUS External vs. Internal Regulators (by Maine Sail): Musings Regarding External Regulation | SailboatOwners.com Forums
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Old 17-10-2015, 12:28   #8
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Re: derating an alternator

I think if you monitor the amps out of your alternator you will almost never see it at the rated 105 amps, and if it does get there it will be dropping within minutes. Assuming you have a normal regulator, I think you are worried about something that is really a non issue.
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Old 17-10-2015, 13:02   #9
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Re: derating an alternator

it seems to me you could use a rheostat (or large ceramic resistor!) in the field wire along with the regulator ... so you still have some regulator control....???
NAPA sells a rheostat meant for old car heaters on a bracket to mount under dash...
Often a 100 amp alternator even with a good regulator will only put out max amps for 15-20 minutes though anyway.
Find yourself a used Ample Power Next Step regulator and you can adjust output I think...
Yes a Rheostat works. Done it a few times testing 12V gen sets etc.
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Old 17-10-2015, 14:00   #10
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Re: derating an alternator

Quote:
Originally Posted by Cheechako View Post
it seems to me you could use a rheostat (or large ceramic resistor!) in the field wire along with the regulator
cheech, he wants to DErate it.

As Grant said:

Quote:
Originally Posted by gjordan View Post
Using rheostats is normally used to get MORE OUTPUT (faster charge) and causes more heat in your alternator.
The Spa Creek AutoMac did just that. I used to have one. Worked fine if you learned how to properly use it.

If the OP reads the link I provided, he may come to the same conclusion that Dennis reported.

His boat, his choice.
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Old 17-10-2015, 14:41   #11
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Re: Derating An Alternator

I just read the Mainsail link from post #7. Excellent explanations. It confirms my feelings that so many people spend lots of money on things that dont really help, or help so little that it doesnt matter. Grant.
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Old 17-10-2015, 14:52   #12
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Re: Derating An Alternator

And if you really want to DErate the alternator, just put a toggle switch in the field wire and turn it off.

One could do this with the cockpit panel key switch (in most cases), but that would turn off your engine hour meter.

If you turn off the field wire, you are simply telling the regulator to stop telling the alternator to provide output. Your tach will drop to zero, too.
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Old 17-10-2015, 15:14   #13
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Re: derating an alternator

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Originally Posted by Stu Jackson View Post
cheech, he wants to DErate it.

.
Ahh... right, I guess lowering the voltage to the field puts out more amps? Too long for me...
Yes on toggle switch, done it on a couple of my boats.
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Old 17-10-2015, 15:23   #14
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Re: Derating An Alternator

No, raising the voltage on the field increases the output - you had it right the first time.

Cutting the field altogether would derate the alternator to 0 - I don't think that is what the OP had in mind.

If this is an internally-regulated alternator, the OP does not have to worry about running it for long times in the tropics - it will take care of itself just fine.

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Old 17-10-2015, 21:01   #15
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Re: Derating An Alternator

The link is post 7 is required reading and states what I want to do:

NOTE: I generally prefer to go much larger with the alt than is needed then derate the output with the regulator. This allows it to work less hard and run cooler. This can be done with Balmar regulators.

I do not see a hot rating for the L/N 105A 8MR series small frame alternator?

Is the Blamer the only regulator with belt/amp manager?
I had been leaning towards a Mark Grasser regulator due to the good things I have read and my past experience with premature Balmer AR5 failure.


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