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Old 03-24-2020, 06:56 PM   #1
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Stupid question of the day...Alternator

I should know this, have been living aboard and cruising full time for over five years...but it appears I donít know, but Iím sure I can get the answer here.

I have a Proline 165 amp alternator, dual belt. Does the current out it it depend on my speed, the requirements or something in between ? While running at my normal 1000 rpms it seems to put out about 50-55 amps.As the demand for my house band goes up, I donít notice an increase in amperage from the alternator.

Is that because the alternator does not see the demand ? Doesnít care ? Or bad alternator?

Guess I try in the morning when the batteries are low, but I typically run my genset in the morning and bring them back up.

Thanks in advance, we are still in Mexico self quarantining inn Barra De Navidad. The plans for Panama got shut down with the virus, better luck in the fall

Mark
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Old 03-24-2020, 07:04 PM   #2
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Originally Posted by Dswizzler View Post
I should know this, have been living aboard and cruising full time for over five years...but it appears I donít know, but Iím sure I can get the answer here.

I have a Proline 165 amp alternator, dual belt. Does the current out it it depend on my speed, the requirements or something in between ? While running at my normal 1000 rpms it seems to put out about 50-55 amps.As the demand for my house band goes up, I donít notice an increase in amperage from the alternator.

Is that because the alternator does not see the demand ? Doesnít care ? Or bad alternator?

Guess I try in the morning when the batteries are low, but I typically run my genset in the morning and bring them back up.

Thanks in advance, we are still in Mexico self quarantining inn Barra De Navidad. The plans for Panama got shut down with the virus, better luck in the fall

Mark


Iím not an electrician but what you are seeing is normal. Higher rpm gives higher amps.
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Old 03-24-2020, 07:21 PM   #3
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Yep, the output of the alternator is related to the speed of the engine. There's work to be done. That energy doesn't come out of nowhere..... As our friends down south say..."No comida gratis"!!!
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Old 03-24-2020, 07:25 PM   #4
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Yes alternator output is rpm dependent. If you bring engine rpm up to say 1800 then the output current should come up if not limited by something else like voltage or temperature.
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Old 03-24-2020, 07:58 PM   #5
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Would I be wrong to assume that my 1982 boat with a delco remy alternator has more in common with an automotive alternator than a more modern boat and alternator system. I haven’t started working on the electrical system yet, but I would bet my 100 amp alternator is probably putting out 15-20 amps at 800-900 idle rpm. I would also assume that my engines max rpm would not bring the alternator anywhere near its max output.
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Old 03-24-2020, 08:04 PM   #6
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Yes and no. Yes there is a minimum RPM for them to start producing. There is an output curve that corresponds to RPM, to a point. There is a limit where the RPM can continue to increase without the output increasing. Most better alternator have output graphs available. The graph refers to the RPM of the alternator, not the engine. Carrying this further, you can increase the output of an alternator with the same engine RPM by charging the ratio of the pulleys. As an example, if you have an alternator that reaches full output at 4,000 RPM, if the pulley on the engine is the same as the alternator (1:1), the engine would have to reach 4,000 RPM for full alternator output. If the engine pulley is twice the size of the alternator pulley (2:1), with 2,000 engine RPM you get full alternator output. If the engine pulley is 3 times the size of the alternator pulley (3:1), with 1,333 engine RPM you get full alternator output.
My boat engine alternator has a 3:1 ratio and get full output at around 1,300 RPM. If you're trying to get full output from the alternator, spinning the alternator faster also improves cooling.

Does your setup include a 3 or 4 stage external regulator?

Ted
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Old 03-24-2020, 08:27 PM   #7
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Well kinda! The alternator's current output is a function of the demand on it - usually the difference between the battery state of charge voltage and the voltage regulators setting. The alternator's ability to meet that demand is usually a function of engine RPM. (and its design) As the state of charge improves, the current levels will start to fall off.

LSS - there are at least two factors that determine Alternators current output Battery SOC and RPM.

Sound like your system is operating normally.
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Old 03-24-2020, 08:52 PM   #8
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So clearly the initial question was actually a smart question - but what about this one:. Because diesel engine have high torque, would a large alternator allow for running the air conditioner while underway without running generator?
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Old 03-24-2020, 09:12 PM   #9
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So clearly the initial question was actually a smart question - but what about this one:. Because diesel engine have high torque, would a large alternator allow for running the air conditioner while underway without running generator?
Torque has nothing to do with it.

I have a 220 amp alternator that will produce full output continuously at 14 volts, which is about 3KW. I can run it through my inverter and produce about 25 amps at 120 VAC. So the answer is yes. In fact, I did that on my charter boat to air condition the bunk room on the way out and back.

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Old 03-24-2020, 09:13 PM   #10
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So clearly the initial question was actually a smart question - but what about this one:. Because diesel engine have high torque, would a large alternator allow for running the air conditioner while underway without running generator?
Check out the SeaPower generator/alternator.

I will say this....diesel engines do in fact have lotsa torque....which means you need hardware to handle it. The brackets that held those SeaPower units never held up. I had two different friends that had them and they were nothing but trouble...not to mention RPM demands. The brackets were always slipping or breaking. The only way to really do it would be to do it through an inverter and a big battery bank to act as some sort of a buffer.
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Old 03-24-2020, 09:51 PM   #11
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I concur, was a smart question, a Captain needs to understand how the systems on his boat operate and what is normal vs abnormal.

OCD is also correct - (don't confuse torque with horsepower). If you have the horsepower(KW), you can use it for whatever - AC (BTU) or prop thrust. How you move that horsepower around efficiently is usually the issue. Converting HP to KW to BTU/H with alternator/battery/inverter system is relatively efficient. Running a compressor directly off the engine (like my truck) is even more efficient.

However, I don't normally need AC when underway - even here in Alabama - humidity center USA - just open the windows. At night on the hook is another story. Fire up the very efficient genset, that burns less than 0.25 gallons of diesel an hour - a shot of JD and sleep like a baby - even in mid August.
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Old 03-25-2020, 05:41 AM   #12
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The alt RPM can certainly be one limit to charge output , the most common limit is an auto brain V regulator , the 3 or 4 stage is best for discharged deep cycle house batts..

Other limits can be the alt output hot , may be half of its cold rating , or the wiring to the batts and not carry the output.

Battery combiners can have ratings from 65A to over 500A .
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Old 03-25-2020, 07:12 AM   #13
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Swizzler- Your alternator "tries" to maintain voltage at the regulator setpoint, usually about 14Vdc. The amps it puts out will vary with system load and battery state of charge.

As long as batt voltage is 13.5-14Vdc, it is doing its job just fine and amps are what they are. If batt voltage is like 12.0Vdc, load is outrunning alt capacity.

And yes, the peak output of an alternator does go up with rpm. So your 165A machine might only be capable of 55A with engine at 1000rpm. But as long as volts are near 14.0, it is still good enough.

Pay attention to batt bank volts, not alt amps.
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Old 03-25-2020, 02:32 PM   #14
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ANd Blue Seas does make a small little panel mount ammeter that you can hook up to your alternator and read amps. I had them installed on my previous boat. It only went up to 99.9 amps and would read OL(overload) above that. I had 160amp alternators. It rarely went to OL but it would if I sat on the inverter for an extended amount of time with the engines off. The alternators did not share the load either. One would be at 0 after things settled and the other at a very low amperage. I noticed something similar when two battery chargers are on one bus. Once it got below a certain point, one charger would go to 0.
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Old 03-25-2020, 04:59 PM   #15
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Quote:
Originally Posted by Dswizzler View Post
I should know this, have been living aboard and cruising full time for over five years...but it appears I donít know, but Iím sure I can get the answer here.

I have a Proline 165 amp alternator, dual belt. Does the current out it it depend on my speed, the requirements or something in between ? While running at my normal 1000 rpms it seems to put out about 50-55 amps.As the demand for my house band goes up, I donít notice an increase in amperage from the alternator.

Is that because the alternator does not see the demand ? Doesnít care ? Or bad alternator?

Guess I try in the morning when the batteries are low, but I typically run my genset in the morning and bring them back up.

Thanks in advance, we are still in Mexico self quarantining inn Barra De Navidad. The plans for Panama got shut down with the virus, better luck in the fall

Mark
A lot of good answers. Did you get the answer you were looking for?

What I see is a 165 amp alternator that is only producing 50-55 amps. Either it is not working at full potential OR the amp load does not increase regardless of RPM. The 50-55 amp could be all the demand OR you do not have a 165 amp alternator anymore.
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Old 03-26-2020, 03:53 PM   #16
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Balmar Power Curves

This may be of use to you. Balmar, shows their ratings of their alternators based off of hot/cold and RPM's. What you will see, is that unless you are running your engines at WOT you will never actually get close to the rated amounts. Probably HALF if your lucky...
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Old 03-26-2020, 03:53 PM   #17
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Here's a link if you want to read more...

https://www.defender.com/pdf/Balmar_2016.pdf
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Old 03-26-2020, 06:19 PM   #18
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This may be of use to you. Balmar, shows their ratings of their alternators based off of hot/cold and RPM's. What you will see, is that unless you are running your engines at WOT you will never actually get close to the rated amounts. Probably HALF if your lucky...

Keep in mind, they're assuming an alternator pulley ratio of 2. On fairly low revving engines, spinning the alternator at 3, 4 or even 5 times engine speed isn't uncommon while still keeping the alternator below max RPM.
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Old 03-26-2020, 08:28 PM   #19
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Just a follow up, and of course you guys were right on. This morning with the batteries low from carrying the house load all night, we needed to move about5 miles. Perfect for a test. At 900 rpm solid 14 volts 55 amps, at 1500 rpm 110-120 amps. Now I know my Detroitís loveto run at that speed, but my retirement account says we run closer to 900-1000 most of the time. I do blow the engine out during long runs and every few days on short runs, but typically cruise between 6-7.5 knots . Thanks for the info
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Old 03-26-2020, 08:53 PM   #20
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Just a follow up, and of course you guys were right on. This morning with the batteries low from carrying the house load all night, we needed to move about5 miles. Perfect for a test. At 900 rpm solid 14 volts 55 amps, at 1500 rpm 110-120 amps. Now I know my Detroitís loveto run at that speed, but my retirement account says we run closer to 900-1000 most of the time. I do blow the engine out during long runs and every few days on short runs, but typically cruise between 6-7.5 knots . Thanks for the info
Might be worth investigating whether you can reduce the alternator pulley. As an example, if the engine has a 6" pulley and the alternator has a 2.5" pulley, the ratio would be 2.4:1. Reducing the alternator pulley to 2" would give you a 3:1 ratio. This would mean at a 1,000 RPM cruise, the alternator would have the same output as if you're cruising now at 1,250 RPM. Could be a nice gain for the price of a pulley and probably new belt(s).

Ted
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