upgrade a generator's alternator

[KDA]

I’ own a 40 year old Taiwanese trawler with a single Lehman 120. It also has a Kubota, 309cc, 4 HP diesel generator – max. 2000 RPM. When the generator was originally bought, it came with a 135 amp large frame alternator rated for 70 amps maximum output.
When I got the boat the Kubota engine wouldn’t produce enough power to get the alternator up to speed. The engine would lug and not attain high RPM. The advice I got from local “experts” was that, if I wanted to keep the Kubota engine, I needed to change to a smaller alternator. I consulted an electrician who installed a 25 amp alternator and I can now get max RPM but it won’t recharge the batteries fast enough.
I want to install the largest alternator on the generator that it can handle but I can’t find anyone who can tell me what size that is. The pulley on the engine is 5.5” and the pulley on the existing alternator is 2.65”.
There must be a formula or rule of thumb that applies here. I don’t want to just buy another alternator to see if it works. Any thoughts?

 

[O C Diver]

I'm assuming the alternator wouldn't spin up when the generator was under full load? Most generators are designed to maximize the engine HP when the generator is under full load.

There are no free lunches. If you add a larger alternator, you probably won't be able to utilize all of the generator 120 VAC capacity. Probably simpler to just plug a battery charger into the generator and practice load shedding when recharging the batteries.

Ted

 

[DavidM]

Well, here is a rule of thumb tempered by experience:

These small generators are very inefficient at converting hp to power. Your 4 hp engine is about 3 Kw. With a 50% efficiency that means it can produce about 1.5 Kw of power. At 14 volts charging voltage that means about 100 amps.

So.... I would buy a Balmar 125 amp externally regulated alternator and get Balmar's external regulator, about $800 for the package. Then I would adjust the pulley sizes so the alternator could run at 5,000 rpm while the engine runs at 2,000 rpm. This means increasing the engine pulley to about 6.5-7" with the existing alternator pulley of 2.65".

The Balmar external regulator can be adjusted to limit maximum current so it doesn't bog down your engine. But it will also charge your batteries optimally.

David

 

[KDA]

Yes, the alternator wouldn't spin up because the engine wouldn't achieve high rev's. I was getting what is called "wet stacking".
Could I get you to explain what you mean by "plug a battery charger into the generator and practice load shedding when recharging the batteries"?

 

[O C Diver]

Yes, the alternator wouldn't spin up because the engine wouldn't achieve high rev's. I was getting what is called "wet stacking".
Could I get you to explain what you mean by "plug a battery charger into the generator and practice load shedding when recharging the batteries"?

Let's assume the generator produces 4 KW (120 volts at 33 amps). If a 120 amp battery charger consumes 15 amps at 120 volts AC, that means that of the 33 amps that the generator produces, you have 18 amps remaining to use for air conditioning, water heating, or whatever you choose. Load shedding implies that the total sum of all your 120 VAC loads exceeds the capacity of the generator. So, you have to shed some. Basically it's prioritizing what gets plugged in when.

The other thing to remember when battery charging is that the amperage requirement diminishes as the batteries get closer to being full. So the 15 amp draw may diminish after an hour or two to 5 amps, allowing you to plug in another item.

If you already have a shore power battery charger, it may be a simple matter of wiring it to also run off the generator.

Ted

 

[Lepke]

Wet stacking damages an engine over time. It might be better to solve the problem and keep the big alternator. Carbon builds up in the exhaust and can foul the valve seat, injectors, and maybe the exhaust elbow. When carbon builds on the valve or seat, it holds open the valve slightly and combustion pressure is released, eventually scaring the valve or seat and causing a power loss. Don't know the engine, but there may be new tips available for the injector. A carbon buildup on the injector can ruin the spray pattern and be the original cause of the engine's power loss.
Running a generator for long hours on a light load keeps the engine from reaching full operating temperature. Wet stacking is one result, but also glazing of the cylinder walls can happen, too. When I run a generator on a light load, I turn something else on. Even if it's a heater put out on deck.

 

[BruceK]

Are there cross purposes here?
The primary purpose of the generator is to power its integral generation end to produce mains type power for appliances,chargers, etc. Opinions here suggest yours has a 3KW output.
As well, the generator engine typically has its own alternator, to charge its own battery, usually the 25 or 35A variety you were advised to replace the 135A one. Perhaps someone was using the genset in a different way, to produce lots of amps in excess of usual via the alternator, when usually the generator is used to power a battery charger to charge batteries,as well as charging its own dedicated battery with its modest alternator.
Perhaps it was an unusual inventive way of boosting output, but in my experience it`s atypical use of a generator.

 

[ksanders]

There is no practical reason to upsize the engine alternator on a properly designed generator.

The engine alternator is only connected to the generator start battery anyway.

On many/most generators the limiting factor to power production is the loading of the engine to point where the RPM and thus the frequency drops below acceptable levels. Adding more load to the engine with a larger engine alternator will not help.

One thing at WILL help with the longevity of a marine generator is oversizing the power production alternator.

Why???

Power production alternators are rated for continuous duty at a certain power production creating a rise in the winding temperature over ambient temperatures. Upsizing a generator end reduces the winding temperatures which can be exaggerated due to the hot confines of a typical marine engine room.

The good thing is that when replacing a generator end you can often upsize the unit to a larger capacity and still keep the same "frame size" which is the diamater of the generator end, and thus the mounting feet height. The extra capacity is made up in the length of the generator end which is something to consider if you have a sound enclosure or tight space requirements.

 

[Russell Clifton]

I agree with BruceK. What are we missing here?

 

[FF]

" When the generator was originally bought, it came with a 135 amp large frame alternator rated for 70 amps maximum output."

Perhaps over the decades the V reg was changed from one that limited the amperage to 70A and one that will try to charge at the 135A rate?

Might explain the overload on the genset motor.

 

[ranger58sb]

If you already have a shore power battery charger, it may be a simple matter of wiring it to also run off the generator.

Yes.

Charger probably already installed. Generator already running anyway. Easy.

-Chris

 

[DavidM]

This has been a confusing thread, including my post above. I am pretty sure the OP is talking about a DC generator not AC which many posts above assume.

So some more questions: Are we talking about DC and if so does the original alternator have an external regulator and if so what kind. If the answers to both are yes, reinstall the original alternator and install a Balmar regulator. That will let you adjust the current to stay within the diesel's capability.

Another probably simpler way is to reinstall the original alternator but decrease the pulley size on the engine to slow down the alternator to stay within the diesel's power limits.

But 309 ccs and 4 hp at 2,000 rpm is a really, really small, low power diesel. Wikipedia notes a 309 cc Kubota but one that produces 7.7 hp at 3,000 rpm. Perhaps with a few Kubota parts (purchased at a Kubota tractor dealership) you can get it up to these specs which probably means an injector pump replacement and maybe the injector.

With that much power and rpms you should have no problem driving your original alternator.

Or maybe the engine is totally carboned up and once it is really burned up and blown out (by running at wot with medium load) it will produce 7.7 hp at 3,000 rpm.

As noted by some above, the problem may be more in the engine than the alternator.

David

 

[KDA]

DavidM, I suspect I may be the source of the confusion. Sorry. Yes, it is a DC generator and intended to recharge the batteries after a day/night on the hook. Unfortunatley, when the small alternator was installed, I was advised that the original one was of no value so it was thrown out. I do not know if it had an external regulator.
So, I expect to have to buy a new alternator and maybe an external regulator. Having said that, should I find a 135 amp large frame alternator and an external regulator or should I find the 135 amp alternator and change the pulley size on the engine? Since I am starting from scratch, should I even be looking for a 135 amp alternator or is there something else out there that would be better suited to my system?

 

[DennisB1]

Your generator can turn a 100 or 200 amp capable alternator under no load. If you look at the alternator output vs rpm chart from Balmar or another supplier you can see that the alternator may need some rpm higher that what your set up is capable to achieve the advertised full load output . The alternator rpm based upon the pulley diameters on the engine and alternator ie. 5.5 inch engine and 2.65 inch alternator = engine rpm x (5.5/2.65) or 2.07 which says the alternator rpm is about 4000 rpm for a 2000 rpm engine. Look at the capacity of the alternator that is the max capability. For continuous operation you would want to derate the output to prevent alternator overheating which can be accomplished with a multistep remote regulator which controls the field voltage to the alternator. Your original set-up suggests that it was set up that way. Select an alternator with good cooling capacity and use a regulator that controls the field current to keep the charging current in an acceptable range for the engine and alternator. I would think the if you could achieve a 70 output and 14.7 vdc with a smart regulator that it would serve you charging requirements. Look at a Balmar or Warpspeed regulator for more details.

 

[DavidM]

I agree with what Dennis is saying above. Buy a Balmar alternator and regulator that matches your engine's mount of about 100 amps. Then play with the engine pulley size to match the engine's output to the alternator's power output.

Remember I hp = 745 watts and alternators are about 50% efficient.

David

 

[FF]

I would visit my local 18 wheel truck supply store. Their 135A Prestolite alts are usually Leece Neville , which is excellent. Should be under $150 , fine for a diesel powered boat.A gasoline powered boat requires a sealed alt , far bigger bucks.

You will do best with an after marked V regulator , as they will be adjustable and 3 or 4 stage for best battery charging, the auto or truck V regs suck at batt charging and do best at operating lights and blowers going down the road.

You will also need to purchase a pulley for the alt , come in a number of diameters and belt sizes , bring your old belt to the shop.

Also observe the rotation direction of the installed alt and purchase the fan required.