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Old 16-09-2013, 06:01   #1
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Reasonable Generator De-Rating?

I have a heavy-duty, low-speed generator rated for continuous duty -- a Kohler EFOZ 6.5. This runs at 1500 RPM to produce its rated power of 6.5kW at 230 volts. The generator head is air cooled, and is powered by a Yanmar three-cylinder, 1000cc engine. The whole thing is quite robust-looking and weighs about a quarter of a ton. It's similar in size and weight to the 13kW standby generator I have at my lake house (which however runs at 3000RPM).

I am highly dependent on this generator as my boat is kept on a mid-river mooring with no shore power. When cruising, I am mostly at anchor, and I have a lot of electrical demand on board. When I bought my boat, the generator had only 160 hours on it, but I have been racking up the hours faster than I am accumulating hours on the main engine. I am already working on 700 hours after only four years. So I want this generator to last for a long time -- I can't afford to just throw it away at 1000 hours or 1500 hours.

I have been limiting the power my boat can take from the generator to 24 amps or 5.5kW -- that's 5 amps and 1000 watts less than the rated power. I can do that with the power limiting function on my charger/inverter, through which all AC power on board is routed.

What do you electrical wizards think -- is that too much? Too little?

The same generator is capable of producing 8kW at 60 herz just by changing some breakers and turning up the engine to 1800 RPM, so it seems to me that the generator is already derated at 6.5kW. Maybe I don't need to worry about it and should just run it at 6.5kW?

On the other hand, I know that heat and load are what kills electrical devices like generator heads, so maybe it will extend life to derate the unit as I have been doing, or perhaps even more.
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Old 16-09-2013, 06:10   #2
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Re: Reasonable Generator De-Rating?

I have the 8kW, 1800 RPM, 60 Hz, 120v, 66.7 Amps AC version...

Around here these commonly run for bazillions of hours, generally more (sometimes way more) than main engine hours.

I've never had to limit amperage; given two aircon units, electric cooktop, toaster, microwave/convec oven, coffee maker -- very occasionally but not usually all on at the same time -- the whole system would likely be self-limiting if I really exceeded genset capabilities

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Old 16-09-2013, 06:15   #3
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Re: Reasonable Generator De-Rating?

I have a 5.5KW Northern Lights and run it continuously when out. I have 2177 hours on it in 11 seasons. We run our HVAC unit for both heating and cooling plus a variety of other loads. Have only replaced impellers and one belt. Change oil and filters every 250 hours and impeller at same time. You should have a long time left. I've heard of Northern Lights going 10,000 hours plus! Don't have experience with Kohler, but your Yanmar should last.
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Old 16-09-2013, 06:17   #4
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Re: Reasonable Generator De-Rating?

Quote:
Originally Posted by ranger42c View Post
I have the 8kW, 1800 RPM, 60 Hz, 120v, 66.7 Amps AC version...

Around here these commonly run for bazillions of hours, generally more (sometimes way more) than main engine hours.

I've never had to limit amperage; given two aircon units, electric cooktop, toaster, microwave/convec oven, coffee maker -- very occasionally but not usually all on at the same time -- the whole system would likely be self-limiting if I really exceeded genset capabilities

-Chris
That is precisely the same generator as mine. Apparently one of the more common models produced by Kohler.

The self limitation in your case would be operation of the breakers on the control unit.

How many hours do you have on yours? Have you ever had any trouble with it?
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Old 16-09-2013, 06:24   #5
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Re: Reasonable Generator De-Rating?

Quote:
Originally Posted by J Clark H356 View Post
I have a 5.5KW Northern Lights and run it continuously when out. I have 2177 hours on it in 11 seasons. We run our HVAC unit for both heating and cooling plus a variety of other loads. Have only replaced impellers and one belt. Change oil and filters every 250 hours and impeller at same time. You should have a long time left. I've heard of Northern Lights going 10,000 hours plus! Don't have experience with Kohler, but your Yanmar should last.
You're using yours at similar rate as I do mine, per annum. I don't run my continuously (two or three hours a day is usually enough), but I spend 90 to 100 days a year on board, rarely with shore power.

I change oil and filter twice a year, once before winter and once before my annual summer cruise, so about once in 100 hours. I change the fuel filter, impeller and anode once a year.

I don't need 10,000 hours (that would take me 50 years, and I don't expect to live that long), but I do need for it to last two or three thousand for sure.
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Old 16-09-2013, 06:27   #6
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Re: Reasonable Generator De-Rating?

Quote:
Originally Posted by Dockhead View Post
That is precisely the same generator as mine. Apparently one of the more common models produced by Kohler.

The self limitation in your case would be operation of the breakers on the control unit.

How many hours do you have on yours? Have you ever had any trouble with it?

Yep, same one, 8EOZ in our case.

Yes, could easily limit with breakers, but actually haven't had to. Just the way we use the boat seems to take care of it. For example, both aircons don't start at the same time, both fridges only run when they need to, even when we're cooking mostly everything isn't actually on at the same time, etc.

I think we're up around 800 hours or so... less than mains, in our case, partly 'cause we don't get a chance to anchor out as often as we'd like, partly because for the first several years of ownership we didn't routinely start the genset every time we leave the dock. That latter is more common here for boats like ours (which sometimes don't have DC fridges), but mostly we're finally succumbing to the 95 temps and running the aircons while underway when Summer conditions are like that.

No problems (knock wood, of course). Just had to replace the water pump this year, since it started a small leak on the shaft side. Otherwise, routine maintenance, on the engine side: new impeller every two years, oil change every year, zinc anode once a year, new fuel filters every couple years (assuming no Racor issues indicating more often is necessary), etc. Plus attention to the sea strainer, from time to time.

-Chris
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Old 16-09-2013, 06:33   #7
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Re: Reasonable Generator De-Rating?

It’s often recommended that diesel generators run at a minimum of 60% - 75% load; which insures the engine runs at maximum efficiency.
A diesel engine that operates on a light load, for long durations, runs the risk of glazing the cylinder bore.

Effects of Diesel Generator Light Load Running
Effects of Diesel Generator Light Load Running | Workspace Technology

Wet stacking: How it happens, what it does, and how to avoid it.
A close look at wet stacking*|*Plant Engineering

What happens when you run a diesel engine for too long without a sufficient load?
Running Diesels with no load.
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Old 16-09-2013, 07:05   #8
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Re: Reasonable Generator De-Rating?

Quote:
Originally Posted by GordMay View Post
Its often recommended that diesel generators run at a minimum of 60% - 75% load; which insures the engine runs at maximum efficiency.
A diesel engine that operates on a light load, for long durations, runs the risk of glazing the cylinder bore.

Effects of Diesel Generator Light Load Running
Effects of Diesel Generator Light Load Running | Workspace Technology

Wet stacking: How it happens, what it does, and how to avoid it.
A close look at wet stacking*|*Plant Engineering

What happens when you run a diesel engine for too long without a sufficient load?
Running Diesels with no load.
Thanks for that, Gord -- it's always good to remind everyone that diesels -- unlike petrol/gasoline engines -- don't like to be run without a load.

I do think, however, that you have mis-quoted your first linked source, which says that "Ideally, diesel engines should be run at least 60-75% of their maximum rated load." An ideal load and a minimum load are two different things. The 1000cc Yanmar in my generator is actually a 23 horsepower engine, so even with the generator at full load, the engine is only running at less than 50% of its max. Likewise, my main engine is rated at 100 horsepower, but might be producing only 20 horsepower at economical cruise speed. Yanmar even approves this engine for trolling at very low speeds, using even less than 20% of max. They do recommend "blowing it out" regularly with a full power run for a few minutes every so often, if you are operating it at a very low load -- which I do.

I do know that Cummins recommend a minimum load of 30% for their large gensets. I don't know whether that applies directly to gensets like ours. But I've never heard of anyone having problems with small marine gensets like ours except in cases where people run them all night long for AC on a cool night when the AC rarely comes on. I never do that (don't have AC, anyway), and bunch my power-intensive tasks so I run my genset for a couple-few hours a day at mostly probably 70% of capacity, on average, but sometimes up to the power limit I have been setting, which is 24 amps or about 80% of max.

I really like what this guy has to say about bore-glazing:



Bore glazing

He is a professional petroleum engineer and seems to look more deeply into the question than most.
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Old 16-09-2013, 07:30   #9
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Re: Reasonable Generator De-Rating?

Quote:
Originally Posted by Dockhead View Post

The same generator is capable of producing 8kW at 60 herz just by changing some breakers and turning up the engine to 1800 RPM, so it seems to me that the generator is already derated at 6.5kW. Maybe I don't need to worry about it and should just run it at 6.5kW?
RPM has a bearing on horsepower (Kw) The cheap, easy way to change frequency is to change RPM; the "real" way is to change the number of poles in the genny. Your genny can handle the additional load but can you folks use 60Hz on that side of the pond?

Many generator mfgs use the same head on several different sets, only the engines and final power delivery switchgear differ.... For instance I have a 2.2Kw set which has a 6Kw generator head. The same company builds a larger output machine with the exact same engine/gen head, by just increasing the engine RPM and changing the drive ratio (belt drive) pulleys; the gen head turns the same RPM as the small machine, only the engine goes faster.
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Old 16-09-2013, 11:23   #10
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Re: Reasonable Generator De-Rating?

Quote:
Originally Posted by capngeo View Post
RPM has a bearing on horsepower (Kw) The cheap, easy way to change frequency is to change RPM; the "real" way is to change the number of poles in the genny. Your genny can handle the additional load but can you folks use 60Hz on that side of the pond?

Many generator mfgs use the same head on several different sets, only the engines and final power delivery switchgear differ.... For instance I have a 2.2Kw set which has a 6Kw generator head. The same company builds a larger output machine with the exact same engine/gen head, by just increasing the engine RPM and changing the drive ratio (belt drive) pulleys; the gen head turns the same RPM as the small machine, only the engine goes faster.
In this case, it is exactly the same generator, same engine, same head, which can be set to produce either 6.5kW at 50hz or 8kw at 60hz (and you can choose 230v and 60hz if you like). 6.5kW is plenty for my needs. My point was that if the generator was designed for 8kW, then perhaps it is still reasonably derated at 6.5kW if I run it at its full rated load?
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Old 16-09-2013, 11:50   #11
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Quote:
Originally Posted by Dockhead View Post

In this case, it is exactly the same generator, same engine, same head, which can be set to produce either 6.5kW at 50hz or 8kw at 60hz (and you can choose 230v and 60hz if you like). 6.5kW is plenty for my needs. My point was that if the generator was designed for 8kW, then perhaps it is still reasonably derated at 6.5kW if I run it at its full rated load?
I think the answer is yes. Mainly the things that reduce life of well designed diesel generators are:

1) lack of proper periodic maintenance.

2) high ambient temperature (>40C).

3) high engine RPM (>2K) (yes, I know this is a design issue)

If you tick any or all of these boxes your generator will not last. If you avoid these problems it will likely run at max rated load 100% of the time until it reaches end of life at >10K hours.

In your case you use the inverter to support motor starting loads and that helps too. I would not worry about running your generator at full smoke all the time. Just keep it cool, give it clean fuel, do the maintenance and test the oil annually or more often.
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Old 16-09-2013, 11:52   #12
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One additional point: Inverters with power factor correction provide a resistive load to the generator. This contributes to longer life too.
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