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Old 17-04-2016, 15:02   #1
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Talking High RPM Generator - is 60Hz worse?

Hi there,

Following is bothering me: Are small, high RPM generators really so unreliable or is this special true for our Americans (aka 60Hz) friends as they run mostly at 3600 rpm? I see very often small high rpm gennys in European boats but not so much in American boats, one reason could be to hunger for more power, simply being wiser and have more $$$ Or is it simply the fact that this small machines run better on 50Hz??

This is the thinking behind my question:

The desirable (but heavier and more $$) low RPM Gensets run at 1500rpm (50hz) or 1800rpm (60hz).
- I think this RPM range is close to the sweet spot most diesel would love to run. Less vibration, stress and so on.
- The difference is that a 60Hz Genset usually has more KW than the 50Hz but can use up to 30% more fuel per h.

Small 3000 or 3600rpm Generators
- That’s where I’m not sure: I think (or maybe my ears do) running a small diesel with 3000rpm is right on the redline.
- Going up to 3600rpm for 60Hz sounds like overkill.

I know the engineers will say, the engine is designed for that, but close to 4000 rpm, for hours?? Could this be a reason that I believe I hear more of small genset problems in US forums or do you think there is no difference between 3000 and 3600rpm?

Appreciate your feedback
cheers
Marco
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Old 17-04-2016, 15:28   #2
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Re: High RPM Generator - is 60Hz worse?

It all boils down to the difference between "intermittent duty", and "continuous duty". The low speed ones are much less stressed and are made to be used 24/7 if necessary. For me almost more important than that, is how they sound -- they sound "happy", as if they would gladly be left on all year. And not "screaming", just begging you to shut them down again. The three cylinder ones are particularly good -- they have a pleasant loping rhythm which doesn't disturb anyone.

The way I use my generator is in fact "intermittent duty" -- I run it a couple of times a day, for a couple of hours. Probably a 3000 rpm job would work. But I know I just wouldn't like it. And that's reason enough to spend more money and accept more weight.

And also the low speed ones are objectively much more reliable, something important for us.
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Old 17-04-2016, 15:40   #3
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Re: High RPM Generator - is 60Hz worse?

Dockhead - that was a great, if non-technical, description! I've had two Fischer-Panda gensets and they both sounded like they were running somewhat beyond their means; although they worked well enough.

I've now got a Cummins-Onan that runs at half the RPM and no matter what load I put on it, the sounds from it seem to tell me "<yawn> What, is that all you want?"...

What it tends to boil down to is that the higher-speed generators pack a lot more Kw per weight ratio into the generator. For boats that are space- or weight- constrained the faster gensets are great. But the price one pays is that there's a lot more mechanical stress placed on those units which consequently tends to mean a shorter lifespan and more problems during that lifespan (all other factors being equal, of course).
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Old 17-04-2016, 15:47   #4
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Re: High RPM Generator - is 60Hz worse?

Many US gens used to be 3600 rpm. But not anymore, now many are 1800.
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Old 17-04-2016, 15:47   #5
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Re: High RPM Generator - is 60Hz worse?

The low rpm generators are much easier to live with. Their exhaust note is lower and more easily muffled. For the most part they are also designed to last longer. Simply less wear at the lower revolutions. The higher rpm variety can be reliable, read Honda, but many are designed/intended to be run for short term, intermittent use like for seldom occurring power outages. The higher rpm generators tend to be lighter so better for portable use. Our low rpm Kohler gas powered generator ran for 10 years with every other day 8-10 hour use and was still going strong when we sold it. The 3600 rpm generator that preceded it blew up in a year.

Haven't heard of any issues with the cycles though it stands to reason that the rest of the world 50hz generators might be slightly more long lived because of the lower rpm to get 50 cycles. For the most part, you get what you pay for in generators. The Kohler 1800 rpm generator cost 3 times or more than a typical high rpm unit.
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Old 17-04-2016, 15:59   #6
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High RPM Generator - is 60Hz worse?

Honda is a variable speed generator as it is an inverter/ generator.
I have never looked, but have always assumed it was a DC gen run through an inverter to get AC at 60 Hz.
If this is indeed how they work, then RPM increases to increase amperage, not Hz rate.
Then there are the machines that drive generators through gearing, the so called mid RPM machines. I believe my Nexgen is run at 2850 RPM, but belt drive the generator at 3600 RPM.

All things being equal, less RPM usually results in less noise and vibration of course, also usually results in a much heavier engine to get the same HP.
There just ain't no free lunch
I would like to see a larger inverter - generator, one that could almost idle if power demand were low.


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Old 17-04-2016, 16:13   #7
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Re: High RPM Generator - is 60Hz worse?

Many small engines designed for generator use are optimised to run all day at 3600 rpm. It stands to reason most high speed (aka 2 pole) generators (or alternators, as is the correct term) would choose these motors and are, by association, happy to run all day at this speed. For reference, the small Honda inverter gensets run full speed/load at 6000 rpm.
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Old 17-04-2016, 16:54   #8
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Re: High RPM Generator - is 60Hz worse?

Crankshafts are good for tens of thousands of RPM, it's actually piston speed more than anything that is the limiting factor, and to a lesser extent the valve train, so high RPM motors with short strokes can turn quite high RPM and keep the piston speed down.
My Husqvarna TE 450 for example has a single piston 97 mm, but it's stroke is only 60 mm giving it I think a 14,000 RPM redline

But for the majority of us, a low speed engine is simply a more relaxing sound.



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Old 17-04-2016, 22:40   #9
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Re: High RPM Generator - is 60Hz worse?

My 5KW Northern Lights runs at 1800 RPM, burns about .25 gallons per hour with HVAC, Sat TV, computers and lots of chargers going. I run it continuously while not at a dock and run HVAC full time, docked, sailing or anchored. It never feels or sounds loaded up. It is reasonably quiet at 65 decibels in the cockpit, slightly less inside the salon, and we don't notice the sound after a while. We have a soft, custom built sound enclosure. After about 10 feet away from the boat, you can't hear it at all with our underwater exhaust, so it is never a concern in an anchorage.

I've chartered with other gensets of the 3600 RPM variety and have not been happy with them. They were noisier and more finicky to use. Also was told by charter company not to run continuous. That wasn't a problem as both times I used one of the 3,600 rpm models, it broke before our charter was over. I do know people that have the 3600 rpm models that have no problems with them, but my experience has not been good. Meanwhile my Northern Lights has 3047 hours with only impeller, oil changes and an injector change and runs like a top.

Most people just use generators to charge batteries and for that purpose, either should work for you, but my wife enjoys the comfort of home in our salon and living area that has full time HVAC, Sat TV and other electrical amenities while off the dock. Making my wife happy, keeps us using the boat an average of 75 days per year. Not possible without the generator.


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Old 17-04-2016, 23:42   #10
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High RPM Generator - is 60Hz worse?

I owned both gensets. Entec Farymann single cylinder 3,600 rpm 120v 60hz. Even with the sound box, you can feel the vibration and nuts and screws do come loose so a rigorous maintenance is mandatory.

Now replaced with the Kohler 1,500 rpm 3 cylinder 230v 50hz to supply power to air cond and water maker besides the usual battery charging etc.

The Kohler, like dockhead says, the genset just hum along.

I used to stick my head out often just to check if I wake up anyone in the anchorage . Not any more. Nice to have the low rpm ones.






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Old 18-04-2016, 01:05   #11
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Re: High RPM Generator - is 60Hz worse?

3600 generators are throw away cheap sets. Most diesel generator engines below 25kw are not made for rebuild. The engines are considered light duty. They have no sleeves, engine kits aren't available, etc. The generator wiring is as light as they can make it.
Engines run at high speed with a light load will glaze the cylinders and start burning oil. Even at 1800. It's better to plan on using as much power as possible when running your generator. That's one reason big boats rafted together will trade off supplying power for the raft.
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Old 18-04-2016, 09:15   #12
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Re: High RPM Generator - is 60Hz worse?

I have an older Universal 3600 rpm 5 kW gen get. It has been confirmed that these have been known to run for 10000 hours before rebuild. However the generator unit usually fails before the Kobeta engine. FYI for 60 cycles a generator without variable alternator needs to run at either 3600 or 1800 to get 60 cycles.
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Old 18-04-2016, 09:52   #13
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Re: High RPM Generator - is 60Hz worse?

Quote:
Originally Posted by KadeyKrogen38 View Post
I have an older Universal 3600 rpm 5 kW gen get. It has been confirmed that these have been known to run for 10000 hours before rebuild. However the generator unit usually fails before the Kobeta engine. FYI for 60 cycles a generator without variable alternator needs to run at either 3600 or 1800 to get 60 cycles.
Or a belt driven alternator, then engine rpm simply needs to match the pulley ratio.
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Old 18-04-2016, 11:15   #14
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Re: High RPM Generator - is 60Hz worse?

I just bought (2) 60Hz/3600 rpm surplus gensets, one with 890 hours and one with 345 hours. The box is 2 1/2 ft wide by 5 ft long by 3 1/2 ft high (approx), runs on natural gas (or propane with a slight mod). Engines are 65 HP Mitsubishi industrial and gen is 20KW. Paid $700 for one and $500 for the other and they are pristine inside. At full 20KW load they use about 4 gals propane/hr and the package weighs 955 pounds. There are only minor issues to address to bring them back to $3500/$4000 condition.

The point to the post is that 3600 rpm/60Hz is good enough for a commercial standby power supply. The engines are good for at least 4000 hrs between major maintenance.

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Old 18-04-2016, 11:46   #15
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Re: High RPM Generator - is 60Hz worse?

Quote:
Originally Posted by trifan View Post
. . . The point to the post is that 3600 rpm/60Hz is good enough for a commercial standby power supply. The engines are good for at least 4000 hrs between major maintenance. . . .
The key word here is "standby". Compare the hours to the age of those gensets and you'll see what kind of duty they're designed for.


I have a 16kW standby generator at my lake house which runs at 3000RPM (for 230v/50hz power). I've put less than 100 hours on it in 12 years. It will rust apart before it wears out, and that's exactly the duty it's designed for. I'm glad I have it, but despite elaborate muffling and sound enclosure, it is noisy and makes an unpleasant sound.

But it would have been stupid to buy a continuous duty generator at 3x the cost just to backup the public power grid.

Duty on an average cruising boat is somewhere between backup power and continuous (prime) power. You can choose either type of generator, but here you get what you pay for.
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