How much HP is lost just turning a portable generator at rated speed?

   / How much HP is lost just turning a portable generator at rated speed? #1  

90cummins

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I just finished installing a (New) Yanmar L70 (7hp) on my portable 5kw generator which originally was equipped with a Yanmar L90 (9hp) diesel.
My question pertains to the fixed losses required to turn the generator at rated speed and energize the field.
Other than the increased power required to turn the larger cooling fan does anyone know the difference in power output of a 5kw versus 10kw generator head using the same input HP?
Advantages/Disadvantages of oversizing the generator head?

90cummins
 
   / How much HP is lost just turning a portable generator at rated speed? #2  
I don't know if it will help, but the 4-cyl Isuzu diesel motor in my generator is rated at 33HP continuous and 36HP standby, which corresponds to ratings of 21kW and 23kW on the generator head.

21kW is 28HP and 23kW is 31HP. So it suggests that there are losses of 5HP (or 3.7kW) in going from the engine power to the electrical output for this particular unit.
 
   / How much HP is lost just turning a portable generator at rated speed? #3  
I have a little experience with this. I have a 7.3KW Yanmar that I use for portable use or as a back up to my back up! I got it burned up and had it rewound. I would say that the increased loss would be negligable. I would think that over sizing the gen end can't be anything but good. Too much JUNK being sold that goes the other way!

Curious what happend to the orig. Yanmar?
 
   / How much HP is lost just turning a portable generator at rated speed? #4  
I would think that the rotational losses would be very small. As you stated, a larger cooling fan would require more HP to turn but other than that you would have some drag from the bearings (not much).
By oversizing the generator head, the engine RPMs will drop as you increase the load beyond what the engine can keep up with. The max KW for 7 HP is around 3.5
 
   / How much HP is lost just turning a portable generator at rated speed? #5  
I wouldn't recommend continuous operation of a modern generator head at more than 90% winter and 80% summer.
Sizing to 1-1/2 HP per KW is more reliable than 2HP per KW.
Too often Bubba Tighwad calculates 3950watts load and purchase a 4000W generator and thinks all is well.
 
   / How much HP is lost just turning a portable generator at rated speed? #6  
^^^EXACTLY! And often the breakers are rated larger then the output, for purposes of surges, motor start ups and just the fact the breakers are only made in certain amperages. So Joe homeowner starts trying to see just how much stuff he can run on his genset, while slowly destroying it!

A clamp on amp meter is a very usefull thing to have when runing a genset!
 
   / How much HP is lost just turning a portable generator at rated speed? #7  
Just my my personal experience being a 309A Red Seal electrician who both uses and works on gensets on occasion.
You , family and equipment will be happier with a 15Kw Winco pto generator on the tractor. Good for a 10KW load and not having to worry about load balancing between L1 and L2.
More motor starting capacity.
Less chance of being stolen.
 
   / How much HP is lost just turning a portable generator at rated speed?
  • Thread Starter
#8  
I have a little experience with this. I have a 7.3KW Yanmar that I use for portable use or as a back up to my back up! I got it burned up and had it rewound. I would say that the increased loss would be negligable. I would think that over sizing the gen end can't be anything but good. Too much JUNK being sold that goes the other way!

Curious what happend to the orig. Yanmar?

The original L90 needs repair and I was acquiring the parts when this new L70 was offered on e-bay.
I also have a L100 that I'm accumulating parts for also.

90cummins
 
   / How much HP is lost just turning a portable generator at rated speed? #9  
I HAD a 15 KW Onan PTO Genset. No doubt about it, it had tons of power for my electrically heated house. I just never liked leaving my JD compact running unattended. It does not have Murphy gauges.

Plus, I could not start and stop it remotely.

I much prefer a proper dedicated 1800 RPM commecial Diesel Genset.

I recently replaced a really JUNKY genend on a Northern Hyd genset. It had the Yanmar engine which I like (as far as 3600 rpm gensets go). But I would say, if the genend isn't anything great in terms of quality, by all means run it at half rated power!
 
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   / How much HP is lost just turning a portable generator at rated speed? #10  
I just finished installing a (New) Yanmar L70 (7hp) on my portable 5kw generator which originally was equipped with a Yanmar L90 (9hp) diesel.
My question pertains to the fixed losses required to turn the generator at rated speed and energize the field.
Other than the increased power required to turn the larger cooling fan does anyone know the difference in power output of a 5kw versus 10kw generator head using the same input HP?
Advantages/Disadvantages of oversizing the generator head?

90cummins

7hp should be good for 5215 watts (745x7) with no losses.. Calculate losses and whatnot, and you're looking at around 4500w in real life. Why would you want to put a 10,000w gen head on that as you will never get past 5,000w?
 
 
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