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

[90cummins]

I have no intentions of putting a 10kw gen head on the 7hp. It was said to help clarify my question.

I'm having some problems with the 5kw gen head and was doing my homework looking for a replacement. I also have a 9 & 10hp Yanmar engine that will mate to the gen head if & when I get them up & running and need more power.
The price difference between a 5kw head and a 7 or 8kw unit is minimal.
My thought was; if there is minimal efficiency loss turning a larger gen head why not? Also in the future I would also be able to swap the 10hp engine onto the gen head if I find I need additional output.
Correct me if I’m wrong but won’t the larger gen head have more surge capacity than a smaller gen head given the same input hp?
The issues with my 5kw head were higher than normal voltages 127/254 then it stopped generating.
I changed capacitors attempting to lower the voltages. The OEM capacitor was rated at 50UF@440 volts so I installed a smaller cap rated at 40UF@440v.
This dropped the voltage to 117/ 234.
This unit is new to me unit so I’ve been running it with a 3kw load for a couple hours at a time to gain confidence that it will perform without problems.
During a run it shut down due to a low oil pressure indication. I installed a gauge and found the oil psi to be good at 70 lbs; it then shut down again due to a low oil pressure indication.
So it’s an electrical issue.
During that same run it stopped generating so I reinstalled the OEM capacitor, still no output.
I then checked the resistance of the windings which were good but when I checked the diodes they were open.
Upon closer inspection I noticed one of the diodes was mechanically defective. I’m thinking that when it failed it took the other diode out also.
I’ve ordered 2 new diodes and will continue the trouble shooting process when I get then installed & running.
For those wondering why I would want to install too small an engine on a generator rather than repair the original I have a couple reasons.
1st this new L70 Yanmar is a Military version with a 3qt deep sump, bypass spin-on filter with an intake grid heater that is capable of running 500 hours unattended.
2nd it was a brand new engine smaller in displacement and hp so it would consume less fuel yet still meet my minimum needs in the event of a prolonged power outage.
3rd It has a J603B mounting configuration which is the same as the 9 & 10hp Yanmar.
4th The selling price was a steal!
90cummins

 

[s219]

Correct me if I’m wrong but won’t the larger gen head have more surge capacity than a smaller gen head given the same input hp?
90cummins

The larger gen head will certainly have more capacity since it has heavier windings/guts. However, you're not going to get more than about 85% of the engine's surge power converted to electrical output. If the smaller gen head can handle that amount within its rating, to me there's no real practical advantage to going with the larger head except that maybe it gives you the option of changing to a bigger motor in the future.

So for example, your 7HP Yanmar will be good for about 4.5kW of electricity. To me, it won't matter if you run it on a 5kW head or a 10kW head, since in both cases your max output is lower than the gen head rating.

 

[Industrial Toys]

Are the genends you have been working with any good or just more typical consumer sheit?

When I was working on that Yanmar by Northern (for a neighbour) Northern told me just to buy the whole genend rather then messing about. I was surprised how cheap it was but I guess you get what you pay for!

 

[Midniteoyl]

The larger gen head will certainly have more capacity since it has heavier windings/guts. However, you're not going to get more than about 85% of the engine's surge power converted to electrical output. If the smaller gen head can handle that amount within its rating, to me there's no real practical advantage to going with the larger head except that maybe it gives you the option of changing to a bigger motor in the future.

So for example, your 7HP Yanmar will be good for about 4.5kW of electricity. To me, it won't matter if you run it on a 5kW head or a 10kW head, since in both cases your max output is lower than the gen head rating.

This.. The motor will stall before you get any practical 'surge' use.. especially if its more than a transient.

 

[90cummins]

The gen set is a 5kw Master/Dayton and the gen end is made in France by DESA.

90cummins

 

[CobyRupert]

It's pretty complicated on whether a genny can ride through the surge. Factors include:
The inductance of the motor being started (high efficiency motors are the worst)
The inertia of the genny/motor set
The inertia (curve) of the load being started (variable torque or constant torque) I.e. Are you starting a fan or a rock crusher.
The response of the governor
The torque curve of the motor ( which has a lot of other factors, including fuel type (gas, diesel, LP)
Contribution, or demand, of other loads.

 

[CobyRupert]

Regarding OP's original question, I don't think much power is required because no (not much) energy is being consumed by anything.

 

[john_bud]

Regarding OP's original question, I don't think much power is required because no (not much) energy is being consumed by anything.

But a demand power surge can stop the engine dead. There there is a risk of the engine compression recoiling and running reversed. A remote application will then run with the oil pump blowing bubbles until it dies. Unless it has a low oil pressure stop circuit. Best option is to match the gen to the engine where possible.

 

[SPYDERLK]

Gen output buckles virtually instantly when you go above its surge rating even if rpm is maintained. Oversizing the gen gives greater surge capacity longer, feeding on inertia while rpm drops. Adding flywheel to the underpowered engine will extend surge time further.
larry

 

[Industrial Toys]

I guess, making sure that your overcurrent protection is working and rated for the engine KW and not the stator KW rating, might be a good idea. If that were taken care of, I personally would have much more confidence in that setup then running your typical over-rated genend at its limit.