Lister Engines > Lister Based Generators

Poll: Oversized generator?


Folks:  I'm NOT asking if this is "right or wrong".  I already know the answer to THAT question.  And any engineer worth his salt will agree (though on only some level)....(sounds like "no good Scotsman to me....)

So, capacitance (actually "Inductance") within a generator head is largely forgotten in the modern vernacular of modern-day generators.  But, many of us know that LRA (locked rotor amperage) is a defining factor in getting a motor to turn, when using a generator.  It ain't the "run" amps that you need to START a device, it's the "LRA" that counts. 

Now, given the MASSIVE flywheels of a Lister-design, and as a function of them, the VAST momentary momentum (instantaneous horse-power (ahem....torque)), these engines are, compared to modern "engineering"....most amazing. 

So then, to a poll I propose:

Given (let's say) a 8 HP lister design...or perhaps a 12 hp (as in my case)....what's the LARGEST generator head anyone has used to pull massive 1 power-factor at some astounding momentary amperage (given the horsepower of the engine)? 

OBVIOUSLY, I'm not talking about using an 8 or 12 hp lister to pull a demand of 12 or 20 KW of power, on a constant-demand basis.  That would be absolutely ludicrous....all would agree.  But...let's say that you've got an air conditioner at 220V, that pulls 60 amps on locked-rotor-amperage.  Demand at that 60 amps is....1 second?  And then, after that, you're running the AC unit at 12 amps.   Nothing substantial for the 8 or 12 hp engine. 

I'm contemplating running a 20 KW ST head on my 12 hp single.  This, to overcome the LRA demands of several "intermittent" electrical motors within  a potential "emergency circuit".  The flywheel on the engine, as well the flywheel on the 20 ST head will provide substantial instantaneous power...

I'm not asking for theoretical opinions about this, from the brotherhood-of-electrical-power-generation.  I'm asking for "real-world" results from this craft, who have bothered to "tickle the dragons belly"....

As you probably know I run a belt driven 15KW head with a 1115 China horizontal that is running  a tick over 1500 RPM so maybe 16?? HP. My reasoning for the large head is as your post here,  It starts our motor loads effortlessly. When I decided to spend on a whole house stand by the wife said I want to be able to keep house same as grid. Our large starting loads are same as most everyone else's, well pump,  A/C,  Plans were to add a flywheel to the generator shaft but that turned out to be unnecessary.  Outside of initial cost there is little reason to not upsize the head for a stand by set. Years ago there were many posts about more drag from the fan and bearings but lets face it they were splitting hairs, same with the serpentine belt deal. Maybe had some importance with a 6/1 running 18 hours a day every day but who does that now? I don't know of anyone. In a stand by situation its pretty much immaterial. Being as I am an engine mechanic and pretty much electrically illiterate I also figured the large head working at half capacity would last longer??  The set up is now about 13 years old and outside of the always bad bridge rectifier replacement it has been trouble free. Largest surge load it has seen was last summer when we lost power, AC was on, wife was washing clothes, lights etc and I needed to run my 6.5HP air compressor. To my supprize the compressor fired right up with a puff of smoke from the 1115. At that time I didn't have a load monitor which I have since installed.

Butch is absolutely correct on oversizing the gen head. I run a 15KW head on a 8/1 knowing I can never produce 15KW but starting currents and inrush is never a problem. The extra mass helps ride through those starting currents much like a large flywheel.

On a side note, oversize your wire as well. Seeing as you have a 20 KW head which could potentially deliver 80 amps at 240 would be #4 AWG copper (70-90 amps depending on wire type). Since you can only generate about 9 KW with the 12 HP that could deliver about 38 amps and would need a wire size of around #10 awg (30-40 amps depending on wire type), you may want to go somewhere in between sizes. I would do at a minimum a #8 if not a #6 between your generator and your transfer switch depending on distance. The increased wire size will help prevent voltage drop or sag with a heavy load inrush. 

Sir Petteroid:
The first time I started up my rebuilt cs 6 with a yanan 9.9 kW alternator I was a tad afraid it was to big for the engine. But once those flywheels build up speed it carries the  mass of that head no problem.
I have not got around to plugging in my 15 cfm compressor yet to see how the engine copes but it should be a good test. Piston compressors draw hard at start up. That much I do know.


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