Pulley size for ST7.5 & 8/1

[cgwymp]

So I'm about at the point where I'm starting to think about generator pulleys.  I've got an ST7.5 in hand, and the engine is slowly going back together.  I can get an ~11" pulley and run the engine at ca. 850 rpm and make the full 8 hp, or I can go for lower fuel consumption and greater longevity at reduced power by getting an 8.5" pulley and running at 650 rpm.  I don't foresee this setup being my primary power source, but I may someday put it supplemental service.  Until then, it'll be emergency use.

My mail loads (in addition to the normal lighting, refrigeration, et al) will be well pump (not submerged), septic pump, and in winter, perhaps furnace fan (propane furnace, but I also have a woodstove that gets used a lot in winter).

Because I'm thinking emergency use, I'm not concerned about trying to pull the electric water heater, oven, or dryer, nor am I thinking of running big tools, the air compressor, etc.

The rating on the septic pump is unknown.  The well pump states: "3/4 hp, 0.55kW, 115/230V, 12.4/6.2A".  It's wired for "115V" so I'll probably set up the ST for 120V only (no 240V).  I don't know the power factor or startup current needs, but there's 1.4kW right there.  Granted, it only runs a few minutes at a time.....

Anyone care to comment?  Ya think 650 rpm will be enough?

Cheers!

[playdiesel]

My 6/1 and 5KW set up is on the back burner while I complete my 1115 and 15KW unit but my desire for the 6/1 is smooth operation and long life being far and above a few tenths of a KW or saving an ounce a fuel per hour. What I plan to do is vary the RPMs of my 6/1 until I find the best RPM, within reason, for smooth operatiion and then shoot for that while driving the gen gead at the correct RPM. I will then adjust loads to the power available at that engine RPM. I have the advantage of being able to machine my own pulleys and the formula to the generator pulley diameter is a simple one. With your 8/1, if the ratings are accurate, you need to remember that the differance between 6 HP (650 RPM) and 8 (850) RPM is actualy quite a bit by percentage, 33% or 1/3 more available power at 850 than that 650. That could make a big differance in meeting you anticipated needs. A common mistake is to make small of the starting requirements of motors. Some tables and sugestions are pretty conservative but dont take it lightly either. Our E generator has been a 3500/4000 watt home cheapo unit and it starts our submersable well pump (220) OK but you dont want the fridge + freezer and too may lights on when it does.
Have I answered any of your questions? Heck I dont know LOL

[cgwymp]

Thanks for the input!  The extra power would certainly be nice if needed, and I'm not discounting the starting surge capacity I might need.  The other side of the coin, though, is that in the "average" case (whatever that is) the engine will more lightly loaded, which might not be a good thing.

Wish I had the resources to make my own pulleys!  :-)

[Crofter]

Oh, if you could just snap your fingers and have two pulleys appear sized for 650 and 850 rpm! It would be interesting to see how much  if any saved any fuel to produce say 2kw at the lower rpm. As long as adequate coolant temperature is maintained, I wouldnt worry too much about light loading conditions. Wear wise might be nearer a toss up too between the choices.

I have a piece of 1 1/2 plate that I got my pulley out of but it was a good part of 2 days messing around before all was said and done. There is someone on the forums here who makes them to order but it would be a fair chunk of change.

[luv2weld]

I just went through exactly what you are asking about.
I'm running an 8/1 with a 7.5k ST.
My first pulley (8 inch) I ran at 650 rpm. Worked fine until a heavy load kicked in.
Slowed down too much and caused my inverter (Xantrex Power Station)to kick out. (I'm off-grid)
So I changed to a 10.3 inch pulley.  Now running at 800 rpm.
Doesn't seem to affect it at all when something heavy turns on.

Ralph