Running the tractor at 2500 rpm the pto at 540 produces 15kw so if i lower the rpm on the tractor i guess it would lower the speed of the pto accordingly correct producing less KW power?
You have a couple of misunderstandings above.
The alternator head on the PTO generator will produce the correct voltage and frequency ONLY when it is turned at it's specified RPM. For a 2 pole alternator head, that required rotational rate is 3600 RPM, and for a 4 pole alternator head, that rate is 1800 RPM.
As you have noticed, neither of these values is the same as your tractor's nominal PTO RPM, 540. Ergo, between your tractor PTO and the alternator head will be a small gearbox -- to "upconvert" the 540 RPM output from your tractor PTO to the 1800 or 3600 RPM input needed at the alternator head.
Let's assume for the moment that you have a 2 pole split-phase (120Vac/240Vac) 60Hz alternator head which requires 3600 RPM; and the gearbox mounted on the face of the alternator head has a 1:6.66 gearing -- each input rotation from the tractor PTO results in 6.66 output rotations to the alternator head.
A reduction in PTO speed from 540 RPM to 486 RPM (-10%) results in a reduction of alternator RPM from 3600 to 3240 RPM (-10%). This will reduce the voltage by 10% (from 120Vac to 108Vac) and the frequency by 10% (from 60Hz to 54Hz) . Neither of these is a desirable situation from the viewpoint of the loads plugged in (such as a refrigerator). An internal voltage regulator on the alternator head will attempt, by increasing the field voltage, to counter the voltage sag from the lower RPM, but a) this is usually bounded by about 5% "improvement", and b) this will not and can not fix the frequency offset.
As you can see, you MUST turn a direct-coupled alternator head at it's specified design RPM -- there is no other option. (Inverter generators are another story). So, a PTO generator requiring 540 RPM input must be spun at 540 RPM (+/- a little bit).
All that said, all we have discussed above is regarding voltage and frequency -- and nothing about power. The output power (in W or KW) of the PTO generator is a function of the input power (PTO HP) and the applied load. As you have seen above, the voltage and frequency of the PTO generator must be maintained for compatibility with the load (e.g., a well pump) but the actual power delivered is according to how much load is attached to the generator. Hence, a 15KW PTO-driven generator makes no power if nothing is connected to it -- although it is making 120Vac/240Vac at 60Hz, there is no net current flow and therefore no net power output.
As others have pointed out above, it is sometimes practical to engage the PTO using the 1000 RPM setting and (nearly) halve the engine RPM to again achieve 540 PTO RPM. However, note that the designer of the tractor set up the internal PTO gearing to achieve 540 RPM just *past* the torque peak of the engine (that is, on the high side of the curve). This means that as the load increases and the engine speed sags slightly against the governor, torque will go UP marginally -- helping the engine recover from the applied load (such as starting a large motor). If the engine is running at half speed with the PTO selector set up for 1000 RPM, torque will go DOWN with applied load and engine speed control will be worse. With enough horsepower available at the engine output shaft, this may not be a problem. You will have to experimentally find out what "enough" means, however.
Wrooster
