The one wire alternators do need more RPM's to charge. They are usually used on boats with run at higher RPM's. You may have problems with this alternator charging at low RPM's. You can fix this problem by changing the pulley to a smaller size which will increase how fast the alternator actually spins.
I think that the alternator should charge without the battery. I think that there is another problem. The charging system of a tractor is the same as a car, a tractor just runs at lower RPM's and requires less amperage. The one wire alternator should be self-exciting.
A standard alternator such as a DELCO 10SI has to have a diode or an small lamp as a load to excite the field and begin charging. Make sure that you really do have a one wire alternator. The standard three wire alternators have one lug type terminal located on the rear of the alternator and two male spade type terminals located toward the rear, but on the side.
The best method of checking the charging system is with an ammeter. If you do not have one, check the voltage of the battery while the tractor is running and you are applying some type of load on the battery such as turn on all the lights. The voltage should be 14.4 if all is well. If you have less, you have a problem. Increase the RPM's, the voltage should stay the same. Lower the RPM's and check the voltage again. If the voltage does not remain close to 14.4, you have a problem that needs to be fixed.
Let me know what you find out. We do these 12 volt conversions and alternator repairs frequently in our shop.