Hi Johndoh, If your generator was set to produce 50 Hz at 3600 RPM, running it at only 3000 RPM will only give you about 42 Hz.
A half horse power pump will pull a lot more than that when it is starting up, it might only pull 1 amp when running but it could, briefly, pull 10 amps while starting. The lower the voltage the higher the current draw will be. Most electric motors have the speed at which they rotate determined by the frequency of the AC supply they are connected to. A 50 Hz motor coupled to a 42 Hz supply will rotate more slowly and pump less water. The head to which you are trying to pump water will also have an impact on how much current it draws.
A lot of smaller generators loose voltage during prelonged running. This is because the wire coils in the alternator warm up under load.The heat increases the resistance in the wire causing the voltage to drop.
My Lister ST2 7.5 KVA generator produces 245 volt when cold, this drops to around 235 volts after about ten minutes. When the domestic water pump kicks in, the voltage will drop to around 220 volt before the governor kicks in to pump more fuel, the voltage then quickly climes back to 235 volts.
hope this helps,
Bob