GRABBING FORCE
Once, current passes through Rotor then it produces Magnetic Field. Change in Magnetic Field over conductor produces electromotive force. Once Electromotive force is produced in windings of Stator then it in return produces Magnetic Field which tries to counter the affect of primary Magnetic field from Rotor that’s why rotor tries to grab the stator and tries to slow its self down.
Most of the generators operate at constant RPM, but once we try to extract more power from Alternator then we experience that engine is taking load. Prime reason, for this is that grabbing force is becoming stronger and engine is feeling difficulty maintaining its RPM.
You were right, as I did not take into account the magnetic field produced by Stator. Stators Magnetic Field tries to oppose the Magnetic Field of Rotor. This is what we are referring to as Grabbing force.
Anyway, I think we can install a smart equipment which counters this affect by controlling the Engine Power and RPM or something which tries to step up the RPM under some conditions so that we may not have to use very heavy engines.
This may be a difference between branded and self made generators.