In synchronous generator, field electromagnet rotates and induces voltage in the stator coils. So, when the generator is loaded, it slows down. If the field is stationary and armature is conducting (as in dc generators), there will be "current carrying conductor in a magnetic field", resulting opposite motoring torque, slowing down the generator. But here there is a magnet(rotor) moving in magnetic field(of the stator conductors). How does this slow down the generator?