Lorentz force opposing mechanical input to a generator?
Energy is preserved all the time, when it works like a motor it ouputs torque to mech. load, when acts like a generator makes a negative torque that has to be feed from mechanical drive. When there is no electrical load at generator output then there is no negative torque, the mech. drive (ex. diesel motor) is unloaded. As soon there is a load present at generator output, then the engine is loaded - it has to supply enough torque to spin the generator.