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I'm not sure what you are referring to, but by guessing what is it from the context of the episodes the term was used, main energizer could be referring to the dilithium crystal matrix that generates the warp plasma from matter-antimatter reactions, or the electro-plasma system that distributes the warp plasma around the ship. The dilithium crystal matrix is in the warp core, and it also regulates the reactions. The electro-plasma system is a series of conduits and regulators all over the ship that directs the plasma. For all we know, the main energizer could be the warp core itself.