We show that the typical 3-3-1 models are only self-consistent if they contain interactions explicitly violating the lepton number. The 3-3-1 model with right-handed neutrinos can by itself work as an economical 3-3-1 model as a natural recognition of the above criteria while it also results an inert scalar triplet (eta) responsible for dark matter. This is ensured by a Z_2 symmetry (assigned so that only eta is odd while all other multiplets which perform the economical 3-3-1 model are even), which is not broken by the vacuum. The minimal 3-3-1 model can provide a dark matter by a similar realization. Taking the former into account, we show that the dark matter candidate (H_eta) contained in eta transforms as a singlet in effective limit under the standard model symmetry and being naturally heavy. The H_eta relic density and direct detection cross-section will get right values when the H_eta mass is in TeV range as expected. The model predicts the H_eta mass m_{H_eta}=lambda_5times 2 TeV and the H_eta-nucleon scattering cross-section sigma_{H_eta-N}=1.56times 10^{-44} cm^2, provided that the new neutral Higgs boson is heavy enough than the dark matter.
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