Can I install only the OpenBridge Modeler? How I do this?

For now, when you install OBD, it is for the complete package with OBM, LBC, LBS and RM. 

communities.bentley.com/.../openbridge-designer-connect-edition

Yes, if you only use the modeling and detailing CAD features you can have an installation of the OpenBridge Modeler application as  OpenBridge Designer is more for analytics. To know more about the differences, check the information at the link below. 

Please verify your license as well, because if it's for OpenBridge Designer users then must use OpenBridge Modeler from inside OpenBridge Designer to load the correct compatible tested version of OpenBridge Modeler.

All software is available at https://connect.bentley.com/, login with your user account, at the top menu there is an option for Upgrade/ and Download now, the same at My Services and Software Downloads, in the Product Line option search for Bridge Analysis option, download, and install it.