We study nonlinear Schrödinger equations of derivative type by variational approach. The main aim of this talk is to investigate the structure of the equation from the viewpoint of solitons. We establish the mass condition for global existence by potential well theory inspired from the classical work of Payne and Sattinger (1975). The important point of our study is to clarify the connection between the mass condition and potential well generated by the solitons. We see that the effect of the momentum plays an essential role in the arguments.