I don't think you are using the word "energy" correctly. First of all, the word "electro" does not mean energy. It refers to electronic force (and you can have force without any change in energy).
An electromagnetic field is not an endless source of energy. It is an energy in = energy out sort of deal. The energy you can extract from an electromagnetic field is positional... it is a type of potential energy. Once it does work, to create propulsion it can't use the same potential energy again unless you add more energy.
Unless you are adding some energy source (i.e. a fuel) then you are proposing a perpetual motion machine... and breaking basic laws of physics.