From Technic Wiki
A Turbine Controller is a 'central command' block, which provides the primary user interface for a Multiblock Turbine. Every turbine requires at least 1; adding more than one has no effect.
Turbine Controllers may be placed on any external face, inside the outer frame. This includes the top and bottom faces of the turbine.
When a turbine is successfully assembled, the controller's screen turns red. When the turbine is active, the screen turns green and the graphical dials change.