This paper develops a design framework for sensorless  flux-vector control of synchronous machine drives. Observer-based  volts-per-hertz (V/Hz) control is presented as a special case in this framework. A complete linearized model is derived, which allows systematic control design and stability analysis. Using the model, passivity conditions are derived. The control methods are experimentally evaluated using a 6.7-kW synchronous reluctance machine (SyRM) and a 2.2-kW permanent-magnet (PM) machine as examples.