This paper introduces a new method to stabilize synchronous reluctance motors (SynRMs) driven by efficiency optimized V/f control utilizing optimized flux. Efficiency optimized V/f driven SynRMs are prone to instabilities. Specifically, it becomes unstable for either fast ramp acceleration or large load disturbances. For example, in an efficiency optimized V/f control scheme, flux is reduced under low load to improve efficiency. However, a large load increase would lead to loss of synchronisation under this condition. In this paper, a stability analysis of efficiency optimized V/f driven SynRM is performed. Instability threshold indicators using power factor and stator current angle are proposed. A novel instability protection scheme is introduced and verified in experiment.