This paper proposes an adaptive controller with an improved high-frequency injection method for sensorless synchronous reluctance drive systems. A mathematical model is presented to show that the use of a high-frequency injection method that takes into account the effects of unequal mutual-inductances and the influence of speed reduces the rotor position estimation error. The adaptive speed control algorithm offers improved transient performance in comparison to typical proportional-integral (PI) controllers that are employed in sensorless drive systems. To demonstrate the viability, the proposed adaptive controller and the modified high-frequency injection method are implemented using a TMS-320F-28335A digital signal processor to control a 500 W prototype synchronous reluctance motor drive. Experimental results are presented to show that the transient, load disturbance, and tracking responses of the proposed drive system are superior in comparison to a drive that uses a PI controller. Both experimental and theoretical analyses clearly indicate that the proposed high-frequency injection method with an adaptive speed-loop controller offers improved performance in adjustable speed synchronous reluctance drive systems.