The condition of a cutting tool is an important factor in any metal cutting process. Several different methods have been developed to monitor the condition of tools in turning. Motor current measurement is one of the indirect tool wear monitoring methods in machining processes. The advantage of motor current measurement method over other methods for detecting malfunctions in the cutting process is that the measuring system has an acceptable cost and the measurement apparatus does not disturb the machining process. In this paper, the effect of tool wear and tool breakage on the harmonics of the AC spindle motor is investigated. Finally a method based on time-frequency marginal integral of current signal is proposed for indirect measurement of tool wear in turning. In order to validate the proposed tool monitoring system, an accelerometer was attached to the shank of the cutting tool to measure vibrations resulting from tool wear in feed direction. A vision system was also used to measure tool wear and traces tool state by observing the surface texture of the workpiece. Performance of the proposed system was validated against different experiments. The results showed improved robustness of the proposed system with high potential for practical application in detecting tool wear.