Lack of a more complete understanding of the system characteristics, particularly thermal effects, severely limits the reliability of high speed spindles to support manufacturing. High speed spindles are notorious for their sudden catastrophic failures at high speeds due to thermal problems without alarming signs. In this paper, a complete power flow model is developed to characterize the power distribution of a high speed motorized spindle, in particular the characterization of the heat generation within the spindle by the built-in motor and the bearings under the influence of speed, preload, and lubrication. The model is verified with a custom-built high performance motorized spindle of 32 KW and maximum speed of 25000 rpm (1.5 million DN) using several systematic test procedures. The results reveal interesting observations of spindle beat generation due to the built-in motor and the bearings.