The motor, together with its load, is a mechanical as well as an electrical system.Figure 7.13 indicates some of the mechanical factors that will effect the motor’s performance. These include the moment of inertia (I ), the bearing friction (F), and thedrag (D). The drag, called windage, comes from air resistance on moving parts. Allthese factors cause resisting torques that the motor must overcome. The moment-ofinertia factor opposes any change in speed, the bearing friction is a constant, and theair drag increases with speed. Equation 7.12 describes the mechanical torque on a freerunning motor, that is, a motor not connected to any external load:
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