5.1.2. Friction torque
The friction pair appears in a rotating object by varying speed, as shown in Figure 5.1. The static friction torque T S appears at boot time with a peak value, drops occur after booting and increases in proportion to speed. The component proportional to the speed, for the second term on the right hand position of the equation 5.1, which is due to viscous friction. The portion indicated by the corresponding T f is the static friction torque, which is independent of speed, and is located in the last term of equation 5.1. The static friction torque T S is not related to the equation 5.1 or 5.2, as these are the equations governing the rotor after startup, but T S is an important factor that affects the starting characteristics.
Figure 5.2. A system for measuring the static friction torque
Figure 5.2 illustrates a system for measurement of T S and T f. The motor shaft is coupled to a pulley that rotates freely, and it is located a thread tension dynamometer, which in turn measures the force in kg, if the force is measured just before starting to rotate is F S (Kg) and the force just after boot is F f (Kg), then T S and T f are determined by:
 \\ T_f\ =\ r*F_f\ (Nm) "Static friction torque")
(5.3)
where
r = pulley radius (m)
