The moment of inertia of the rotor J can be determined using more methods, ones of them could being applied if the rotor is dismounted (the torsion oscillations method, the oscillator pendulum method).

A simple method, which do not impose the detaching of the rotor and which allows to determine also the viscous friction coefficient k_v and the dry frictions torque T_f, is the free stop method. In addition, by using this method, it could be determined not only the mechanical parameters of the motor, but of the assembly motor-driven machine.

The method consists in plotting and processing the evolution of the speed during the free stop (Figure 10). Initially, the motor is supplied with the rated voltage and suddenly it is disconnected.

Figure 10

The curve is obtained between W₀ and stop (W = 0), at the instant t_s.

Taking into account the general equation of the movement in the instant of the motor's disconnection

,

and the fact that the torque T_s0 is due only to the mechanical looses, it results

J = 0,0222 kgm².

Staring from the general equation of the speed during the free stop

,

written for the instant t = t_s, it results the coefficient of the viscous frictions

k_v = 0,003262 Nms

and the dry frictions torque

T_f = 0,1 Nm.