Читать книгу Analysis and Control of Electric Drives - Ned Mohan - Страница 36

The gravitational force on the mass is shown in Fig. 2-4b. For the lever to be stationary, the net force perpendicular to the lever must be zero, i.e. f = M g cos β where g = 9.8 m/s² is the gravitational acceleration. Note in Fig. 2-4b that β = θ. The holding torque T_h must be T_h = f r = M g r cos θ. Substituting the numerical values,

Fig. 2-4 (a) Pivoted lever and (b) holding torque for the lever.

In electric machines, the various forces shown by arrows in Fig. 2-5 are produced due to electromagnetic interactions. The definition of torque in Eq. (2-10) correctly describes the resulting electromagnetic torque T_em that causes the rotation of the motor and the mechanical load connected to it by a shaft.

Fig. 2-5 Torque in an electric motor.

In a rotational system, the angular acceleration, due to the net torque acting on it, is determined by its moment‐of‐inertia J. The example below shows how to calculate the moment‐of‐inertia J of a rotating solid cylindrical mass.