temperature of these types of constructions is 250-300'C. During heating the electrical conductivity of the rotor, cr, is decreased, and consequently, the attenuation coefficient is decreased. For heating of 100"C the initial slope of the momentum characteristic of the brake is decreased by about 36%, although the maximum momentum changes insignificantly. Large problems associated with the heating of the rotor arise during testing of the brakes to determine their momentum characteristics. Special test units are required which make it possible to accelerate the brake to the maximum velocity in portions of a second and measure the brake momentum at the stable velocity for 1-2 seconds; however, even in such a short time the rotor is heated by 100-150'. 6.4.4. Issues Regarding the Construction of Electromagnetic Brakes and Other Types of Brakes A number of specific problems are solved in the construction of electromagnetic brakes. To increase the energy of the magnetic system the external and internal parts of the stator are made in the form of permanent magnets. This configuration complicates the schematic of the full rotor, its attachment to the output shaft and the position of supports. The small gaps between the stator and the rotor, the large speeds of rotation, and the significant dynamic loads impose strict requirements on durability, accuracy, and the rigidity of structural elements. Moreover, it is necessary to insure stability of the gap during significant heating of the rotor. In the construction of the electromagnetic brake for the APDA (Figure 6.9) a support is used with four ball bearings. Due to the use of magnets with high specific characteristics and more optimal design, the power of the electromagnetic type brakes was significantly increased, and the time constant was decreased. The conditional specific energy capacity (for displacement of the rod by 0.1 m with a velocity of 0.6 m/s and a speed of rotation of the rotor of 104 rev/min) for the electromagnetic brake used in the APDA is about 500 J/kg, which is close to the value of w for friction brakes and is significantly larger than for spring mechanisms.
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