208 



ALTERNATING CURRENTS 



diameters, the brake may be made to fit different sizes of motor 

 spindles. The stationary part consists of a number of electro- 

 magnets (8) and two external yoke rings.* The cores of the electro- 

 magnets are provided with expanded pole-pieces, which are supported 

 by brass plates bolted to a casting fitted with a gunmetal bush which 

 rides loose on the bush supporting the brake rings. Screwed into 

 the casting, diametrically opposite to each other, are two tubes, 



VENTILATIN6 DUCT 



OUNME.TAL BUSH 



INNER BUSH TO FIT SHAFT 



OF SHAFT 



FIG. 136. Morris and Lister's Eddy-current Brake. 



a short one to carry a counterpoise, and a long graduated one which 

 forms the brake lever. The external yoke rings are attached to the 

 magnet system by means of aluminium castings, which overarch the 

 brake rings (the mode of attachment is not shown in Fig. 136). 

 The brake rings rotate in the narrow gaps between the magnet poles 

 and the yoke rings. The coil of each magnet is provided with a 

 ventilating duct in the middle. 



122. Measurement of Slip. Stroboscopic Methods 



We shall next consider the measurement of slip. For the sake 

 of securing high efficiency, the slip is arranged to be small at all 

 ordinary loads, not exceeding, as a rule, some 5 per cent, at full load 

 even in small motors, and being much less in larger motors ( 67). 

 Hence its measurement by a direct comparison of the actual speed 

 with the speed of synchronism is incapable of yielding accurate 

 results, as the error involved in the speed measurement is quite 



* The poles of the electromagnets on the same side of the brake are alternately 

 north and south. 



