ABSORPTION DYNAMOMETERS 205 



power nocessary for carrying out brake tests becomes a serious 

 consideration. When dealing with motors of small size, it may be 

 flrsinible to supplement the no-load and short-circuit tests by brake 

 tests and measurements of the slip, which furnish additional checks 

 on the accuracy of the circle diagram. We shall now consider some 

 of these supplementary tests. 



For carrying out direct brake tests, one of the most convenient 

 appliances is Soames' Testing Brake, shown in Fig. 133. This brake 

 may be used up to about 5 h.p. It consists of a rigid tripod stand, 

 at the top of which is a nut fitted with a hand-wheel. Passing 

 through this nut is a screwed shank, which ends in a fork fitted with 

 hardened steel plates on which rest the knife-edges of the brake 

 lever. The middle portion of the lever is graduated, so that the two 

 sliders supporting the brake band may be set at equal distances from 

 the knife-edge, each distance being equal to the radius of the pulley. 

 The tripod is adjusted so as to bring the knife-edge vertically over 

 the centre of the shaft. The brake band passing around the pulley 

 consists of a thin braided band of hemp. A suitable weight depend- 

 ing on the load required is hung from one end of the lever, so that 

 when the brake band is tightened the motor tends to lift the weighted 

 end of the lever. The displacement of the lever in either direction is 

 limited by the height of the rectangular opening in the supporting 

 frame through which it passes on one side of the fork. By means of 

 the hand- wheel the friction between the pulley and the brake band 

 is adjusted until the lever is gently swaying in a horizontal position. 

 From the weight W, its known distance d (in feet) from the lever 

 knife-edge, and the speed m in revs, per min., the b.h.p. is easily 

 calculated by means of the formula 



m 



brake horse-power = - oo 



but since ; rf^ is a constant whose value is supplied by the 

 00,000 



makers, all that is necessary is to multiply the product Wm by this 

 constant. The use/id torque * exerted by the motor is, in lb.-ft., "Wd. 



120. Air-friction Dynamometer 



An exceedingly compact, simple, and ingenious form of brake 

 dynamometer is the one shown in Fig. 134. This is the invention 

 of Mr. W. G. Walker, and may be used up to about 20 h.p. It 



* This torque ia not the same aa the T in the formulae given in connection with 

 the circle diagram. The latter is the total torque, and includes the torque required to 

 overcome the resistance's due to friction and hysteresis in the rotor core. 



