THE INDUCTION MOTOR. 383 



above. A direct-current generator was coupled to the 

 motor by a belt, and its armature was connected to a 

 variable resistance in series with an ammeter; a volt- 

 meter read the terminal voltage. This generator was 

 separately excited, the excitation being kept constant by 

 means of a regulating resistance. The motor was supplied 

 at a constant voltage of 120 throughout the test, and the 

 generator was allowed to send gradually increased cur- 

 rents, so as to increase the load on the motor. For each 

 value of the load on the generator, watts supplied, rotor 

 and stator currents, speed and generator voltage were read. 



The final readings, at and near the point of stoppage 

 of the motor, were taken by supplying the direct-current 

 machine armature with current from, an external source 

 in series with a resistance and reversing the fields, so as 

 to make it tend to drive the alternating-current motor in a 

 reverse direction. By this means the armature current 

 corresponding to the torque at stopping was obtained. 



A test on the direct-current generator was then made 

 to ascertain the current equivalent to the frictional and 

 other losses at various speeds. The machine was run as 

 a motor at the same excitation as before, and the current 

 taken by the armature at various speeds was noted, both 

 with the belt coupling it to the motor and with the belt 

 thrown off. The readings taken with the belt on included 

 the friction of the induction motor and belt as well as of 

 the direct-current generator. 



The results of this test are shown in Fig. 187. In 

 plotting the curves of Fig. 186 the no-load current 

 required to overcome the total losses of both machines 

 (as shown on the upper curve) were added to the current 

 actually given out by the generator. The frictional 

 losses of the induction motor itself are thus counted as 

 torque exerted by the rotor. 



In order to determine the relation between the 

 horizontal scale of amperes in Fig. 186 and torque, 

 measured in Ib.-ft. or kg. cm., it is only necessary to 

 observe the voltage generated in the direct-current 

 machine when running unloaded, and its speed. In the 

 present case the no-load voltage at 1,320 revs, per minute 

 was 210. 



Thus, at an output of 10 amperes, and the speed 

 observed of 1,160, the power generated (including 



