382 



THE INDUCTION MOTOR. 



current required to drive both machines at each speed. 

 In this way the horizontal scale is a scale of torque, but 

 measured in terms of the armature current of the direct- 

 current generator. If the generator field remained 

 constant during the experiment for all currents, the 

 armature current would have been exactly proportional 

 to the amperes thus plotted. In order to make 

 this as nearly true as possible, the excitation of the 

 generator was carefully adjusted to a constant value, and 

 the brushes were fixed in the neutral position to avoid 

 much weakening of the field at higher current densities. 

 Some weakening of the field due to magnetic distortion 

 must have taken place ; but as the machine was not worked 

 up to its maximum output this was probably not serious. 

 This assumption was made after testing the proportionality 

 between speed and voltage of the generator, when 

 on open circuit and when fully loaded, and finding 

 that the observed loss of voltage determined with the 

 heaviest armature current at the reduced speed was nearly 

 all accounted for by the resistance of armature and 

 brushes. In any experiment carried out in the same 

 manner this should be similarly tested. 



3-0 



2.0 



i-o 



200 



1000 



400 600 800 



Revolutions per minute. 



FIG. 187. NO-LOAD CURRENT TAKEN BY D.C. MACHINE. 

 I. Driving Induction Motor by belt. 

 II. Belt thrown off. 



The method of obtaining the curves in Fig. 186 was 

 briefly as follows : 



The motor was connected to the supply of three-phase 

 current exactly as described in the instructions given 



