2/0 INDUCTION MOTORS. [Exp. 



equivalent single-phase current and line voltage; or, (3) current 

 per phase and voltage per phase. See 28-30, Exp. 6-A. 



40. Measurements on Generator. Measure terminal voltage, 

 armature current and speed. Also measure field current, which 

 is to be kept constant* during the run by adjusting the field rheo- 

 stat. A strong field is desirable so as to minimize the effect of 

 armature reaction on heavy load, which will change the core loss 

 and so introduce error. The generator may be self excited when 

 separate excitation from a constant source is not available ; see 

 4oa. When the generator is self excited, the armature current 

 is equal to the external current plus field current. 



41. Readings. Readings should be taken as follows: 



(1) Take readings with belt off. The watts input gives the 

 no-load losses of the motor ; these losses include the rotation losses 

 W (iron loss, friction and windage) plus the no-load copper 

 losses. (Should it be desired to determine W Q alone, the no-load 

 copper losses are computed (44) and deducted.) 



(2) Take readings with belt on, the generator being without 

 excitation. The increasef in the watts input of the motor gives 

 the belt loss plus friction and windage of the generator. 



(3) Take readings with the generator excited to the constant 

 excitation (or constant terminal voltage) used throughout the 

 test. (The difference between (3) and (2) would give the iron 

 losses of the generator, a value, however, which is not used by 

 itself in this test.) 



42. The difference between (3) and (i) gives the total rota- 

 tion losses W Q of the generator (iron loss, friction and windage),. 



* (403). The test may also be conducted by varying the field current 

 for each load so that the terminal voltage of the generator is constant, 

 42a. In some ways this is simpler, particularly with a self-excited gen- 

 erator, but is not quite as accurate. 



t(4ia). This assumes that the motor losses remain constant, which 

 will be practically true for such a small increase in the motor load. 

 Should the increase in the copper losses of the motor seem to be appre- 

 ciable, an allowance for it may be made. 



