TESTS OF SYNCHRONOUS MOTORS 



135 



electric power applied at the terminals of the motor is measured by a 

 watt -meter. This method has the objection of requiring the expendi- 

 ture of too much energy in the case of large motors. 



The author's method, described further on, for the study of con- 

 ditions of operation, then finds its application. In such cases, the 

 generator and motor are mounted and coupled together in such manner 

 that the generator is driven by its pulley, and also tends to be driven 

 by the motor which is connected to the other end of the shaft and 

 which takes the lead and jdrags the dynamometer. The latter 



JOC 



140 ass> 



ISO Z40 



100 ZOO 



60 /6O 



< 



40 &>\ 



B / 



zooo 



field '-Exc/faf/or? , Amperes 



FIG. 67. Curves of 15 H.P. synchronous motor. Normal voltage 240 volts. 

 A = characteristic curve without load. 

 B= short-circuit current. 



then measures directly the work done by the motor, and all that is 

 necessary is to measure, by means of a watt-meter, the electric power 

 supplied to the motor by the generator. If we let C equal the resisting 

 torque, n the number of turns per second, and w the electric power 

 measured by the watt-meter, the efficiency, ij, will be expressed by 

 the following equation: 





The segregation of the different losses, in a synchronous motor, is 

 a much more complex problem than in the case of direct current machines 

 because the parasite effects (hysteresis and eddy currents) in the 

 field-pole pieces depend not only on the induced current and on the 



