316 



ELECTRICAL MACHINERY 



is called the " pull-out " point; generally this is between 

 75 and 150% overload. 



Use of Synchronous Motors. Speed-load curves for two 

 motors are given in Fig. 204. The shape of these curves 

 shows that the synchronous motor is entirely unsuited for 

 loads requiring a variable speed, such as railway work, 

 or for driving machine tools. Its principal use is in 

 frequency-changing motor-generator sets. 



Such motor-generator sets are generally used in connec- 

 tion with 25-cycle transmission lines. A 25-cycle synchro- 

 nous motor is direct connected to a 60-cycle generator 



100* 

 Load 



FIG. 204. Speed-load Curves for Synchronous Motors. 



which furnishes power to local lighting circuits. This 

 transformation is necessary because 25 cycles per second 

 is too low to use for lamps as bad flickering results. The 

 synchronous motors of these sets are also used to regulate 

 the power factor of the transmission line, as described in a 

 later paragraph. 



82. Phase Characteristics or " V " Curves. Suppose a 

 synchronous motor is running light and that the field 

 current is at its normal value; the armature current will 

 be quite small, in fact, just enough^to supply the no-load 

 //, now, the field current is altered, either above or 



