THE SYNCHRONOUS MOTOR. 149 



both P r and P" will be positive, that is, both machines will 

 on the average give out power. 



If, however, machines A and B are in exact synchronism 

 the value of <f> remains constant, and either P 1 or P" may 

 be negative, that is, either machine A or machine B may take 

 in power steadily (motor action), according to the value of the 

 angle <. 



Therefore, if an alternator (motor) is to take power steadily 

 from another alternator (generator) the two machines must run 

 in exact synchronism, and it is for this reason that an alternator 

 when used as a motor is called a synchronous motor. 



71. Stability of running of a synchronous motor. The neces- 

 sity of exact synchronism of running of an alternator which is to 

 be used as a motor may seem to be an impracticable condition ; 

 but if an alternator is once started to operate as a motor, exact 

 synchronism is automatically maintained, unless the load on the 

 motor becomes excessive. This stability of operation may be 

 shown by the help of Fig. 1 29, remembering that any kind of a 

 motor whatever, steam or electric, will show stability of running 

 if a momentary decrease in its speed causes it to take more power 

 from boiler or electric generator, whereas the running will be 

 unstable if a momentary decrease in its speed causes it to take 

 less power from boiler, or electric generator. 



Suppose that machine A is driven at constant speed by a 

 governed engine or water wheel, and suppose that machine B is 

 in synchronism with A with the point P, Fig. 129, at any given 

 position on the circle. If the point P is anywhere between s 

 and M a momentary decrease of speed of B will cause the point 

 P to move towards M (B falling behind A in phase), and the 

 power intake of machine B will increase, and thus counteract the 

 momentary decrease of speed due for example to a sudden in- 

 crease of motor load on machine B. If, on the other hand, 

 machine B runs momentarily faster than A, then the point 

 P, Fig. 1 29, will move away from M and towards s (B get- 

 ting ahead of A in phase), and the power intake of machine B 



