CONVERSION OF ALTERNATING CURRENT. 179 



the point P is carried beyond the point M, and the machine falls 

 out of synchronism and ceases to operate. The maximum intake 

 of power is 2 times Ma times Bji/R 2 -f &> 2 Z 2 .* 



A synchronous motor can operate for a fairly wide range of 

 values of voltage B, the supply voltage A being given, pro- 

 vided there is considerable inductance in the circuit or, in case of 

 a polyphase machine, in each circuit ; but this possible variation 

 of B shows itself as a variation of the terminal voltage only when 

 there is inductance (or resistance) between the supply mains and 

 the synchronous motor. Therefore, if it is desirecj to control the 

 voltage on the direct-current side of a synchronous converter by 

 changing the field excitation, inductance (or resistance) must be 

 placed in each circuit between the supply mains and the machine. 

 Then a change of field excitation changes the terminal voltage of 

 the machine on both sides. Inductance is always used rather 

 than resistance when it is desired to control the voltage of a 

 synchronous converter without changing the supply voltage. 



The hunting action of the synchronous motor is described in 

 Art. 72. The synchronous converter is especially prone to de- 

 velop hunting oscillations as stated in Art. 72, and the methods 

 for obviating this trouble are there described. 



When the synchronous converter is driven as a direct-current 

 (shunt) motor and loaded by delivering alternating current, it is 

 called, usually, an inverted rotary, and when so operated the 

 machine exhibits one peculiarity which should never be lost sight 

 of, namely, the machine tends to run at an excessively high speed 

 when it is overloaded. This rather paradoxical behavior is due 

 to the weakening of the field by the demagnetizing action of the 

 alternating current which is taken from the machine, and, like 

 any shunt motor, its speed rises when its field is weakened, since 

 there is but little mechanical opposition to the motion. 



* What is here said concerning the maximum intake of power of a synchronous 

 converter and its stoppage due to overload is based on the assumption that B is con- 

 stant, whereas, in fact, B may be greatly reduced by armature reaction when the 

 load on the machine is excessive. In fact, a synchronous converter can scarcely be 

 stopped by overload. 



