218 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



It is seen, from the diagram, that, at light loads, 6 being then small, 

 the increment of Id is small in comparison with the increment of /,, 

 whereas with heavy loads, being then large, the reverse is the case. 



The observed phenomenon can now be explained. 



If the voltage at the terminals of the converter remained constant 

 the torque would decrease when the speed increases. But each increase 

 of voltage causes an increase of torque. At light loads the first effect 

 is greater than the second; therefore, since the torque decreases, 

 there is an automatic regulation of the speed. 



Beyond a certain load, the second effect preponderates, and there 

 is an increase in the torque. The converter therefore continues to 

 gain in speed until the torque ceases increasing. It is then in a state 

 of unstable equilibrium and it then loses speed until the torque ceases 

 decreasing. 



Experiment shows that the amplitude of the speed-oscillations 

 increases rapidly with the load and that it then becomes impossible 

 for the machine to operate. The oscillations can be diminished by 

 taking advantage of the fact that the torque of the converter diminishes 

 with its excitation. If we take a compound excitation comprising a 

 shunt-winding connected to the terminals of the converter and a series- 

 winding connected differentially, it will be seen that whenever there 

 is an increase of voltage at the terminals of the converter, there will 

 be an increase of the load-current. This current, acting upon the 

 series-winding, will cause an increase in the series-excitation which is 

 opposed to the shunt-excitation. Therefore the total excitation will 

 decrease, and the torque will also decrease. These different effects 

 can be calculated and adjusted so as to balance or overbalance the 

 increase of torque. Actual test shows that this can be done, and that 

 the converter can be made to carry full load even when the induction- 

 regulator is used, if the series-winding is reversed. It appears, there- 

 fore, that, in the case of a rotary converter which is transforming 

 direct current into alternating current, it is desirable to have a com- 

 pound winding in which the shunt and series excitations add themselves, 

 or act cumulatively. 



On the other hand, a rotary converter serving to transform alternat- 

 ing current into direct current should have a compound winding in 

 which the series and shunt excitations act differentially. 



