140 SYNCHRONOUS MOTORS 



or it can be deduced from an oscillograph diagram. We then have all 

 that is necessary to draw the diagram. The results are sufficiently 

 concordant with the theory, but they indicate no new phenomenon, 

 except the discontinuity already mentioned. For more complete 

 details see the author's original paper. 



Advantages and Disadvantages of Synchronous Motors. The 

 characteristic features of synchronous motors, constituting their advan- 

 tages and disadvantages, may be summarized as follows: 



(1) Their construction is as simple as that of alternators, and it 

 enables them to be connected directly to high voltage circuits. 



(2) Their specific output and their efficiency are also the same, 

 and are quite satisfactory. 



(3) When once put in operation they have perfectly uniform speed. 



(4) Their power-factor is as good as may be desired when their 

 E.M.F. is properly selected. By using them, all reactive currents can 

 be eliminated, and currents can even be made to lead in phase, by 

 suitable regulation of the excitation. 



(5) It is difficult to start these motors, especially single-phase 

 motors; special methods and apparatus must be employed and also 

 an idler-pulley or clutch. Even when starting without load, which 

 is the only practical way, a very high current is required. 



(6) Speed-regulation is impossible. 



(7) A very sudden overload causes the motor to fall out of phase 

 and brings about a short-circuit. 



(8) Any irregularity of operation of the generator causes great 

 oscillations of speed (" pumping ") which make the voltage of the 

 system fluctuate. If the excitation is not well regulated, the motor 

 produces a more or less important reactive current, which may bring 

 about a drop of voltage in the system. 



(9) If the E.M.F. curve of the motor is different from that of the 

 supply-system, harmonics will be produced, which will uselessly increase 

 the line-current and compromise the stability of operation. Stability 

 of operation can only be attained by the addition of inductance to the 

 circuit; and it remains weak, at best. 



(10) A synchronous motor reacts more or less on the rest of the 

 system, to which it can communicate its oscillations, or of which it 

 can distort the E.M.F. curve. The construction of synchronous 

 motors should therefore be carefully studied, not only with reference 

 to their operation by themselves, alone, but with reference to their 

 effect on the distributing system from which they are to be supplied. 



