298 INDUCTION MACHINES. [Exp. 



necessity of using a synchronous machine in conjunction with it. 

 It has the advantage* of rugged construction, with no commu- 

 tator, brushes or slip rings. The squirrel-cage rotor requires no 

 moving coils of wire and practically no insulation. The machine 

 gives a smooth wave of electromotive force and tends to damp 

 out, rather than to produce, harmonics and surges. On short 

 circuit the machine gives no voltage, which is an advantage in 

 operation. 



8. When an induction generator is operated in parallel with 

 a synchronous generator, the frequency and voltage are determined 

 by the latter. The load taken by the induction generator depends 

 upon its slip, that is, its speed with reference to the speed of 

 the synchronous machine. 



This characteristic may prove desirable or not according to cir- 

 cumstances. It would, for example, be obviously undesirable if 

 the induction and synchronous generators were driven at constant 

 speed; for, as the load increased, the induction generator would 

 not take its share. On the other hand it would prove desirable 

 if a station with induction generators driven by water power 

 were connected in parallel with a station composed of syn- 

 chronous machines driven by steam power. It could be so ar- 

 ranged that the induction generators would tend to speed up 

 and take all the load up to the limit of the water power, the 

 steam-driven synchronous machines carrying only the excess of 

 load. 



9. When an induction generator is operated in parallel with 

 a synchronous motor or converter, the frequency will depend upon 

 the speed of the generator (see Fig. 5) but will vary also with the 

 slip, that is, with the load. At constant speed the frequency would 

 diminish with the load; or, for constant frequency, it would be 

 necessary for the speed to increase with the load. The voltage 



* For a discussion of the induction generator and its use, see a paper by 

 W. L. Waters, A. I. E. K, Vol. XXVII., pp. 157-180 and the discussion 

 pp. 217-254. 



