350 



ELECTRICAL MACHINERY 



than synchronous speed (in the same direction it had as a 

 motor) the machine will act as a generator and so wlil 

 give off electrical power instead of absorbing it. An induction 

 machine so used is called an induction generator or some- 

 times an asynchronous generator. Such an induction 

 machine cannot operate as a generator unless it is connected 

 to a supply line of fixed voltage and frequency and is then 

 driven above synchronous speed for this frequency. 



. Load Taken by an Induction Generator. The amount 

 of load such a machine furnishes to the line is determined 

 entirely by its speed; if it is driven ^JJabove synchronous 



lyo 6 -f, o-pi TP V 7 w^ 



Negative Slip 

 FIG. 228. Operating Curves of an Induction Generator. 



speed it will furnish about twice as much power as though 

 it had only 2% greater than synchronous speed. In the 

 case of the induction generator the rotor is turning faster 

 than the magnetic field and the slip (i.e., difference between 

 the field speed and the rotor speed) is said to be negative. 

 Advantages of the Induction Generator. The operating 

 curves for a large induction generator are given in Fig. 228. 

 The chief advantages of this type of generator and the 

 reason for its increasing use in large power stations is its 

 short-circuit characteristic. If an ordinary alternator was 

 short circuited, it would burn up in a few seconds but 



