THE INDUCTION MOTOR 351 



if an induction generator is short circuited it stops generating 

 and so cannot harm itself. 



Short-circuit Risk of a Power Station. The operating 

 engineer often uses the phrase " short-circuit risk of a 

 station; " by this term is meant the danger of burning 

 up the apparatus of a station when a short circuit occurs 

 on the bus-bars. In case the short circuit occurs in a place 

 where the protective apparatus cannot operate, there is 

 much danger of burning up the synchronous generators 

 in the station. But when the short circuit occurs the 

 voltage on the bus-bars drops and so whatever induction 

 generators are connected to the bus-bars stop generating 

 and so do themselves no harm. 



A synchronous generator (i.e., an ordinary alternator) 

 on the other hand, if short circuited would be very quickly 

 damaged, either by excessive mechanical strains or else 

 by overheating. 



A station having 50,000 kw. capacity of synchronous 

 generators and 50,000 kw. capacity of induction generators 

 in parallel with them has a short risk of only 50,000 kw. 

 while its operating capacity is 100,000 kw. It is thus 

 advantageous to have a good share of the generators in a 

 large station of the induction, or asynchronous type; but 

 too many induction generators would result in a low power 

 factor in the station and hence it is not advisable to have 

 more than about half of the generators of this type. 



The regenerative action of an induction motor running 

 above synchronous speed is used to some extent in electric 

 railway installations; a car running down hill may drive 

 its induction motors at higher than synchronous speed 

 and thus furnish power to the rest of the system. In addition 

 to the saving in power of such a scheme there is the advantage 

 that the motors are acting as brakes, thus saving the 

 braking equipment. Induction motors installed for hoisting 

 purposes may be used as induction generators in the same 

 way. 



