348 ELECTRICAL EQUIPMENT 



effectively conduct away the heat and the supply should then be 

 increased. 



For a further consideration of the ventilation of the generator 

 room, see page 173. 



Brakes. In large, modern water-power installations the units 

 are very often provided with brakes, in order to stop them quickly. 

 Foreign material may obstruct the gates, preventing their closure; 

 so that, unless a brake is provided, it may not be possible to stop 

 the wheel without closing the emergency gates. The brakes are 

 generally applied to the generator rotor, the wooden face bearing 

 directly on the field rim, and the required pressure being obtained 

 by means of the oil pressure which is used for operating the gov- 

 ernors, or air pressure from the compressed air system. 



A band brake is also sometimes used, consisting of a flanged 

 pulley mounted on the main shaft and rotating in a steel brake 

 band into which are bolted blocks of maple. The band may be 

 tightened around the pulley by a worm gear operated by a hand 

 wheel on the main generator floor above. 



3. INDUCTION GENERATORS 



Output and Excitation. The induction generator is simply 

 an induction motor driven above its synchronous speed. It 

 requires a wattless exciting current for its operations and can, 

 therefore, not be operated as a self-contained unit, but always in 

 connection with synchronous machines, generators or motors. 

 These machines will then furnish the necessary excitation, and 

 also entirely govern the voltage and frequency of the induction 

 generator. 



The output depends on its speed above synchronism, and, with 

 the speed of the induction generator constant, it can only be 

 increased by decreasing the speed and thus the frequency of the 

 synchronous machinery. There can be no permanent short-cir- 

 cuit current flowing inasmuch as the exciting current disappears 

 when a short circuit takes place, and the momentary current rush 

 is also very small. 



Comparative Capacity of Induction and Synchronous Gen- 

 erators. Inasmuch as the induction generator cannot furnish 

 any wattless exciting current for the inductive load on the system 

 or for its own excitation, it follows that this must be furnished 

 entirely by the synchronous machines, thus increasing their 



