266 ALTERNATING CURRENTS 



An alternator, by itself, cannot deliver power unless its field is 

 excited. The same is true of the induction generator. The 

 alternator usually receives its excitation from a direct-current 

 source or its equivalent and the resulting north and south poles 

 are rotated mechanically. The flux in the induction generator 

 is produced by the polyphase exciting currents in the stator wind- 

 ings and the resulting north and south poles rotate in the air gap 

 at synchronous speed. The currents which excite these north 

 and south poles come from the line. Therefore, the induction 

 generator does not receive its exciting current from a separate 

 source, but from the same lines that conduct away the energy 

 that it generates. The induction generator cannot generate its 

 own exciting current but the exciting current must be supplied by 

 the line. For this reason it is necessary to have either a static 

 condenser or synchronous apparatus in parallel with the induc- 

 tion generator for supplying its excitation. A static condenser, 

 however, is seldom practicable. 



Generator Motor 



FIG. 248. Vector diagram of induction motor and induction generator. 



Moreover, the induction generator can deliver only leading 

 current. If a load requires a lagging current, a synchronous 

 machine in parallel with the induction generator must supply 

 the lagging component of this current. 



The reason for this is as follows: 



Let V, Fig. 248, be the terminal voltage of an induction ma- 

 chine operating as motor. The counter or generated electro- 

 motive force E is approximately 180 from V. Let I be the 

 quadrature exciting current lagging 90 behind V and let l' m be 

 the motor energy current. The total current taken by the motor 

 is the resultant current I m , lagging behind V (see page 244, Fig. 

 233). When the rotor speed is increased by a sufficient amount, 

 the machine passes from motor to generator action. The mag- 

 nitude and the phase of the air-gap flux alter by only a slight 

 amount during this transition, just as the flux of a shunt motor 



