224 ALTERNATING CURRENTS 



delivered at terminals of generator, with stator current, slip, and 

 torque, we obtain the elegant graphical representation shown in Fig. 

 143. The ordinates of the three curves drawn correspond to stator 

 current, slip, and torque respectively, the abscissas representing power 

 absorbed from, or supplied to, the mains. 



Between the regions of generator and motor action the generator 

 action extending, in the circle diagram of Fig. 142, to a point a little 

 to the left of B (vertically over 0'), and the motor action to P we 

 have a region, corresponding roughly to the arc P S B, but extending 

 slightly beyond B, within which the machine is giving out neither 

 electrical nor mechanical energy, but is absorbing both, acting as a 

 sink of energy pure and simple. This region, it need hardly be 

 pointed out, is of purely theoretical interest, not only because for 

 obvious reasons it would never be used in practice, but because, even 

 if we did attempt to reach it,* the insulation of the coils would be 

 rapidly destroyed by the excessive currents. This portion of the 

 diagram is marked "heat radiator," as the only function then 

 performed by the machine is to absorb energy and convert it 

 into heat. 



132. Characteristic Features of Induction 

 Generator 



It must be carefully noted that the generator action of an induc- 

 tion motor depends wholly on the fact of its stator terminals being 

 in connection with a polyphase generator which supplies the necessary 

 exciting current required to maintain the rotating field. An induction 

 generator is thus not a self-exciting machine ; it can only be run in 

 parallel with a polyphase generator of ordinary construction. A 

 remarkable feature of the induction generator is the fact that its 

 frequency is independent of the speed, and that for small values of 

 the slip its output is roughly proportional to the slip as is immedi- 

 ately evident from the "slip" curve of Fig. 143. 



If we suppose the induction generator to be working on a non- 

 inductive load, then the current supplied to the load will be repre- 

 sented by the component O'P' in the diagram of Fig. 142. The 

 wattless component of the current, obtained by taking the projection 

 of O'P on the vertical, will be considerable, amounting, even in the 

 neighbourhood of maximum power factor, to some 30 per cent, of 

 O'P'. An induction generator thus requires a very large exciting 

 current ; and a plant consisting of induction generators would have 

 to include a supplementary plant of synchronous machines, of 



* The point K is, in any case, inaccessible for purely mechanical reasons, corre- 

 sponding, as it does, to an infinite speed. 



