LATOUR ALTERNATOR 265 



We thus have a twofold method of altering the speed in one, 

 \\< alter the p.d. impressed on the brushes; in the other, we alter the 

 position of the brushes. 



158. Case of Synchronous Speed 



The case of synchronous speed is of particular importance from a 

 practical point of view. At this speed, the rotor winding contains 

 no induced e.m.f., and therefore behaves as if it were a simple non- 

 indiictire resistance. The rotor current is equal simply to the 

 quotient of the p.d. by the resistance, and is in phase with the 

 p.d. The winding must, therefore, have a sufficiently high resistance 

 to withstand the application of the full p.d. without damage.* The 

 rotor currents apart from minor fluctuations are continuous 

 currents (i.e. of zero frequency), and the rotor behaves as if continuous 

 current were supplied to it through three slip-rings, instead of 

 alternating current through brushes bearing on a commutator. It is, 

 further, clear that at synclironous speed the motor entirely ceases to 

 act as an induction motor it becomes a synchronous motor pure and 

 simple. At a speed slightly above or below that of synchronism, it 

 may be regarded as a combination of a synchronous with an induction 

 motor. 



The importance of maintaining nearly synchronous speed is due 

 to the fact that at this speed sparking troubles are reduced to a mini- 

 mum. In Latour's compensated induction motors, no special devices 

 are employed to suppress sparking ; in Heyland's motors, the commu- 

 tator segments are shunted by non-inductive resistances. 



Although compensated motors of this type may be made to have 

 a higher power factor than ordinary induction motors, they have not 

 come into use to any large extent, mainly on account of the additional 

 complications involved. Their designers have, however, been led by 

 a study of their peculiarities to the construction of self-exciting and 

 compounded alternators. 



159. Latour Alternator 



The armature winding of a polyphase generator does not in any 

 way differ from the stator winding of an induction motor. Now, we 

 have just seen that if a rotor of the commutator type be supplied 

 with three-phase currents, when running at nearly synchronous speed 



* Since this would necessitate the use of very fine wire ami many turns on the rotor 

 if the brashes were directly across the mains, in motors of this type as actually 

 constructed a transformer for reducing the stater p.d. is used. The stutor winding 

 itself is made to act as the primary of this transformer, the secondary consisting of a 

 few turns embedded in the stutor slots and having the rotor winding for its load. 



