4 SYNCHRONOUS MOTORS 



The efficiencies of the three-phase motors are a little higher than 

 those given for the single-phase motors. The horse-powers given in 

 this table correspond to a frequency of 42 periods, but these motors 

 can be also used at frequencies between 40 and 60 periods, and their 

 power then increases with the frequency. 



As the table shows, types Nos. 14 to 30 are made with 4 poles, 

 self-exciting. For higher powers, the number of poles increases, and 

 the excitation is obtained by means of a small direct current exciter 

 mounted on the same base. Above type 90 the armature is stationary 

 and the fields turn inside. The fields are of mild cast steel, the arma- 

 tures being slotted. 



FIG. 4. 



As an example of these large motors may be cited several from 50 

 to 100 H.P.. giving the best of results on the power-transmission system 

 around Grenoble, notably at Voiron, a distance of 30 kilometers from 

 the generating station. Their efficiency is from 90 to 92 per cent. 

 One of these motors even works in parallel with a steam-engine of the 

 same power, and it compensates for the variation of angular velocity 

 of the engine as it passes the dead centers. 



All these motors are provided with a clutch and with an idle pulley 

 for starting, as will be explained later. When running, they can 

 undergo considerable variations of load without falling out of step. 



Attention should also be called to another interesting type of syn- 

 chronous motor, the Maurice Leblanc type, which is characterized 

 by the addition of dosed circuits in the pole-pieces to insure a perfect 

 damping of oscillations, as will be seen later. 



