268 



ELEMENTS OF ELECTRICAL ENGINEERING. 



Supply taain 1 



converter by inherent induction motor action, as explained in 



Art. 132. 



Figure 237 shows the interior of a three-phase starting com- 

 pensator in the lower part of 

 which is a cylinder switch ior 

 making the successive connec- 

 tions above mentioned. Fig- 

 ure 238 is a diagram of 

 connections of a three-phase 

 compensator in which one in- 

 termediate voltage is provided 

 for. To start the motor, switch 

 S f is closed, and then switch 

 vS is closed on the starting 

 side, thus connecting the low 

 voltage taps of the compen- 

 sator to the motor. When the 

 motor reaches a moderate 

 speed, switch 5 is suddenly 

 thrown to the running side, 



'.low voltage taps 



Xf- connection 



Fig. 238. 



thus connecting the full supply voltage to the motor, and then 

 switch S f is opened, leaving the compensator entirely discon- 

 nected. The trip coils actuate a trigger which opens a circuit 

 breaker if the current exceeds a prescribed value. 



129. Speed control of the induction motor. The speed n of 

 the stator magnetism of an induction motor is fixed by the fre- 

 quency / of the alternating-current supply and by the arrange- 

 ment of the stator windings (number of polar regions on stator). 

 Therefore the zero-load speed of an induction motor, which is of 

 course equal to the speed of the stator magnetism, is invariable, 

 unless the frequency of supply or the arrangement of the stator 

 windings is changed. 



On the other hand, the rotor speed n' of an induction motor 

 with given load may have any value between zero and normal 

 full speed if the rotor resistance is increased to the proper value. 



