120 



ALTERNATING CURRENTS 



number of slots to be doubled, so as to bring all the conductors into 

 the same layer, we have the following winding table : 



1254973 

 2265074 

 3275175 

 428 52 76 

 5295377 

 6305478 

 7_315579 

 8325680 



The voltage for which a rotor is wound is immaterial * so far as 

 the behaviour of the motor is concerned, and is merely a matter of 

 convenience from a manufacturing point of view. It generally lies 

 between 100 and 400 volts across the slip-rings when the rotor is at 

 rest and the slip-rings are open-circuited. 



63. Starting Resistances for Rotors. Internal 

 Short-circuiting and Brush - lifting Gear 



The usual form of starting resistance for a three-phase rotor 

 is shown diagrammatically in Fig. 93. It consists of a three-armed 



switch provided with a triple 



S 1"7^M/ se ^ ^ con ^ ac ^ s an d resistances. 



"^ n rs . 7%. The movable arms of the switch 



form the neutral point of the 

 starting resistance. The termi- 

 nals marked Si, S 2 , and S 3 are 

 in connection with the slip- 

 rings. As the switch is moved 

 over the contacts, the resistances 

 are gradually cut out. 



In the case of large motors, 

 it becomes necessary to provide 

 a considerable number of con- 

 tacts, in order to avoid violent 

 fluctuations in the current (and 

 consequent disturbance of the 

 voltage of the supply system). 

 This increases the cost of the 

 The same end may, however, be attained with a 



FIG. 93. Starting Resistance for Wound 

 Eotor. 



starting switch. 



comparatively small number of contacts by means of the arrangement 



* Assuming the same amount of copper to be used iu each case. 



