346 ELECTRICAL MACHINERY 



the rotor always turns with a speed somewhat less than 

 that of the revolving field and that the speed of rotation 

 of the field is determined by the frequency of the power 

 supply. A motor designed for 25 cycles will run at more 

 than twice its rated speed if. connected to a 60-cycle line. 

 In fact the speed of the motor is practically proportonal 

 to the frequency of the power supply. 



A motor designed for a low frequency can safely be 

 run on higher frequency lines, provided the mechanical 

 strains in the rotor are not dangerous at the higher speed. 

 But a motor designed for 60 cycles should never be operated 

 on a 25-cycle line because of the excessive densities result- 

 ing in the magnetic circuit and the corresponding high 

 hysteresis loss. The multiple frequency scheme for speed 

 control never came into prominence owing probably to the 

 complexity of the wiring and station apparatus. 



Varying the Number of Poles. In another scheme for 

 speed control the windings of the stator are connected 

 through a series of switches; by proper operation of these 

 switches it is possible to change the number of poles on the 

 stator. Thus a motor might have its windings so connected 

 through these switches that when they are thrown one way 

 the motor has eight poles and when thrown in the opposite 

 way the motor might have six poles. This scheme, while it 

 gives an efficient speed control, requires rather complicated 

 connections on the coils and so is not used as yet to a great 

 extent. 



Cascade Connection. In some installations (notably 

 railway equipment) two motors are used connected in 

 concatenation or cascade. The stator of the first motor 

 is connected to the line, the slip rings of the first rotor are 

 connected to the stator windings of the second, and the 

 second rotor circuit is closed through a variable resistance. 

 When all the resistance is cut out of rotor No. 2, both 

 motors operate at half the speed they would have if directly 

 connected to the line (with rotor short circuited). This 



