SPEED CONTROL 227 



(which is the same as that of the currents in rotor I) steadily 

 decreases, and if we suppose that the speed has nearly reached half 

 the speed of synchronism (for motor I) then motor II will be running 

 at nearly synchronous speed. Hence at this speed the torque 

 exerted by it will be very small, the current being also small and 

 nearly wattless. The current in stator II will, therefore, also be 

 small and nearly wattless, and so will the current in rotor I. The 

 torque exerted by motor I will thus also be small. We therefore 

 find that as half the speed of synchronism is approached the 

 combined torque decreases, so that two motors mounted on the same 

 shaft and coupled in cascade will run at approximately half the speed 

 of synchronism.* If both motors are connected in parallel across the 

 mains, they will run at nearly synchronous speed, with the result 

 that the speed will be doubled. Thus by changing from the parallel 

 to the cascade grouping we are able to halve the speed. 



It may be pointed out that in the cascade arrangement motor I 

 performs two functions ; it acts as a motor, and also as a frequency 

 transformer, practically reducing the frequency of supply to half its 

 value. The rotor winding of motor I must, of course, be wound for 

 the voltage of the supply mains, if the change from the tandem to 

 the parallel grouping is to be feasible. 



Various modifications of this arrangement may be used. Thus, 

 motor II may have a different number of poles from motor I, so as 

 to give a speed greater or less than half the speed of synchronism ; 

 and a third speed will be obtainable by fitting motor II with a pole- 

 changing switch.! 



Closely allied to the cascade method is what we may describe 

 as the multiple motor method.} This is used by Messrs. C. Wiist & 

 Co., of Switzerland, and consists in mounting several rotors (in 

 practice, from 2 to 4) on the same shaft, each rotor being surrounded 

 by a corresponding stator. By choosing suitable numbers of poles 

 for the different stators, as many different speeds as there are motors 

 may be obtained by using each motor singly. Other speeds are 

 obtainable by using two or more motors simultaneously, and the 

 speeds will be different according as the torques developed by the 

 motors all act in the same direction or oppose each other. 



* So far as motor II is concerned, this speed ia, of coarse, its speed of synchronism. 



t A largo motor of this type, designed by Mr. E. Danielson, has recently been 

 installed at the iron-works at Sandviken, Sweden (Elektrutechnische Zeitschrift, vol. xzv. 

 p. 43(1904)). 



J Elektrotechnitche Zeitschrift, vol. xxiv. p. 694 (1903). 



