THE IM)l'('TION MOTOR . 261 



A detailed analysis of the operation of this apparatus involves 

 a somewhat complicated vector diagram and is beyond t lie scope 

 of this lx>ok. However, it can be shown that the commutator 

 motor develops an electromotive force in each phase which is 

 nearly in phase-opposition to its respective phase current. This 

 emf. is practically independent of the slip, if the speed of the 

 commutator motor be held constant by its load, such as the 

 generator G. Therefore, neglecting impedance drops, the rotor 

 of the main motor will slip until its emf. is equal to the counter 

 en if. of the commutator motor. As the rotor emf. is proportional 

 to the slip, the rotor slip will be constant at all loads if the impe- 

 dance drops be neglected. 



When a current flows in opposition to an emf. (as in a direct- 

 current motor) it gives up energy. Therefore, the current de- 

 livered to motor B gives up energy, some of which is returned to 

 the line through generator G. Ordinarily, in the wound-rotor 

 type of motor this energy is lost in heating a resistance. The fact 

 that this counter emf. is constant gives the motor a practically 

 const ant -speed characteristic for any one adjustment. Speed 

 adjustments are made by changing the positions of the taps of 

 the auto-transformer B. Because of the cost of two extra 

 machines, this method has been but little employed in this 

 country except in the very large units used in steel mills, where 

 the method is now coming into general use. 



Change of Frequency. Commercial power systems operate at 

 constant frequency and it is impossible to control the speed of 

 induction motors by change of frequency when the motors take 

 t heir |H>\ver from such systems. In a few special instances, such 

 as in the electric propulsion of battleships, (General Electric 

 Review, April 1919), the motors arc the only loads connected to 

 the turUn-Mheniatora. Therefore, it is possible to obtain speed 

 control by changing the speed of the turbines them- 

 Kven hen- the range of speed variation is limited, because the 

 efficiency of turbines decreases very rapidly when th 

 departs from the speed for which they a- '-d. 



Change of Poles. By means of a suitable >\\itch. (1 

 DOnnectioni may be chaimed in such a manner that the number 



of pole- i> ohaaged This champs th'' synchronous -peed of the 

 motor and then-fore tin- >pe-d .f the rotor. If the poles be 



