CHAPTER IX. 



INDUCTION MACHINES: FREQUENCY CHANGERS AND INDUCTION 



GENERATORS. 



EXPERIMENT 9-A. Operation and Test of a Frequency 

 Changer (Secondary Generator). 



i. Principles of Operation. The frequency of the current 

 in the secondary of an induction motor depends upon the speed 

 of rotation of the rotor; within limits any desired secondary fre- 

 quency can be obtained by giving the rotor the proper speed. 

 The usual form of frequency changer or frequency converter 

 consists merely of an induction motor and a separate driving 

 motor for driving the rotor at the proper speed. 



The primary is furnished with polyphase current and produces 

 a rotating magnetic field as in any induction motor. The second- 

 ary is phase-wound and delivers, usually, polyphase current to 

 the receiving circuit. The driving motor may be of any type, 

 but in commercial practice a direct-connected synchronous motor 

 is commonly used, so that the delivered current has a definite 

 fixed frequency. (Since the induction machine takes a lagging 

 current of low power factor, the synchronous motor by taking 

 leading current will raise the power factor of the set.) 



2. The secondary frequency is increased or decreased accord- 

 ing as the rotor is turned in the opposite direction or in the same 

 direction as the rotating field ; the secondary voltage is increased 

 or decreased in proportion to the secondary frequency. Aside 

 from losses, the secondary current (with a i: I ratio) is equal to 

 the primary current, either for an increased or decreased fre- 

 quency. 



Frequency changers are generally used to change from a lower 

 to a higher frequency, for example, from a low-frequency 

 power circuit to a 6o-cycle lighting circuit. 



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