CHAPTER VII. 



PHASE CHANGERS, POTENTIAL REGULATORS, ETC. 

 EXPERIMENT 7-A. Polyphase Transformation. 



i. Possible Kinds of Transformation. The transmission of 

 power in a single-phase system is pulsating, no matter what the 

 character of the load. This can be readily seen by sketching 

 assumed curves for the instantaneous values of an alternating 

 electromotive force and current, and plotting the products of the 

 ordinates from instant to instant as a power curve.* In a bal- 

 anced polyphase system, however, the pulsations of power in the 

 different phases are seen to so combine that the total transmis- 

 sion of power is uniform,f without pulsation. (See 2, Exp. 

 6-A.) 



2. Polyphase to Single-phase Transformation not Possible. 

 In a transformer, neglecting the slight modification due to losses, 

 the power given into the primary at any instant is equal to the 

 power taken out of the secondary at that instant. It is not pos- 

 sible, therefore, simply by means of transformers to change a 



* ( la). The area included between the power curve and time axis rep- 

 resents energy, this energy being positive (supplied to the line) or nega- 

 tive (returned from the line) according to whether the current and electro- 

 motive force have, at the time, like or unlike signs. It is instructive to 

 sketch curves for currents differing in phase from the electromotive force 

 by o, 45 and 90 degrees. 



t ( ib). This can be shown for a 2-phase system by drawing, for each 

 phase, sine curves for electromotive force and current and plotting the 

 product as a power curve. Two power curves are thus obtained, one for 

 each phase, and it will be seen that the crests of one correspond to the 

 hollows in the other, the algebraic sum of the two power curves being 

 constant. The sum of the three power curves for a 3-phase circuit can 

 be shown to be constant in the same way. 

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