,,'IS 



ELECTRICAL MEASUREMENTS 



It 18 obvious that a different design of generator and trans- 

 former must l>c used. 



To preserve a sinusoidal wave form, the generator should be 

 a 7-wound three-phase machine with non-salient poles; it should 

 have a distributed, fractional-pitch (%) winding and be provided 

 with damping grids. 



The use of the Y- wound three-phase machine eliminates the 

 third harmonic in the e.m.f. wave; the fractional pitch greatly 

 reduces the fifth and seventh harmonics and the damping grids 

 tend to damp single-phase pulsating armature reaction. 



The transformer should be designed to work at low saturation. 



Tests of such a machine and transformer (designed by C. A. 

 Adams) fail to show any appreciable distortion of the wave form 

 under the most exacting conditions. No testing set should be 

 installed which is incapable of maintaining a sinusoidal test 

 voltage under all circumstances. Questions as to peak voltages 

 and abnormal dielectric losses are thus eliminated. 



Measurement of Peak Voltage. Those interested in the pur- 

 chase and installation of cables should be able to satisfy them- 



Aii Condenser 



nnn 



Fio. 439. Chubb and Forteseue arrangement for measuring high 



peak-voltages. 



selves as to the maximum voltage applied to the cable during 

 the breakdown test, for as shown by the above oscillograms it 

 may happen that the effective value of the voltage may give 

 little information as to its peak or maximum value. Also inter- 

 ested parties may be skeptical as to the maintenance of the sinu- 

 soidal wave form, even when the proper testing equipment is 

 used, and hence must be satisfied. 





