6-A] 



GENERAL STUDY. 



219 



If, instead of coils with iron, three resistances are used, the har- 

 monics cannot be heard; the fundamental will no doubt be heard 

 due to lack of perfect symmetry, and will become louder if the circuits 

 are thrown more out of balance. 



45. If an electrostatic voltmeter is available, connect it between 

 O and 0' (in place of the telephone) and measure the hysteresis 

 voltage, H. Measure also from one line wire X, the voltages 

 OX and O'X, the latter being the larger on account of hysteresis 

 harmonics. 



Compute O'X from the formula 0'X = V(OX) 2 H- -H* and com- 

 pare with the measured value. It is to be noted that OX = E-- \/3 

 and O'X = s. By what per cent, is s greater than E ~ \/3 ? 

 What per cent, is EH of E -=- V3 ? 



46. With a voltmeter* measure the line voltage XYZ and con- 

 struct a triangle as in Fig. 16. Measure also the three star voltages 

 O'X, O'Y, O'Z and lay them off as shown, each one twice. A supply 

 neutral is not necessary for this test. Cut 

 out the diagram on the heavy lines and 

 fold on the light lines, bringing the three 

 points 0' together so as to form a pyramid. 

 The height of the pyramid represents the 

 voltage EH due to hysteresis harmonics. 



47. The foregoing illustrates the fact 

 that vectors in a plane can exactly represent 

 only currents and electromotive forces 

 which are simple sine functions; the error 

 due to harmonics is commonly neglected. 



48. Generator Coils. If there is a third harmonic in the generated 

 electromotive force, with the generator coils delta-connected it cannot 

 appear in the line but will appear as a circulating current in the delta. 

 This may cause appreciable heating if the harmonic is large. 



49. The third harmonic can appear on the line only in case the 

 generator coils are y-connected and have the neutral connected to 

 ground or a 4th wire. If the line is not grounded also at the receiv- 



*Use an electrostatic voltmeter; although this is not important with 

 large transformers, it becomes necessary in case the coils or transformers 

 are small, as the current taken by an ordinary voltmeter may cause con- 

 siderable error. 



