1 64 ALTERNATING CURRENTS 



system, be calculated from the measured resistance between two 

 terminals and the current in each line wire, without any knowledge 

 of how the windings are connected. For if V = voltage between line 

 wires, I = current in each line wire, and r = measured resistance 

 between any two terminals, then in a mesh-connected system we 



have for the resistance per phase f r, for the current per phase =., 



\/3 

 and for the voltage per phase V, so that the percentage drop is 



3 



100 ^f = 100 . 2jy fl. In a star-connected system, the resistance 



per phase is r, the current per phase is I, and the voltage per phase 

 V 



is -, the percentage drop amounting to 100 -2- = 100 . _ >I 



Vz 



an expression identical with that obtained for a mesh-connected 

 system. 



It must be "remembered, in dealing with alternating-current 

 machinery, that the resistance as measured by means of a continuous 

 current is always less than the true resistance corresponding to an 

 alternating current, on account of eddy-current loss. With conductors 

 of small cross-section, the difference is inappreciable ; but with large 

 conductors the eddy-current loss may be considerable, amounting to 

 as much as 20 per cent, of the loss calculated from the resistance as 

 determined by means of continuous currents. 



89. Insulation Tests 



A considerable amount of importance was formerly attributed to 

 insulation resistance tests. Such tests are now, however, regarded as 

 of little value. The insulation resistance is an extremely variable 

 quantity, and measurements of it are liable to be misleading, both 

 when the results are very high and somewhat low.* A much more 

 important test is that for dielectric strength. By means of a suitable 

 testing transformer, a voltage considerably in excess of the normal 

 working voltage is applied to the winding for a definite period, and 

 the test may be regarded as satisfactory if no breakdown takes place 

 during that time. The dielectric strength test is applied to the insula- 

 tion between the winding and the core, and also to the insulation 

 between different windings if there are several on the same core. The 



* It is, in any case, unreasonable to expect the insulation resistance to exceed 

 1 megohm. 



