CHAPTER XI 



88. Resistance measurements of alternating-current machinery 89. Insulation 

 t< stM 90. Experimental determination of equivalent impedance of transformer 

 91. Transformer efficiency tests 92. Separation of core losses 93. Heating 

 test of transformer 94. Ageing of transformer core 95. Determination of 

 open-circuit and short-circuit characteristics of alternator 98. Uopkinson 

 efficiency test 97. Determination of no-load losses 98. Determination of total 

 losses 99. Retardation method of determining losses 100. Heating test of 

 alternator. Mordey's and Behrend's methods 101. Goldschmidt's method 

 102. Hobart and Punga's method. 



88. Resistance Measurements of Alternating- 

 current Machinery 



SOME of the tests which have to be carried out in connection with 

 alternators and transformers are common to almost every class of 

 electrical machinery. Thus we have to determine the resistances of 

 the windings and the insulation resistances, and to test for dielectric 

 strength. We have, further, to find the maximum temperature rise 

 under normal working conditions. 



The conductor resistances are most conveniently determined hy 

 sending a suitable current from a battery of secondary cells through 

 the coil or coils under test, and measuring the drop of potential. In 

 the case of three-phase apparatus, we may have either a mesh or a 

 star connection of the windings. With the former arrangement of 

 circuits, the resistance of each phase (i.e. of each side of the triangle 

 formed by the windings) is clearly equal to 1 times the measured 

 resistance between two terminals, while with a star connection the 

 resistance of each phase is one-half of the measured resistance between 

 two terminals.* 



The voltage absorbed by the resistance of the windings if expressed 

 as a percentage of the total voltage may, in the case of a balanced 



* Let x = resistance of each phase of a A system. The resistance between any two 

 terminals is then the joint resistance of one phase connected in parallel with the other 



z . 2x 

 two joined in series. It is, therefore, r = Jr, so that x = ij x measured resistance. 



J" ~T " J" 



In the case of a star-connected system, it is obvious that between any two terminals 

 there are two phases in series with each other, so that the measured resistance is twice 

 that of one phase. 



