HEATING OF ALTERNATOR 175 



ioo. Heating Test of Alternator. Mordey's and 

 Behrend's Methods 



In order to determine the temperature rise of an alternator, the 

 machine must be kept running under normal full-load conditions 

 until the temperature approaches a steady value. Since the period 

 dining which an alternator of large output would have to be run for 

 this purpose is very considerable, such a heat test would prove very 

 expensive if the machine were actually run fully loaded. Numerous 

 methods have, therefore, been devised by means of which the heat 

 test may be carried out with the expenditure of only a relatively 

 small amount of energy. 



If two similar machines are available, the arrangement used in 

 connection with the Hopkinson test ( 96) furnishes a ready solution 

 of the problem. Or the method of driving one of the machines as an 

 under-excited generator, and running the other as a synchronous 

 motor ( 98) may be used. 



Where only a single alternator is available for the test, the 

 method originally suggested by Mordey,* and recently improved by 

 Behrend,f may be resorted to. 



In Mordey's method, the armature winding is divided into two 

 unequal portions, which are coupled in series so that their e.m.f.'s 

 oppose each other, the entire winding being then short-circuited 

 through an ammeter. The short-circuit current so obtained is due to 

 the difference between the e.m.f.'s in the two sections of the winding, 

 and its value may be adjusted by means of a suitable choking coil 

 included in the armature circuit. Instead of driving the alternator 

 mechanically, it may be driven electrically, by supplying to it an 

 alternating current at a much lower p.d. than the normal p.d. of the 

 alternator, the auxiliary source of alternating e.m.f. (a small alternator 

 or transformer) being introduced into the local circuit of the two 

 unequal sections of the armature winding, which are connected so 

 as to be in opposition to each other. The stronger section (i.e. the 

 one having the higher e.m.f.) then acts as a motor, the weaker one 

 acting as a generator. 



Instead of dividing the armature winding into two unequal 

 sections, the field winding may be similarly divided, the connections 

 being arranged so that two opposing unequal e.m.f.'s are induced in 

 the armature winding. 



Neither of these arrangements can be successfully employed in 

 connection with a modern revolving field alternator, on account of 



* Journal of th* Inttitution of Electrical Engineer^ vol. xxii. p. 116 (1893). 

 t Electrical World and Engineer, vol. xlli. p. 715 (1903). 



