EQUIVALENT LOAD TESTS 47 



lagging current in the armature circuit. This is known 

 as the zero power-factor method. The rise in temper- 

 ature on the fields during open circuit run, and on the 

 armature during the short circuit run, is practically 

 the same as will obtain during operation under load. 

 The rises in temperature obtained from a circulating 

 open delta run are also so considered. 



With induction motors, it has been found that the 

 temperatures on low voltage runs when combined with 

 temperatures at no load and normal voltage, give very 

 nearly the same results as an actual load test. 



Except in the case of commutating pole machines, 

 it is often necessary to shift the brushes to get good 

 commutation while under load. The point at which 

 the best commutation is obtained is known as the 

 running point. Its position should be plainly marked 

 on both the rocker arm and the frame by means of a 

 chisel. 



Compounding consists in placing a shunt across the 

 series field terminals, in order to obtain the proper 

 voltage at no load and full load. The contacts of the 

 shunt should be perfect. In making a no-load field 

 setting on the machine, the voltage should be raised 

 about 15 per cent above normal no-load voltage, and 

 then reduced to normal. With the rheostat left in 

 this position, the load is thrown on, and if the com- 

 pounding is high, the resistance of the german silver 

 shunt should be reduced, a new no-load reading taken, 

 and the operation repeated. This should be continued 

 until the machine compounds according to specifications. 



To take final temperatures after a heat run requires 

 the greatest care. Arrangements should be made so 

 that no delay results in placing the thermometers on 

 the proper parts. Temperature readings should be 

 made every few minutes until all temperatures begin 

 to drop, when the thermometers may be removed. 

 When final temperatures are being taken the hot resist- 

 ance of the machine should be measured. After all 

 the necessary tests are made, the wiring should be 

 removed and the high potential tests applied while 

 the machine is still warm. 



