CORE Loss TESTS 21 



the power to drive such machines will be the volt- 

 ampere input. Wattmeters may be used in addition 

 to check the volt-ampere readings. This measurement 

 includes friction and windage losses, together with 

 open circuit core loss plus the C 2 R loss in the armature. 

 If the value of the core loss need not be separated from 

 the other losses, the test is useful for checking up full 

 load efficiencies. For direct current apparatus this 

 test is obtained quickly and at less expense than by 

 other more elaborate methods, and the results are just 

 as accurate and satisfactory. 



The "belted core loss" method separates the core 

 loss from the bearing friction, brush friction and windage. 

 A small direct current motor is used to drive the machine 

 under test as a generator at its rated speed. A 

 belt drive between these machines is most commonly 

 used. However, wherever great accuracy or a high 

 speed is necessary, direct driving by a coupling is often 

 used. 



The driving motor for this test must be carefully 

 chosen. It must be operated with good commutation 

 and with a fixed setting of the brushes, through the 

 range of load required for the core loss test. Ordinarily, a 

 safe rule to follow is that the motor should be of approx- 

 imately 10 per cent of the capacity of the machine 

 under test. The maximum load which this motor 

 should carry with the heaviest field on the machine 

 under test, should not exceed 50 per cent of its normal 

 rated capacity. The driving motor should be operated 

 as nearly as possible at its rated speed and field strength ; 

 the brushes should be carefully set at the best position 

 for good commutation at all the loads required by the 

 test; and the commutator surface should be in first 

 class condition. 



The weight and width of the belt must be selected 

 to give minimum loss. When testing motor-generator 

 sets, rotary converters, and other apparatus that does 

 not use belts, the tension of the belt must be kept as 

 low as practicable, so that the bearing friction is not 

 increased due to the belt pull. Endless belts should 

 be used in preference to laced belts. 



