ALTERNATOR LOSSES 173 



unity. A test at any other power factor may be carried out by 

 ah riing the coupling so that there is a corresponding phase difference 

 between the alternatm- e.m.i'.'s. 



97. Determination of No-load Losses 



The losses due to hysteresis, friction, and eddy currents may be 

 measured by coupling the alternator to a standardized motor (i.e. a 

 motor of known efficiency), and finding the power required to drive 

 the alternator on open circuit when excited with the normal exciting 

 current. If to this is added the armature copper loss, which may be 

 calculated from the known values of the armature resistance and 

 current, the total loss taking place in the machine is determined, and 

 the efficiency may be calculated. 



Another simple method of finding the open-circuit or no-load 

 losses consists in running the alternator as a synchronous motor, the 

 normal exciting current being used, and the p.d. across the armature 

 terminals being adjusted to correspond to minimum armature current. 

 The power supplied to the alternator when corrected for the small 

 armature copper loss gives the power lost in hysteresis, friction, and 

 eddy currents. 



98. Determination of Total Losses 



It must be remembered, however, that owing to field distortion 

 the actual hysteresis and eddy-current losses under full-load running 

 conditions will in general be somewhat greater than those determined 

 by either of the two methods just explained. If two similar alter- 

 nators are available, the total losses under full-load running conditions 

 may be approximately determined as follows. One of the alternators 

 is coupled to a standardized motor, and is driven as a generator, 

 supplying current to the other alternator, which runs as a synchronous 

 motor. By sufficiently increasing the exciting current of the generator, 

 and reducing that of the motor, the full-load current may be made to 

 circulate between the two machines. The power supplied by the 

 standardized motor then corresponds approximately to twice the total 

 full-load loss in either machine (exclusive of excitation). 



Another variety of this method of determining the total full-load 

 loss consists in running one of the alternators as a synchronous motor 

 at normal excitation, the armature p.d. being adjusted by varying the 

 generator excitation so as to cause the current to assume its full-load 

 value. The power supplied to the motor armature, which corresponds 

 to the total full-load loss, is measured by means of a wattmeter. 



