ALTERNATORS. 253 



The losses (b) should be determined by a special experi- 

 ment. Some of the most-used methods are given below. 



An approximate calculation of the iron losses can be made 

 in the same way as that described in the case of transformers, 

 from the volume and induction of the iron subjected to the 

 alternating flux. 



The losses (c) can be directly calculated from the resistance 

 of the field windings and voltage of the excitation. 



The losses (d) can usually be determined at the same 

 time as the iron losses. 



Determination of Losses. For the copper losses (a and c] 

 the only determination to be made is that of the resistance 

 of the windings, which is usually performed by sending a 

 measured direct current through them and measuring the 

 fall of potential in the windings. In each case watts lost = 

 (current) 2 x resistance. 



In order to determine the frictional and iron losses, the 

 simplest method is to run the alternator as a synchronous 

 motor. After running the machine up to normal speed and 

 switching on the supply, the field is adjusted until the 

 current taken is at its Jeast value, and the power-factor of 

 the driving circuit is consequently high. The power taken 

 by the armature under these conditions is read on a watt- 

 meter, and is the sum of the losses due to friction, hysteresis, 

 and eddy currents. The copper losses in the armature due 

 to the driving current will probably be too small in most 

 oases to be worth subtracting. Allowance can easily be 

 made for them from the known resistance of the armature 

 and the value of the current, which must in any case be 

 observed in order to ascertain whether the excitation is 

 correct. Assuming (which is nearly though not accurately 

 true) that the friction and iron losses remain the same at 

 all loads, the total loss at any load is obtained for any load 

 current by adding together the watts thus measured and the 

 calculated copper losses in the armature and field. 



Another method which is easily carried out, if a direct- 

 current motor of known efficiency is available, is the following: 



The motor is coupled to the alternator, and made to 

 drive it at the full speed of the alternator. If the alternator 

 fields are unexcited, there will be practically no iron losses, 

 and the whole of the power supplied by the motor will be 

 due to mechanical friction. If the normal excitation of. thq 



