64 ELECTRICAL MACHINERY 



If carbon brushes are used on the machine, we must put 

 Efficiency is less than 



E -2 

 E ' 



(17) 



Now if Eg is, say, 6 volts, the efficiency of the generator 

 would have to be less than 66% ; as a matter of fact, it 

 would probably be about 40%. On higher voltage machines 

 this drop of 2 volts at the brush contacts is not such an 

 important factor in determining the efficiency. 



17. Field Windings. The field windings of a dynamo 

 electric machine serve to force the flux through the whole 

 of the magnetic circuit. The field coils are always placed 

 on the poles; the m.m.f. produced by these field coils must 

 be sufficient to force the magnetic flux through the poles, 

 yoke, air gaps, and armature core. If the length, area, 

 and permeability of each portion of the magnetic circuit 

 are known, the number of turns required in one field coil 

 and the current necessary can easily be calculated. 



Ampere-turns. The term ampere-turn will be used 

 frequently in our discussion; its significance is easily seen 

 by a few illustrations. A coil of 50 turns in which a current 

 of 2 amperes is flowing has 100 ampere-turns; a coil of 

 one turn having a current of 100 amperes has 100 ampere- 

 turns; a coil having 2000 turns in which a current of 0.8 

 ampere is flowing has 1600 ampere-turns. The number of 

 ampere-turns of a coil is equal to the product of the number 

 of turns in the coil and the current flowing in the coil. 



Referring to formulas (7) and (9), pp. 16 and 17, we 

 may write 







_ f- 



