PARTS OF A DYNAMO-ELECTRIC MACHINE 71 



to be determined. Here we at once notice a peculiarity 

 in the problem. So far as the magnetizing effect of the coil 

 is concerned, with a given supply voltage, it makes no difference 

 how many turns are used for the field coil; no matter how many 

 turns are used the m.m.f. of the coil will be the same. 



Suppose that we put on 1000 turns and that this number 

 of turns has a resistance of 50 ohms. If the voltage of the 

 field current supply is 100 volts the field current will be 

 2 amperes and the ampere-turns of the coil will be 2000. 

 If, now, 2000 turns are used the resistance of the coil 

 increases to 100 ohms, the field current decreases to one 

 ampere and the ampere-turns in the coil will be 2000 as 

 before. 



Proper Number of Turns. There must be some method 

 of determining the proper number of turns to use; and 

 this is fixed by the safe allowable temperature rise in the coil. 

 It is apparent that if, for example, only one turn were used 

 in the field coil, the proper number of ampere-turns would 

 be obtained but the current through the one turn would 

 be so great that the wire would melt. As the number of 

 turns is increased, the required current decreases; when 

 the number of turns has been increased to such an extent 

 that the current which flows through the coil does not 

 heat the wire more than 50 C. above room temperature 

 (the limit fixed by the A.I.E.E.), we may say that the 

 field coil has been properly designed. 



The process of predicting the temperature rise from the 

 power used up as heat in the coil (I 2 R loss in the coil) and 

 the calculated radiating surface requires judgment and 

 experience, because some radiating surfaces are more 

 effective than others, etc. 



It has been found, however, from the results of many 

 tests that a safe figure to use in fixing the number of turns 

 is obtained by allowing 1200 circular mils per ampere in 

 the winding. The proper sized wire has already been found 

 and .so the cross-section in circular mils is known. Suppose 



