THE MAGNETIC CIRCUIT 



193 



ing factor. In the latter case, the allowable current density 

 will be about the same as in the shunt coils, unless the series 

 turns are next to the pole shoe, in which case a slightly higher 

 density would be permissible because of the better ventilation. 

 If the current to be carried exceeds 100 amp., the coils may be 

 made of flat copper strip wound edgewise by means of a special 

 machine. For smaller currents, cotton-covered wires of square 

 or rectangular section are commonly used; the round wire being 

 rarely employed, unless the diameter is less than that of No. 8 

 B. & S. gage. 



On account of the method of connecting the series coils be- 

 tween adjacent poles on a multipolar dynamo, there may be an 

 odd number of turns per pair of poles, or a whole number plus 

 a half turn on each pole. This will easily be understood by re- 

 ferring to Fig. 75 which is supposed to show a portion of the 



FIG. 75. Diagram of series field winding. 



crown of poles, looking down through the yoke ring onto the 

 cylindrical surface of the armature. The number of ampere- 

 turns required per pole being known, the number of turns in 

 the series winding can easily be calculated. It may not be 

 possible to put this exact number on the pole, and a slightly 

 greater number of turns is, therefore, provided, the excess of 

 current being shunted through a diverter. This is merely a re- 

 sistance connected as a shunt to the series winding. Thus if the 

 required series ampere-turns per pole are 2,000, and the current 

 300 amp., [the calculated number of turns per pole is 6.66. 

 Since 6> turns will not be sufficient, the winding may consist 

 of 7K turns, and the current required through the coils is, there- 

 fore, 2,000/7.5 = 267 amp. The balance of 33 amp. must be 

 shunted through the diverter, the resistance of which is easily 

 calculated after determining the resistance of the series coils 

 from the known cross-section and computed length of the winding. 

 59. Temperature Rise of Field Coils. On account of the 

 proximity of the shunt and series windings, it is advisable to 



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