280 ALTERNATING CURRENTS 



In the alternating-current series motor this difficulty is elimi- 

 nated by laminating the field structure. Even with laminated 

 field-cores, however, losses in the iron occur with alternating cur- 

 rent which do not occur with direct current. 



(b) There is a relatively large voltage drop across the series fields, 

 due to their high reactance. This limits the current and also re- 

 duces the output and power-factor to such low values as to make 

 the motor impracticable. 



In the alternating-current motor this difficulty is partially 

 overcome as follows: 



A low frequency is used, since reactance, X, is 2irfL, where / 

 is the frequency and L the inductance. Even when the field 



Compensating 

 Winding 



FIG. 257. Windings of an alternating-current series motor. 



inductance, L, is made as low as is practicable, the field reactance, 

 X, will be considerably too high unless the frequency /is made low. 

 The usual lighting frequency of 60 cycles is much too high, except 

 for motors of fractional horsepower rating. Difficulty is ex- 

 perienced in designing a series motor for a frequency of 25 

 cycles, even. To obtain satisfactory operation, frequencies 

 of 12Jx<2 and 15 cycles are commonly used abroad for this type of 

 motor. 



The inductance varies as the square of the number of turns. 

 The turns per pole must therefore be reduced to a minimum in 

 order to keep the inductance, and therefore the reactance, low. 

 To obtain sufficient flux with few ampere-turns per pole, the 



