THE SINGLE-PHASE SERIES MOTOR. 305 



(b) In the second place, it is to be noted that the reactance 

 voltage X f l which is lost in the field winding is proportional to 

 the product of the number of field turns and the maximum value 

 of the alternating field flux <l>, exactly as in the case of the pri- 

 mary coil of a transformer, and we may assume that we have a 

 motor in which a specified maximum flux is to be produced ; 

 then to make the reactance voltage X f l a minimum, the machine 

 must be designed so that the desired flux may be produced by 

 the least possible number of field turns. That is to say, the re- 

 luctance of the magnetic circuit must be small, or in other words 

 the gap space between the pole faces and the armature core must 

 be short and the iron of the field magnet and armature core must 

 be operated at a low degree of magnetic saturation. Of course 

 there is no clearly defined value of magnetic saturation which 

 gives the best results, and all that can be said in regard to this 

 second point is that low degree of magnetic saturation and short 

 air gaps are important features of the single-phase series motor. 



(c) In the third place the value of E e at a given speed may be 

 made fairly large, even though the value of <I> be rather small, 

 by increasing the number of armature conductors. Therefore 

 the use of a large amount of armature copper is an important 

 feature of the single-phase series motor. 



(d) In the fourth place, suppose that we have a single-phase 

 series motor running at given speed with given value (effective) 

 of current and given value (effective or maximum) of field flux. 

 Under these conditions, the counter electromotive force is definite 

 in value (effective), the mechanical power EJ is definite, and a 

 reduction of the frequency of the supply voltage would produce 

 a proportional reduction of the reactance voltages X f l and XJ 

 without affecting the value of E e ; that is, a reduction of frequency 

 would increase the power factor of the motor. Therefore it is 

 important to use a low frequency alternating current for a single- 

 phase series motor. A low frequency will be found to be de- 

 sirable in order to reduce the tendency to spark at the 

 brushes. 



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