78 PRINCIPLES OF ELECTRICAL DESIGN 



It is not always possible or desirable to provide a square pole 

 face, and indeed it is necessary to check the dimensions of the 

 armature core by calculating the peripheral velocity. If a 

 suitable value for the peripheral velocity can be assumed, the 

 diameter is readily calculated because 



20. Number of Poles Pole Pitch Frequency. For calcula- 

 ting the relation between the length and diameter of armature core 

 by formula (46), the number of poles p must be known. The 

 selection of a suitable number of poles will be influenced by 

 considerations of frequency and pole pitch. 



Frequency of D.C. Machines. The frequency of currents in the 

 armature conductors and of flux reversals in the armature core 

 generally lies between 10 and 40 cycles per second in continuous- 

 current generators. Higher frequencies are allowable, but should 

 be avoided, if possible, because on account of increased losses 

 in the iron, or increased weight to limit these losses the use of 



p N 

 high frequencies is uneconomical. The frequency is / = X 



Z OU 



whence 



P - ^ (47) 



This relation is useful for determining the probable number of 

 poles when the diameter, and therefore the peripheral velocity, 

 are not known. 



Pole Pitch. The width of the pole pitch is limited by armature 

 reaction. It will readily be understood that the armature 

 ampere-turns per pole will be proportional to the pole pitch, 

 except for variations in the specific loading (q). With a large 

 number of ampere-turns per pole on the armature, it is necessary 

 to provide a correspondingly strong exciting field in order that the 

 armature shall not overpower the field and produce excessive 

 distortion of the air-gap flux, resulting in poor regulation and 

 sparking at the brushes with changes of load. A good practical 

 rule is that the ampere-conductors on the armature shall not 

 exceed 15,000 per pole; i.e., in the space of one pole pitch. 



Ampere-turns on Armature. Exactly what is meant by the 

 expression "ampere-turns per pole" when applied to the arma- 

 ture winding should be clearly understood. In a two-pole 



