COMMUTATION 



171 



61. Example of Interpole Design. The numbers and dimen- 

 sions used to illustrate the method of calculation outlined above 

 will be chosen without reference to an actual design of interpole 

 dynamo, and they must not be considered representative of 

 modern practice. Assume the leading particulars of the machine 

 to be as follows: 



Output = 200 kw. 



Volts = 440. 



R.p.m. = 500. 



Number of main poles p = 6. 



Armature core diameter D = 30 in. 



Armature core length l a = 11 in. = 28 cm. 



Total number of slots = 120. 



Number of slots per pole n = 20. 



Slot pitch X = 0.785 in. 



Slot width s = 0.39 in. 



Slot depth d = 1.5 in. 



Style of winding: full-pitch, multiple. 



Current per armature path I c = 76 amp. 



Number of conductors per slot = 8. 



Total number of conductors Z = 120 X 8 = 960. 



Number of commutator bars = 240. (There are four coil- 

 sides per slot, or two coils, giving two turns between adjacent 

 commutator bars.) 



Diameter of commutator = 20 in. 



Pitch of commutator bars = 



7TX20 



240 



= 0.262 in. 



Number of bars covered by brush = 3.5. 

 Thickness of brush (brush arc) W = 0.262 X 3.5 

 Brush arc referred to armature periphery 

 0.916 X 30 



0.916 in. 



W 



20 



= 1.375 in. = 3.5 cm. 



Assuming the same number of commutating poles as there are 

 main poles, the flux entering the armature teeth in the commutat- 

 ing zone of width W a should have the value given by formula (85). 

 The end flux cut by the short-circuited coils is given approxi- 

 mately by formula (72) in which the coefficient k may be given 

 the value 2. Thus: 



= 0.4\/2 X 2 X 4 X 76 X 20 X 3.5 X (3.69 - 1) 

 = 64,800 maxwells. 



