DYNAMO DESIGN 7:; 



simple matter to design a field system to furnish the necessary 

 magnetic flux. The designer is usually given the following data: 



Kw. output, 



Terminal voltage, 



Speed revolutions per minute. 



Sometimes the proper speed has to be determined by the de- 

 signer, as in getting out a Irhe of stock sizes of some particular 

 type of machine; in that case he will be guided by the practice of 

 manufacturers and the safe limits of peripheral speed. Other 

 conditions such as temperature rise, pressure compounding, 

 sparkless commutation of current, may be imposed by specifica- 

 tions, but if the designer can evolve a formula which will give him 

 an approximate idea of the weight or volume of the armature, 

 this will be of great assistance to him in determining the leading 

 dimensions for a preliminary design. Modifications or correc- 

 tions can easily be made later, after all the influencing factors 

 have been studied in detail. Many forms of the output formula 

 are used by designers. The formula is based on certain broad 

 assumptions, and is used for obtaining approximate dimen- 

 sions onty. Attempts to develop exact output formulas of 

 universal application should not be encouraged because it is not 

 possible to include all the influencing factors. The art of 

 designing will always demand individual skill and judgment, 

 which cannot be embodied in mathematical formulas. 



In developing an output formula it is not necessary to enter into 

 details of the armature winding, provided the total number of 

 conductors, together with the current and e.m.f. in each, are 



known. 







Let $ = maxwells per pole. 

 p number of poles. 

 N revolutions per minute. 

 Z = total number of armature inductors. 

 E c = volts per conductor. 

 I c = amperes per conductor. 



The output of the armature, expressed in watts, will be 



W = ZEJ C (39) 



where I c should include the exciting current in the shunt coils 

 of the field winding; a refinement which need not, however, 

 enter in the preliminary work. 



