DYNAMO DESIGN 



Hence (BA) X S 



E m = 



, 



where all quantities are expressed in absolute C.G.S. units. If 

 we put 3>for the flux (BA) in maxwells, and express the e.m.f. 

 in the practical system of units, we have 



*- - volts 



For the condition S = unity, this formula is clearly seen to 

 express the well-known relation between rate of change of flux 

 and resulting e.m.f., namely that one hundred million maxwells 

 cut per second generate one volt. This is the fundamental law upon 

 which all quantitative work in dynamo design is based. The 

 procedure for obtaining a given amount of flux was explained 

 in previous chapters, and we now see that the voltage of any 

 dynamo-electric generator may be calculated by applying formula 

 (37). For the rest, the electrical part of the designer's work 

 consists in providing a sufficient cross-section of copper to carry 

 the required current, and a sufficient cross-section of iron to carry 

 the required flux, in order that the machine shall not heat ab- 

 normally under working conditions. There are other matters of 

 importance such as regulation, efficiency, economy of material, 

 and in D.C. machines commutation, which require careful 

 study; but it is hardly an exaggeration to say that apart from 

 mechanical considerations, which are not dealt with in this book 

 the work of the designer of electrical machinery is based on two 

 fundamental laws: (1) the law of the magnetic circuit, namely, 

 that the flux is equal to the ratio of magnetomotive force to reluc- 

 tance, and (2) the law of the generation of an e.m.f., namely, that 

 one hundred million lines cut per second generate one volt. 



At any particular moment it is the rate of change of the flux in 

 the circuit that determines the instantaneous value of the 

 voltage, or, in symbols, 



d$ 

 instantaneous volts in circuit of one turn = -^ X 10~ 8 



where the negative sign is introduced because the developed 

 e.m.f. always tends to set up a current the magnetizing effect of 

 Which opposes the change of flux. 



Consider a dynamo with any number of poles p. Fig. 23 

 shows a four-pole machine with one face-conductor driven 



