CHAPTER IV 



DYNAMO DESIGN FUNDAMENTAL CONSIDERATIONS. 

 BRIEF OUTLINE OF PROBLEM 



18. Generation of E.m.f. It has been shown in previous 

 chapters how the strength and amount of the magnetic field 

 produced by an electric current may be calculated, and the next 

 step in the development of the dynamo is to consider how the 

 desired terminal voltage may be obtained by causing the armature 

 conductors to cut the magnetic flux which crosses the air gap from 

 pole to armature core. 



The D.C. motor is merely a dynamo of which the action has 

 been reversed; that is to say, instead of providing mechanical 

 energy to drive the armature conductors through the magnetic 

 field, an electric current from an outside source is sent through 

 the armature winding which, by revolving in the magnetic field, 

 converts electrical energy into mechanical energy. In the design 

 of a D.C. motor, the procedure is exactly the same as for a D.C. 

 generator, and in the following pages the dynamo will be thought 

 of mainly as a generator. 



Consider a flat coil of insulated wire of resistance R ohms, 

 consisting of S turns enclosing an area of A square centimeters. 

 Let this coil be thrust into, or withdrawn from, a magnetic 

 field of density B gausses, the direction of which is normal to the 

 plane of the coil. The quantity of electricity which will be set 

 in motion is expressed by the formula 



o- BAS 



Q - ~R~ 

 a relation that can be proved experimentally. 



But =7..X-^X 



where t = the time required to enclose or withdraw the flux 



(*-BA), 

 I m = the average value of the current in the coil during 



this period, and 



En the average value of the e.m.f. causing the flow of 

 electricity. 



70 



