192 PRINCIPLES OF ELECTRICAL DESIGN 



Output of machine, kilowatts Exciting current, percentage of 



total current 



10 3.5 



20 3.0 



50 2.4 



100 2.0 



200 1.7 



300 1.6 



500 1.5 

 1,000 and larger 1 . 3 to 1 . 



The current density in the field-magnet windings of D.C. 

 machines will usually be between 900 and 1,400 amperes per 

 square inch. 



Series Windings. The series winding on the field magnets 

 carries the main current from the machine and thus adds to the 

 constant excitation of the shunt coils a number of ampere-turns 

 generally in accordance with the demand for a stronger field. 

 It is not usual to wind the series turns on the outside of the 

 shunt wire; but this may be done in small machines. The series 

 turns are usually placed at one end of the pole, either up against 

 the pole shoe, or more commonly near the yoke ring. Space 

 must, therefore, be left for the series winding at the time when 

 the dimensions of the shunt coil are decided upon. The total 

 winding space available may be divided in proportion to the 

 ampere-turns required on the shunt and series coils respectively. 

 The coils near the yoke ring, in a machine with revolving arma- 

 ture, are frequently made to project from the pole core farther 

 than the coils near the pole shoe, partly because the space 

 available between the poles increases with the radial distance 

 from the center, but also because the cooling effect of the air 

 thrown from the rotating armature will be greater if the field 

 windings are stepped out in this manner. 



The construction and insulation of field windings deserves 

 careful attention; but for details of this nature, the designer 

 must rely largely upon the practice of manufacturing firms and 

 his own common sense. The pressures to be considered in 

 D.C. designs are not high, and the chief points requiring atten- 

 tion are the proper arrangement and the insulation of the start- 

 ing and finishing ends of the coils. 1 



The size of conductors for use in the series winding may be 

 determined by considerations of permissible voltage drop, or, 

 if this is unimportant, the temperature rise will be the determin- 



1 Much useful information regarding the insulation of windings will be 

 found in Chap. IV of "Insulation and Design of Electrical Windings," by 

 A. P. M. FLEMING and R. JOHNSON (LONGMANS & Co.). 



