72 ELECTRICAL MACHINERY 



it was 2400 circular mils; this wire could safely carry 

 2 amperes so that if 3000 ampere-turns were required 

 per pole the proper number of turns = 3000/2 =1500 per 

 coil. The figure 1200 circular mils per ampere, is good 

 only for field coils where the wires are packed tightly 

 together and the heat cannot easily escape. In armature 

 windings, where the ventilation is better, it is customary 

 to allow 750 circular mils per ampere, and if the wire were 

 stretched in the open, probably 200-300 circular mils would 

 be a sufficient allowance. 



Construction of Field Coils. The kind of wire to be 

 used in making a field coil and the method of winding it 

 depends upon the machine for which it is to serve. For 

 small machines double cotton covered wire or enamel 

 covered wire is wound on a properly shaped wooden form. 

 It is then taken off and taped, and then dried and impreg- 

 nated with some insulating compound. When cold the 

 coil is ready for assembly on the machine. 



For practically all revolving fields, copper ribbon is 

 used. This is wound on the spool edgewise. Only one 

 layer of winding is used so that each turn has an edge 

 exposed to the outside air. This construction serves to 

 more readily cool the winding, and, therefore, less than 

 1200 circular mils per ampere is required for ribbon wound 

 coils. Another reason for using ribbon, wound edgewise, 

 on revolving fields is that a field coil must be mechanically 

 strong to stand the centrifugal force exerted on its outer 

 edge (where it comes in contact with the pole shoe) when 

 the field is revolving. The ribbon wound coil stands 

 this strain easily even on very high speed machines while 

 a wire wound coil might possibly crush under the excessive 

 pressure due to centrifugal force. 



The copper ribbon used for these field coils is insulated 

 by attaching a strip of paper on one side only, this being 

 sufficient to keep the adjacent turns from touching one 

 another. The outside edge has on it no insulation at all, 



