84 PRINCIPLES OF ELECTRICAL DESIGN 



more common types; but it is not suitable for high voltages, 

 and the friction and PR losses in the brushes are very large. 1 



22. Ring- and Drum- wound Armatures. The GRAMME ring 

 winding is now practically obsolete. In this type of machine 

 the coils form a continuous winding around the armature core 

 which is in the form of a ring with a sufficient opening to allow 

 of the wires passing down the inside of the core parallel to the 

 axis of rotation. The objections to this winding are the high 

 resistance and reactance of the armature coils due to the large 

 proportion of the " inactive" material per turn. This has a 

 bearing not only on the cost and efficiency of the machine but 

 also on commutation, because the high inductance of the wind- 

 ings is liable to cause sparking at the brushes. 



In the drum winding all the conductors are on the outside 

 surface of the armature, and although space is usually provided, 

 even in small machines, between the inside of the core and the 

 shaft, this space is used for ventilating purposes only and is not 

 occupied by the armature coils. The drum winding is simply 

 the result of so arranging the end connections that the e.m.fs. 

 generated in the various face conductors shall assist each other 

 in providing the total e.m.f. between brushes. Both windings, 

 whether of the ring or drum type, are continuous and closed 

 upon themselves, and if the brushes are lifted off the commutator 

 there will be no circulating currents because the system of 

 conductors as a whole is cutting exactly the same amount of 

 positive as of negative flux. 



23. Multiple and Series Windings. Nearly all modern 

 continuous-current generators are provided with former-wound 

 coils. These coils are made up of the required number of turns, 

 and pressed into the proper shape before being assembled in 

 the slots of the armature core. Smooth-core armatures are very 

 rarely used, and the slotted armature alone will be considered. 

 In all but two-pole machines (which are rarely made except for 

 small outputs or exceptionally high speeds) there is practically 

 only one type of coil in general use. This is generally of the 



1 For information on the homopolar type of machine see "Acyclic 

 (Homopolar) Dynamos," by J. E. NOEGQERATH, Trans. A. I. E. E., vol. 

 XXIV (1905), pp. 1 to 18, and the article by the same writer, "Acyclic 

 Generators," in the Electrical World, Sept. 12, 1908, p. 575. Also "Homo- 

 polar Generators, " by E. W. Moss and J. MOULD, Journal Inst. E. E., vol. 

 49 (1912), pp. 804 to 816. 



