PARTS OF A DYNAMO-ELECTRIC MACHINE 81 



The two ends of any one coil connect to adjacent commu- 

 tator bars in the lap winding. In Fig. 42, for example, 

 the coil made up of inductors 4 and 9 has its terminals 

 connected to bars / and a which are adjacent. Suppose 

 the armature has revolved one-twelfth of a revolution from 

 the position shown in Fig. 42, in the direction indicated by 

 the arrow; the brush A will at that time rest half on 

 segment a and half on segment /. Hence coil 4-9, the 

 terminals of which are connected to segments a and /, is 

 temporarily short circuited by the brush. At the same 

 time brush B is short-circuiting coil 3--10. 



E.m.f. in Short-circuited Coils. If at the time a coil 

 is short circuited by a brush there is an appreciable e.m.f. 

 being generated in the coil, a large current will circulate 

 through the path made up of the coil, the two brush con- 

 tacts, and part of the brush itself. This short-circuit cur- 

 rent, as it is called, will produce disastrous sparking at the 

 brush contact if it is large; hence the brushes must be so placed 

 on the commutator that the coil which they short circuit occupies 

 such a position in the magnetic field that there is no e.m.f. 

 induced in it while it is short circuited.* Evidently the 

 position of the brushes shown in Fig. 42 satisfies the require- 

 ment because at the time a coil is short circuited the induct- 

 ors are moving in the interpolar space, where the flux is 

 practically zero. 



In Fig. 43 is shown a four-pole machine having 24 induct- 

 ors on a lap wound armature. There are therefore four 

 paths and four brushes required. (In these figures of arma- 

 ture windings the brushes are shown on the inside of the 

 commutator for clearness only. Of course they really 

 bear on the outside surface.) There is one turn per coil 

 and so twelve coils; the commutator has twelve segments. 

 The position of the brushes satisfies the rule previously 

 given. 



