CH; XII AKMATUEE REACTION 279 



^ 



We see that the smaller the ratio the greater is 



H t 

 H g , and consequently the greater is the winding required 



on the magnets. We must then make H g as small as 



A 



possible, consistent with having the ratio as nearly as 



Ha 



possible equal to 0'132. We may then take A = 7 20, so 

 that 1^ = 5,000, H p = 2,600, and the useful lines per pole, 

 N, is 3-56 x 10 6 . 



The conditions under which motors have to work 

 generally demand that the brushes shall be set once for 

 all, and operate sparklessly when the armature is running 

 in either direction. The brushes must therefore be placed 

 at the points where the magnetisation due to the magnets 

 is nothing. It follows from this that the magnetisation 

 at the centre of the brush is at all times simply that due 

 to the armature reaction. 



Now in order to commutate sparklessly, each coil 

 as it comes under the brush, and is there short-circuited, 

 ought to find itself in a magnetic field of such a sign as will 

 tend to check the current flowing in it. In the case of a 

 motor, the current flowing in any coil is flowing there in op- 

 position to the tension induced in the coil, i.e. the induced 

 tension tends to check the current. Hence the proper 

 sign in which to commutate a coil in a motor armature is 

 that of the magnetic field in the gap through which the 

 coil has just been passing. 



A reference to Fig. 64 will show that the sign of the 

 armature magnetisation under the brush in a motor is the 

 same as that due to the magnets in the gap ahead .of the 

 brush, and is consequently always of the wrong sign for 

 commutation. It is clear, then, that sparkless commuta- 



