ARMATURE WINDINGS AND SLOT INSULATION 99 



The average length per turn of one coil will be 2l a + k where l a 

 is the gross length of the armature core. A small addition should 

 be made for the connections to the commutator, especially if 

 the coil has few turns. 1 The resistance of each coil, and there- 

 fore of each electrical path through the armature, may now be 

 readily calculated. In arriving at the resistance of the armature 

 as a whole it is important to note carefully the number of coils 

 in series in each armature path, and the number of paths in 

 parallel between the terminals of the machine. As a check on 

 the calculated figures, the IR drop, or the PR loss, in the arma- 

 tures of commercial machines, expressed as a percentage of the 

 terminal voltage or of the rated output, as the case may be, is 

 usually as stated below: 



In 10-kw. dynamo 3 . 1 to 3 . 8 per cent. 



In 30-kw. dynamo 2 . 6 to 3 . 2 per cent. 



In 50-kw. dynamo 2.4 to 3.2 per cent. 



In 100-kw. dynamo 2.1 to 2.8 per cent. 



In 200-kw. dynamo 2 . to 2 . 6 per cent. 



In 500-kw. dynamo 1 . 9 to 2 . 1 per cent. 



In 1000-kw. dynamo 1 . 8 to 2. per cent. 



1 There is another type of end connection, known as the involute end 

 winding. It is not much used ; but those interested in the matter are referred 

 to the first volume of " The Dynamo " by HAWKINS and WALLIS (WHITTAKER 

 & Co.), where the manner of calculating the length of these end con- 

 nections is explained. 



