SINGLE-PHASE MOTORS 



283 



. C. Field 



..Sbort-CIrcuited 

 Coll 



-ity for reducing this armature react ance drop to a small 

 value is tin* principal reason for using a distributed compensating 

 winding rather than a more concentrated one. 



(e) In the alternating-current series motor a commidaling diffi- 

 cult]/ occurs which is not present in the direct-current nu.tor. 



Figure 2(i() shows a eoil in the neutral plane undergoing commu- 

 tation. The coil is there- 

 fore short-circuited by the 

 brushes. The plane of this 

 coil is perpendicular to the 

 direction of the main field, 

 which is alternating, SO 

 that the alternating flux of 

 this field links the coil. 

 The short-circuited coil 

 acts as the secondary of a 

 transformer, of which the main field-winding is the primary, 

 and therefore has voltage induced in it. As this coil is short- 

 circuited by the brushes, and has a low impedance, a large cur- 

 rent flows. This current causes severe sparking at the brushes. 

 In addition, it opposes the main flux and so lowers the torque. 

 To reduce this induced current to as low a value as possible, 



FIG. 260. Transformer electromotive force 

 in coil undergoing commutation. 



*- 



Commutator 

 Segment! 



H 



Load Current 



!. Resistance leads inserted t<> improve r<>niinut:iti<>n. 



mce leads are often in-cried between the armature coils 



and the commutator segment,-, as shown in 61. Such 



by incre.-isinir the impedance of the short-circuited 

 reduce the slu >rt -ci rcu i t current. It will be noted. Pig, L.V, I , that 

 BOfarai tl -///current i> concerned, two such lead- are 



to, while so far as tl. <d or load cunvni 'Tned, 



