SINQLR-PHASE MOTORS 281 



reluctance of the magnetic circuit must be reduced to a mini- 

 mum. This is accomplished by operating the iron at low flux 

 densities and therefore at high permeabilities, and by using a 

 very short air-gap. Because of the small number of field ampere- 

 turns and the very low flux density, a very short pole of large 

 - section is necessary, as indicated in Fig. 257. 



(c) The armature of an aUernatinQ-cwrrent series motor of a 

 rating has an unusually large number of conductors. A 



motor of fixed horsepower and speed must develop a correspond- 

 ing torque. The torque developed by a motor is proportional 

 to the product of the field flux and the armature ampere-con- 

 ductors. Therefore, if the total flux of the alternating-current 

 motor is less than the total flux of a direct-current motor of the 

 same rating, the armature ampere-conductors of the alternating- 

 current motor must be correspondingly increased in order to 

 obtain the required torque. This is one reason why the arma- 

 ture of the alternating-current motor is larger than that of the 

 direct-current motor of the same rating. 



(d) The alternating-current motor has a lesser number of field 

 ampere-turns and a greater number of armature ampere-turns than 

 the corresponding direct-current motor. That is, the motor has a 

 strong armature and a weak field. This means that the arma- 

 ture reaction is unduly large. Therefore, the effect of the arma- 

 ture cross-magnetizing turns, unless compensated, is to produce 

 unusually great field distortion. As this distortion of the field 

 by the cross-magnetizing armature ampere-turns would make 

 commutation practically impossible, this cross-magnet i/ing 

 action must he neut rali/ed. This is accomplished by means of a 

 compensating winding placed between the main poles, as shown 

 in I-'iir. -~>7. this winding being embedded in the pole faces (also 

 see Thompson-Ryan winding. Vol. I, page 275, Fig. 2-17). In 

 order to reduce the reactance of the armature, also, this compen- 

 sating winding should not only neutrali/e the cross-magnetizing 

 field ot' the armature as a whole, but it should neutrali/e it at 

 every point. Although it is impossible to secure complete neu- 

 tral i/ation at every point, a close approximation to this is ob- 

 tained by distributing the compensating winding over the pole 



d by making each group of pole-face conduc- 

 carry a current equal and opposite to the current in the 



