278 ALTERNATING CURRENTS 



reactance (due to leakage) is very much less than that of an arma- 

 ture without a compensating winding.* 



Since a properly designed compensating winding will destroy the 

 greater -portion of the armature reactance, it follows that very little 

 additional advantage will be gained by the use of further devices 

 having the same object in view. There is, for example, no longer the 

 same need to use salient poles, and the stator may be similar to that 

 t of an ordinary induction motor. There is, further, very little advan- 

 tage gained by increasing the number of poles, i.e. by running 

 considerably above the speed of synchronism. 



165. Methods of obtaining Sparkless Running 



A difficulty which proved very serious with the earlier types of 

 series-wound motor was that of sparking. The flux through the 

 armature core being an alternating one, a coil short-circuited by a 

 brush is subject to the inductive action of this flux, and may become 

 the seat of excessive induced currents. The difficulty is greatest 

 when the armature is at rest or running at a low speed, since at lower 

 speeds the alternating flux is stronger,! and the coil remains longer 

 short-circuited under its inductive action. 



In modern series- wound motors, the difficulty has been overcome 

 by (1) using as many commutator segments as possible; (2) using 

 narrow carbon brushes, so that only one coil is short-circuited at a 

 time; (3) introducing high-resistance connections (which may be 

 placed at the bottom of the slots) between the winding and the 

 commutator ; (4) providing special reversing pole-pieces. 



Since the width of carbon brush and commutator segment cannot 

 be reduced below a certain limit for purely mechanical reasons, it 

 follows that if the subdivision of the winding is carried as far as 

 possible i.e. with only one turn per coil the number of armature 

 conductors will be limited by the least possible width of segment. 

 Hence it will be impossible, for a given speed, to construct a motor 

 beyond a certain voltage without overstepping the limitations imposed 

 by considerations of sparkless running. This accounts for the fact 

 that such motors have generally been constructed for voltages not 

 exceeding 200. 



As regards both power factor and commutation, better results 

 will be obtained at a lower frequency. Motors of the type under 

 consideration have, for this reason, been generally used on circuits of 

 frequency not exceeding 25. 



* By some writers, the term damping coil is applied to this winding. 

 t Corresponding to the larger torque, and hence larger current. 



