ALTERNATING-CURRENT COMMUTATOR MOTORS 367 



At start, when the drop across C is small, the current is large 

 and the main field F is very strong. The repulsion therefore 

 gives a good starting torque. The field at start will be decreased 

 to a certain extent by the current in the short-circuited coil un- 

 dergoing commutation, as in the series motor. 



232. Commutation. In the single-phase series motor and the 

 repulsion motor there are two [currents to be commutated, (1) the 

 load current and (2) the short-circuit current produced in the coil 

 under the brush by the alternating flux of the main field. 



(1) To reverse the load current a m.m.f. is required opposing 

 the m.m.f. of armature reaction and strong enough to produce a 

 flux in the opposite direction to the armature reaction flux. Such 

 a flux can be produced by interpoles placed between the main 

 poles and excited by a winding in series with the main field or it 

 can be produced by a compensating winding. The conductively 

 compensating winding is the only one which can give perfect 

 commutation since its m.m.f. can be made stronger than the 

 armature m.m.f. Commutation is assisted by the use of high-re- 

 sistance carbon brushes. 



(2) To eliminate the short-circuit current in the coil under the 

 brush an e.m.f. must be generated in the coil equal and opposite 

 to the e.m.f. producing the short-circuit current. The neutralizing 

 e.m.f. cannot be generated by the alternation of a magnetic flux 

 through the coil, since that would require a flux equal and opposite 

 to the field flux and would destroy the field of the motor. The 

 required e.m.f. can, however, be generated by the rotation of the 

 armature through a commutating field of the proper intensity and 

 position, but the field must be in quadrature with the main field 

 in both time and space. In the repulsion motor under running 

 conditions such a field is produced in the compensating coil C. 

 The intensity of the field varies with the speed of the motor. 

 Near synchronous speed the e.m.f. is entirely neutralized and the 

 current is wiped out. Below synchronous speed the current is 

 reduced and above synchronous speed another current is produced 

 as objectionable as before and commutation becomes bad again. 

 At standstill no neutralizing e.m.f. is produced. 



In the single-phase series motor there is no field in quadrature 

 with the main field in time and so no neutralizing e.m.f. can be 

 produced but the short-circuit current is reduced by using high- 

 resistance preventive leads as explained in Art. 230. 



