284 ALTERNATING CURRENTS 



they are in parallel. This makes the resistance of these leads to 

 the short-circuit current four times as great as it is to the load 

 current. Except when starting, such leads are in the circuit 

 but a small part of the time. If the starting period is too long, 

 the leads in circuit at that time may overheat. 



Reactances for reducing this transformer current have been 

 suggested in place of resistances, but the difficulty of finding room 

 for such reactances on a rotating armature has prevented their 

 use. The induced voltage per turn in the armature coil under- 

 going commutation is proportional to the flux per pole. In 

 order to keep this voltage within allowable limits, the total flux per 

 pole must be made as small as possible. Therefore, the number 

 of poles must be increased in order that there be sufficient total 

 flux to develop the required torque. For this reason an alter- 



.E7=Baok Emf 



= Arm, Resistance Drop 

 IX a = " Reactance 

 J/?s = Series Field Resistance Drop 

 IXa'= " " Reactance " 

 IR C Comp. *, Resistance > 

 IX c = " Reactance 



IR 8 

 FIG. 262. Vector diagram for alternating-current series motor. 



nating-current series motor ordinarily has more poles than a cor- 

 responding direct-current motor. 



In order to improve commutation still further, the voltage 

 between commutator bars is kept down to a low value. This 

 requires a large number of commutator segments and a cor- 

 respondingly large commutator. The voltage between commu- 

 tator segments is still further reduced by operating the motor at 

 low voltage, usually not over 250 volts. 



Figure 262 shows the vector diagram for this type of motor. 

 The resistance drop, IR 8 , of the main field, is in phase with the 

 current /. The reactance drop, IX 8 , of the main field, is in 

 quadrature and leading the current /. IR a and IR C) the resist- 

 ance drops of the armature and compensating field, are in phase 

 with the current. IX a and IX C the reactance drops of the 

 armature and compensating field, are in quadrature with the 

 current and leading. The reactance drop of the series field 



