POWER FACTOR OF SERIES MOTOR 275 



163. Conditions Determining Power Factor 

 of Series Motor 



A reference to Fig. 165 shows that the power factor (cos A) increases 



AV 



\\iih increase of the ratio ^-r. It is therefore desirable, in order to 



OA 



secure a high power factor, to make this ratio as large as possible. 

 Now, the reactance- voltage CA may be reduced by reducing the number 

 of turns on the field which will reduce that part of the reactance 

 voltage depending on the field winding and by designing the magnetic 

 circuit so as to interpose a large magnetic reluctance along the path 

 of the flux due to the armature current, and thereby also reduce the 

 part of the reactance voltage which corresponds to armature self- 

 inductance. 



The first method of reducing the reactance is very generally 

 adopted in practice, and leads to the design of motors in which the 

 armature ampere-turns bear a ratio of from 2 to 5 to the field ampere- 

 turns. It must be noted that, beyond a certain point, an increase of 

 power factor by a reduction of the field ampere-turns can only be 

 obtained at a sacrifice of efficiency ; for in order to secure a given 

 torque with a very weak field, a larger current will be required 

 (corresponding to a heavier copper loss) than would otherwise be 

 necessary.* It is thus not advisable to reduce the field-turns below 

 a certain limit. 



In order to tlirottle the armature flux as much as possible, it is 

 usual to employ stators or field-magnets having well-defined or salient 

 poles, such as are typical of continuous-current motors. The reluct- 

 ance to the armature flux may be further increased by making the width 

 of the pole small in comparison with the length of the inter-polar 

 arc, ancl by working the poles at a high induction so as to reduce their 

 permeability. A further improvement consists in slotting the poles 

 down the central line, an additional air-gap being thereby interposed 

 in the path of the armature flux. Fig. 167 represents a form of con- 

 struction due to Dr. Finzi, the field core being built up of stampings 

 shaped as shown ; there is a wide gap down the middle of each pole- 

 piece, and a further throttling effect is obtained by chamfering the 



* This is on the assumption that the field-turns arc reduced without any change 

 in the armature turns. The same result us regards increased armature loss is, however, 

 also arrived at by supposing that the current remains unaltered, and that the original 

 value of the torque is maintained by increasing the number of aimature conductors in 

 the same ratio as that in which the field has been weakened. The current remaining 

 unaltered, and there now being more conductors on the armature, i.e. a larger armaturo 

 resistance, it is evident that the copper loss in the armature will have been increased. 

 The increase in the armature copper loss is only to a very feeble extent counterbalanced 

 by the decrease in the field copper loss. 



