310 ELEMENTS OF ELECTRICAL ENGINEERING. 



in the direct-current dynamo * and which are due to the necessity 

 of a sudden reversal of the current in a given armature section 

 as the terminal bars (commutator bars) of the section pass under 

 a brush ; and (&) the effects of the excessive current produced in 

 each short-circuited section by the electromotive force which is 

 caused by the pulsations of the field flux. The effects (a) need 

 not be considered because they are less serious than effects (^), 

 and because anything which obviates the great difficulties due to 

 effects (b) will likewise obviate the lesser difficulties due to 

 effects (a). 



The effects of excessive short-circuit currents due to pulsations 

 of the field flux are, to heat the armature winding, to heat the 

 commutator, to heat the brushes, and to roughen the commutator 

 by sparking, and these short-circuit currents represent a con- 

 siderable loss of power. The cure for these undesirable effects 

 is, of course, to prevent excessive short-circuit currents and to 

 shorten their duration as much as possible. 



The electromotive force induced in a short-circuited armature 

 section by the pulsating field flux is proportional to the number 

 of turns of wire in the section, proportional to the maximum value 

 reached by the pulsating field flux, and proportional to the fre- 

 quency of the pulsating flux. Therefore the short-circuited cur- 

 rent may be reduced by reducing the number of turns of wire per 

 armature section (the resistance of the short circuit not being re- 

 duced in proportion to the turns because of the relatively large 

 resistance at the brush contacts), by reducing the maximum value 

 of the pulsating flux, and by reducing the frequency. It is essen- 

 tial, therefore, in the design of the single-phase series motor to 

 provide for few turns of wire per armature section (many commu- 

 tator bars), to provide for small field flux per pole (many field 

 poles), and to use alternating current of low frequency. 



The duration of a short circuit of an armature section by a brush 

 is proportional to the thickness of the brush, and therefore it is 

 desirable to use thin brushes. If the armature winding is of the 



*See pages 161 to 164 of the first volume of this text. 



