312 THE POLYPHASE CIRCUIT. 



that the common terminal of the wattmeters is connected 

 to the source of supply. Under these conditions watt- 

 meters of the usual construction will read in the correct 

 direction. If it is found that the reading of the instru- 

 ment is reversed, the shunt coil connections should be inter- 

 changed and the readings of the wattmeter considered 

 negative and must be subtracted from the value recorded 

 by the positively reading wattmeter. 



Armature Reactions in a Polyphase Alternator. In a single- 

 phase alternator the main field is only acted upon by the 

 armature currents periodically, since these currents vary 

 in value from zero to a maximum. The main field will 

 consequently be alternately distorted and remain undis- 

 turbed according to the periodic variations of the armature 

 current. In a polyphase alternator the conductors of the 

 second phase come under the poles as the conductors of 

 the first phase leave them. Also the maximum value of 

 the currents in successive phases follow each other, so 

 that the armature is never without current. In a three-phase 

 armature the total current is always the same. The 

 result is that the armature reactions of a polyphase alter- 

 nator are much more uniform, and also on the average 

 stronger than in a single-phase machine. 



