244 



PRINCIPLES OF ELECTRICAL DESIGN 



71. Star and Mesh Connections. Consider the armature 

 winding of an ordinary continuous-current two-pole dynamo. 

 If we imagine the commutator of such a machine to be entirely 

 removed, the winding whether the armature be of the drum 

 or ring type will be continuous, and closed upon itself. If the 

 armature be revolved between the poles of separately excited 

 field magnets, there will be no circulating current in the windings, 

 because the magnetism which passes out of the armature core 

 induces an e.m.f. in the conductors exactly opposite, but equal 



in amount, to that induced by the 

 entering magnetism. 

 . If we now connect two points of 

 the winding from the opposite ends 

 of a diameter to a pair of slip rings, 

 the machine will be capable of de- 

 livering an alternating current. If 

 we provide three slip rings, and 

 connect them respectively to three 

 points on the armature winding dis- 

 tant from each other by 120 de- 

 grees, the machine will become a 

 three-phase generator. 



In this manner polyphase cur- 

 rents of any number of phases can 

 be obtained, and if the windings 

 FIG. 91. Collection of three- and field poles are symmetrically 

 arma e ture. rentSfr0mbi " P larring arranged, there will be no circu- 



lating current. 



This method of connecting up the various armature circuits 

 of a polyphase generator is known as the mesh connection. In 

 the case of three-phase currents it is usually referred to as the 

 delta connection. 



The diagram, Fig. 91, shows the three equidistant tappings 

 from armature winding to slip rings, required to obtain three- 

 phase currents. It is evident that the potential difference be- 

 tween any two of the three rings will be the same, since each 

 section of the winding has the same number of turns, and occupies 

 the same amount of space on the periphery of the armature core. 

 Moreover, the variations in the induced e.m.f. will occur suc- 

 cessively in the three sections at intervals corresponding to 

 one-third of a complete period. 



