ALTERNATORS. 261 



The generators are connected to separate equal windings 

 on a small transformer. The lamp or voltmeter is connected 

 to a third winding having fewer turns than the other windings, 

 and consequently giving a lower voltage. 



The two high-tension windings will produce a magnetic 

 flux in the core of the transformer. This flux will be the 

 sum or the difference between the fluxes which would be 

 produced by the windings acting singly, according to the 

 relative direction of the currents in the windings. It depends 

 how the alternators are connected to these primary windings, 

 whether the lamp in the secondary will be bright or dull 

 when the machines are in parallel. In this country it is 

 usual to choose the connection so that the voltmeter reads 

 its maximum, or the lamp burns most brightly, when the 

 machines are in phase. In America the opposite practice 

 has generally been adopted, and the machines are switched 

 " in " when the lamps are out. 



Thus, on referring to Fig. 124, with the connections as 

 shown, the current in the left-hand winding of the syn- 

 chronising transformer under the conditions indicated will 

 produce a magnetic flux in the transformer core such that 

 a N-pole would be urged in an anti-clockwise direction in the 

 interior, of the core. The current in the right-hand winding 

 will at the same time produce a similar flux, on account of 

 the direction of winding and direction of current assumed. 

 If the applied voltages in both windings are equal, the 

 resultant flux will be the sum of that due to either winding 

 acting alone and the centre winding connected to the lamp 

 has a maximum voltage induced to it. Under these con- 

 ditions we may suppose the machines are in phase for correct 

 parallel working. If now the connections between alter- 

 nator G. 2 and the right-hand winding of the transformer 

 are reversed, so that the current produced by the machine 

 flows in the opposite direction through this winding, the 

 flux produced by this winding will be clockwise, and will 

 consequently be subtracted from that formed due to the 

 current in the other winding. Hence the same relative 

 phases of the machines would produce zero flux, instead of 

 a maximum flux, through the third winding. 



When more than two alternators have to be connected 

 so as to be capable of being run in parallel, as is usually the 

 case at a large generating station, the connections have to 



