ALTERNATORS. 257 



several values of the excitation, the iron losses at varying 

 induction are obtained. 



Two Similar Alternators Connected in Parallel. The Hopkin- 

 son method of testing which is so much used in the 

 case of direct -current machines is also applicable to 

 alternators. The alternators are connected in parallel, 

 but are not mechanically coupled, and supplied with cur- 

 rent of suitable voltage and frequency to drive them both 

 as synchronous motors. If the machines are similar and 

 equally excited, they will take equal small currents, and 

 run in phase with each other. The power supplied, when 

 measured on a wattmeter, will give the no-load losses of 

 both machines. 



By changing the excitation of one machine, it may be 

 made to lag behind or lead the other in phase, and in doing 

 so it will supply current to the other machine or receive 

 current from it. By suitable adjustment of the excitation 

 it is thus possible to fully load the machines, the power given 

 out by one being spent in driving the other. For any value 

 of the load the power supplied from the mains is that spent 

 in the losses occurring in both. The efficiency of each is got 

 by taking the square root of the joint efficiency of both. 



Running Alternators in Parallel. Alternators can be run 

 in parallel if brought to the same speed and voltage, and if 

 switched together on to a single supply circuit when exactly 

 coincident in phase. This is owing to the fact that if one 

 machine tends to lag behind the other, it will receive current 

 from the other which tends to drive it as a motor, and it is 

 thus prevented from falling further out of step. If the 

 machines are not driven with exactly the same regularity, 

 they will vary slightly in phase, and in doing so the leading 

 machine will always supply current to the lagging one, which 

 will have the double effect of slightly retarding the leading 

 machine and accelerating the lagging machine. The difference 

 in phase between the two machines must never exceed 

 period, or they will fall out of step, since in that case the 

 direction of the current supplied by the leading machine will 

 be reversed, and tend to still further retard the lagging 

 alternator. 



The condition for parallel running is, therefore, that with 

 a small angular displacement between the machines the 



