2.54 ALTERNATORS. 



magnets is applied, the increase of power given to the 

 motor in order to maintain the same speed will be equal to 

 the power taken up by the iron losses of the alternator. 



If the alternator is provided with a direct-coupled exciter, 

 it is convenient to employ this exciter as the motor in 

 the test, after having made a careful determination of its 

 efficiency. The quantities to be measured are small and 

 liable to variation ; great precautions must therefore be 

 taken to run the alternator as nearly as possible under work- 

 ing conditions as regards temperature, &c., and only to take 

 readings after the machine has been rotating for some time, 

 so that the bearings may be thoroughly lubricated. The 

 first readings should be repeated several times between the 

 other readings, so as to insure that the conditions have 

 remained constant. 



This test is often carried out in the following way : 

 The motor is made to drive the alternator unexcited. 

 The power supplied to the motor W^ is noted. The alternator 

 fields are then excited, and the power taken by the motor to 

 maintain the same speed is noted ( = W). The motor is dis- 

 connected from the alternator, and the power required to 

 drive it when running light is noted ( = w). Then the friction 

 losses of the alternator are taken to be (W l w), and the iron 

 losses (W^ Wi). The excitation of the motor should be 

 maintained practically constant during the three measure- 

 ments. It is assumed that the losses in the motor are the 

 same at no-load and when driving the alternator. The motor 

 should be large enough to make the alternator only a light 

 load for it, in order that these assumptions may be justified. 



The Retardation Method of Measuring Iron Losses. This 

 is a method which is especially applicable to alternators 

 of the flywheel type. The machine is brought up to speed by 

 means of a motor and belt, or by its own exciter. The belt 

 is then slipped off, or current shut off from the exciter, as 

 the case may be, and observations are taken of the rate at 

 which the machine slows down, by reading a tachometer at 

 equal intervals during the retardation. Curves plotted from 

 these readings give the information required to enable the 

 iron losses to be determined. If the machine is not of the 

 flywheel type, it can sometimes be conveniently tested by 

 the same method after mounting on the shaft a flywheel of 

 sufficient weight to render the time of retardation sufficiently 



