174 ALTERNATING CURRENTS 



99* Retardation Method of determining Losses 



A method equally applicable to continuous and alternating-current 

 generators is that originally suggested by J. R. Ashworth,* and first 

 fully described by Eoutin.f It depends on the determination of the 

 retardation which takes place in the rotating member or rotor of a 

 machine when the driving torque is removed. Let the alternator 

 under test be run up, on open circuit and without any excitation, to 

 a speed somewhat above its normal speed, and let the supply of power 

 driving the machine be then cut off (this may be done by throwing 

 off the belt if a belt drive is used, by switching off the current 

 supplied to the direct-coupled exciter if this is used for running the 

 alternator up to the desired speed, or if the machine is driven 

 electrically as a synchronous motor by switching off the armature 

 current, and immediately afterwards the field current). The rotor 

 begins to slow down, and readings of its speed are taken at definite 

 intervals ; this may be done by means of a tachometer or a dead-beat 

 voltmeter, which forms a very good speed indicator. The curve 

 connecting the angular velocity w (radians per sec.) with the time is 



then plotted, and from it the value of -^-, the angular retardation, is 



found for that value of w which represents the normal speed of the 

 alternator. If I = moment of inertia of rotor, in C.G.S. units, then 



the retarding couple due to frictional resistances is 1-^*, and the 



power lost by friction is Iw-^ x 10 ~ 7 watts. If I were known, we 



could thus find the frictional losses. In order to determine I, the 

 experiment is repeated, an additional known retarding couple C being, 

 however, applied to the rotor (by means of a suitable rope or band 



brake). If ( -^ ) stand for the new value of the angular acceleration 



\ Ctv J 1 



at the normal speed, then we must have C = I j (-5^) -rH, so that 



^\&o * i Ctt J 



I may be determined. By next repeating the first experiment, but 

 with the field fully excited, we find for the frictional and iron losses 



Io> (-^) X 10 ~ 7 where (-rr) is the new value of the angular- 

 vat /2 \ac/2 



retardation. 



* The Eledririan, vol. xxx. p. 459 (1893). 

 t Edairagu Jjfcrtrique, October 24, 1896. 



