14 \\n.\RrHZATION RULES OF Till A.l.E.E. 



duced to, the temperature which the apparatus assumes under continuous 

 operation at rated load, referred to a room temperature of 25 deg. cent. 

 See Sees. 267-202. 



98 With apparatus intended for intermittent service, the efficiency should 

 be determined at the temperature assumed under specified conditions. 



99 b. POWER FACTOR. In determining the efficiency of alternating-current 

 apparatus, the electric power should be measured when the currnr 

 phase with the voltage, unless otherwise specified, except when a definite 

 phase difference is inherent in the apparatus, as in induction motors, 

 induction generators, frequency converters, etc. 



100 c. WAVE SHAPE. In determining the efficiency of alternating-current 

 apparatus, the sine wave should be considered as standard, except where a 

 difference in the wave form from the sinusoidal is inherent in the operation 

 of the apparatus. See Sec. 80. 



(Ill) MEASUREMENT OF LOSSES. 



101 LOSSES. The usual sources of losses in electrical apparatus and the 

 methods of determining these losses are as follows: 



102 (A) BEARING FRICTION AND WINDAGE. 



The magnitude of bearing friction and windage (which may be consid- 

 ered as independent of the load) is conveniently measured by driving 

 the machine from an independent motor, the output of which may be 

 suitably determined. See Sec. 94. 

 (B) COMMUTATOR BRUSH FRICTION. 



103 The magnitude of the commutator brush friction (which may be con- 

 sidered as independent of the load) is determined by measuring the dif- 

 ference in power required for driving the machine with brushes on and 

 with brushes off (the field being unexcited). 



(O COLLECTOR-RING BRUSH FRICTION. 



104 Collector-ring brush friction may be determined in the same manner 

 as commutator brush friction. It is usually negligible. 



(D) MOLECULAR MAGNETIC FRICTION AND EDDY CURKI 



105 These losses include those due to molecular magnetic friction and eddy 

 currents in iron and copper and other metallic parts, also the losses due 

 to currents in the cross-connections of cross-connected armatures. 



106 In MACHINES these losses should be determined on open circuit and 

 at a voltage equal to the rated voltage +/ r in a generator, and / r in a 

 motor, where / denotes the current strength and r denotes the int 

 resistance of the machine. They should be measured at the correct 

 speed and voltage, since they do not usually vary in any definite pro- 

 portion to the speed or to the voltage. 



107 NOTE. The TOTAL LOSSES in bearing friction and windage, brush fric- 

 tion, magnetic friction and eddy currents can, in general, be determined 

 by a single measurement by driving the machine with the field excited, 

 either as a motor, or by means of an independent motor. 



108 RETARDATION METHOD. The no-load iron, friction, and windage losses 

 may be segregated by the Retardation Method. The generator should 

 be brought up to full speed (or, if possible, to about 10 per cent above 

 full speed) as a motor, and, after cutting off the driving power and 

 excitation, frequent readings should be taken of speed and time, as the 

 machine slows down, from which a speed-time curve can be plotted. A 

 second curve should be taken in the same manner, but with full field ex- 

 citation; from the second curve the iron losses may be found by subtracting 

 the losses found in the first curve. 



109 The speed-time curves can be plotted automatically by belting a small 

 separately excited generator (say 1/10 kw.) to the generator shaft and 

 connecting it to a recording voltmeter. 



() ARMATURE-RESISTANCE Loss. 



110 This loss mav be expressed by p 7 l r\ where r -resistance of one arma- 

 ture circuit or branch, 7 = the current in such armature circuit or branch, 

 and p-the number of armature circuits or branches. 



