314 ELECTRICAL EQUIPMENT 



urement, the current being based on the rated Kv.A. and power 

 factor. The resistance of the windings should be taken at 75 C., 

 or the values corrected for this temperature. It is important that 

 this is followed. 



Stray Load Losses. These include iron losses, and eddy- 

 current losses in the copper, due to fluxes varying with load and 

 also to saturation. 



Stray load losses are determined by operating the machine on 

 short circuit and at rated-load current. This, after deducting 

 the windage and friction and I 2 R loss, gives the stray load loss 

 for polyphase generators. For single-phase generators they are 

 much larger. 



Field-Rheostat Losses shall be included in the generator losses 

 where there is a field rheostat in series with the field magnets of 

 the generator, even when the machine is separately excited. 



In making efficiency tests after installation in the power 

 station, it may occasionally be possible to drive the unit by its 

 exciter when the same is direct connected; but for large units 

 and when the direct-connected exciters are not provided the 

 retardation or deceleration testing method is resorted to. This 

 test is based on the principle that every moving body possesses 

 a certain definite amount of energy, due to its motion. It is 

 described in detail in an article by Mr. R. Treat in the General 

 Electric Review for June, 1916. 



A convenient and most satisfactory method of determining 

 the efficiency of a generator after installation may be employed 

 where there are two or more units in the power house available 

 for the use of the test, or where the unit under test may be varied 

 in conjunction with some other unit of sufficient size located 

 elsewhere in the system but which may be segregated for the 

 purpose. The method for determining the core losses and the 

 friction windage losses consists in operating the generator as 

 a synchronous motor and measuring the input by watt- 

 meters. 



When the retardation method of testing is used, it is to be 

 recommended, if possible, to check such tests by means of the 

 input method. 



A new method of artificially loading generators for tests in 

 hydro-electric power stations is described in an article in the 

 General Electric Review for April, 1917. 



