384 ELECTRICAL EQUIPMENT 



testing ambient temperature of 30 C. would be corrected to 

 50X1.04 = 52 C., this being the temperature rise which would 

 have occurred had the test been made with the standard ingoing 

 cooling-air temperature of 40 C. 



Efficiency. The efficiency of a transformer is the ratio of the 

 kilowatts output measured at the secondary terminals to the kilo- 

 watts input measured at the primary terminals. The difference 

 between these two values equals the losses, which consist of the no- 

 load losses, the I 2 R losses and the stray-load losses. The no-load 

 losses consist of the hysteresis and eddy-current or core loss in 

 the laminations, the PR loss due to the exciting current and the 

 dielectric hysteresis loss in the insulation. The PR losses should 

 include the copper loss in all the windings, primary as well as 

 secondary, and the stray-load losses consist of the eddy-current 

 loss in the windings and core, due to fluxes varying with the load. 

 They should also include the stray loss in other parts of the trans- 

 former. In determining these losses care shall be taken that they 

 are corrected to a reference temperature of 75 C. 



The efficiency is generally given at unity power-factor, but 

 can readily be figured out for any power-factor as the losses are 

 independent of the same as, long as the Kv.A. is not changed. 

 For example, assume a 1000 Kv.A. transformer having a total loss 

 of 14 Kw. or 1.4 per cent based on 1000 Kw. at unity power-factor. 

 Based on 800 Kw. 80 per cent power-factor the loss would be 

 1.75 per cent. In the former case the efficiency at full-load would 

 be 98.62 and the latter 98.28, which illustrates the importance of 

 basing the efficiency identically. 



The efficiency depends upon the voltage and the size of the 

 unit and varies from about 97 to as high as 99 per cent for trans- 

 formers generally used in hydro-electric work. For 25 cycles the 

 losses are somewhat higher and the efficiency somewhat lower on 

 account of the larger amount of material required for this fre- 

 quency as compared to 60 cycles. 



Sometimes the all-day efficiency of a transformer is required 

 for comparison, and this may readily be figured from the following 

 simple formula: 



All-Day Efficiency - 



Kv.A. Hours per Day Output 



Kv.A.Hre. per Day Output + 24 x No-load Loss + No. of rs.x/2tf + Stray-load Loss. 



Voltage. In regard to the use of the terms high-voltage, 



