| output | 10 | |||||
| efficiency | = | = | = | 96.9 per cent. | ||
| input | 10 + .16 + .16 |
At three-quarters load the output will be 7.5 kilowatts; and as the iron loss is practically constant at all loads and the copper loss is proportional to the square of the load, the
| output | 7.5 | |||||
| efficiency | = | = | = | 96.8 per cent. | ||
| input | 7.5 + .16 + .09 |
The matter of efficiency is important, especially in the case of large transformers, as a low efficiency not only means a large waste of power in the form of heat, but also a great increase in the difficulties encountered in keeping the apparatus cool. The efficiency curve shown in fig. 1,975, serves to indicate, however, how slight a margin actually remains for improvement in this particular in the design and construction of large transformers.
Fig. 1,975.—Efficiency curve of Westinghouse 375 kw., transformer. Pressure 500 to 15,000 volts; frequency 60. Efficiencies at different loads: full load efficiency, 98%; ¾ full load efficiency, 98%; ½ full load efficiency, 97.6%; ¼ full load efficiency, 96.1%; regulation non-inductive load, 1.4%; load having .9 power factor, 3.3%.
The efficiency of transformers is, in general, higher than that of other electrical machines; even in quite small sizes it reaches over 90 per cent., and in the largest, is frequently as high as 98.5 per cent.
To measure the efficiency of a transformer directly, by measuring input and output, does not constitute a satisfactory method when the efficiency is so high. A very accurate result can be obtained, however, by measuring separately, by wattmeter, the core and copper losses.
The core loss is measured by placing a wattmeter in circuit when the transformer is on circuit at no load and normal frequency.
The copper loss is measured by placing a wattmeter in circuit with the primary when the secondary is short circuited, and when enough pressure is applied to cause full load current to flow.