The component of drop due to reactance is best diminished by sub dividing the copper or by bringing the conductors closer together. It is little affected by change in size of conductors.
An idea of the manner in which changes of power factor affect drop is best gotten by an example. Assume distance of transmission, distance between conductors E. M. F., and frequency, the same as in the previous example. Assume the apparent power delivered the same as before, and let it be constant, but let the powerfactor be given several different values; the true power will therefore be a variable depending upon the value of the power factor. Let the size of wire be No. 0000. As the apparent power, and hence the current, is the same as before, and the line resistance is one-half, the resistance E. M. F. will in this case be Combining these on the chart for a power factor of .4, and deducing the drop, in per cent, of the generator E. M. F., the value obtained is 15.3 per cent; with a power factor of .8, the drop is 14 per cent; with a power factor of unity, it is 8 per cent. If in this example the true power, instead of the apparent power, had been taken as constant, it is evident that the values of drop would have differed more widely, since the current, and hence the resistance- and reactance-volts, would have increased as the power factor diminished. The condition taken more nearly represents that of practice.
If the line had resistance and no reactance, the several values of drop, instead of 15.3, 14, and 8, would be 3.2, 5.7, and 7.2 per cent respectively, showing that for a load of lamps the drop will not be much increased by reactance; but that with a load, such as indue tion motors, whose power factor is less than unity, care should lx taken to keep the reactance as low as practicable. In all cases it is advisable to place conductors as close together as good practice will permit.
When there is a transformer in circuit, arid it is desired to obtain the combined drop of transformer and line, it is necessary to know the resistance- and reactance-volts of the transformer. The resist ance-volts of the combination of line and transformer are the sum of the resistance-volts of the line and the resistance-volts of the trans former. Similarly, the reactance-volts of the line and transformer
are the sum of their respective reactance-volts. The resistance- and reactance-E. M. F.s of transformers may usually be obtained from the makers, and are ordinarily given in per cent.* These per centages express the values of the resistance- and reactance-E. M. F.'s when the transformer delivers its normal full-load current; and they express these values in terms of the normal no-load E. M. F. of the transformer.
Consider a transformer built for transformation between 1,000 and 100 volts. Suppose the resistance- and reactance-E. M. F.'s given are 2 per cent and 7 per cent respectively. Then the corresponding voltages when the transformer delivers full-load current, are 2 and 7 volts or 20 and 70 volts according as the line whose drop is required is connected to the low-voltage or high-voltage terminals. These values, 2-7 and 20-70, hold, no matter at what voltage the trans former is operated, since they depend only upon the strength of cur rent, providing it is of the normal frequency. If any other than the full-load current is drawn from the transformer, the reactance- and resistance-volts will be such a proportion of the values given above as the current flowing is of the full-load current. It may be noted, in passing, that when the resistance- and reactance-volts of a trans former are known, its regulation may be determined by making use of the chart in the same way as for a line having resistance and reactance.
As an illustration of the method of calculating the drop in a line and transformer, and also of the use of table and chart in calculat ing low-voltage mains, the following example is given: Problem. A single-phase induction motor is to be supplied with 20 am peres at 200 volts; alternations, 7,200 per minute; power factor, .78. The distance from transformer to motor is 150 feet, and the line is No. 5 wire, 6 inches between centers of conductors. The transformer reduces in the ratio 2,000 200 , has a capacity of 25 amperes at 200 volts, and, when delivering this current and voltage, its resistance-E. M. F. is 2.5 per cent, its reactance E. M. F. 5 per cent. Find the drop.