Fig. 1,344.—Method of drawing the power curve from the pressure and current curves. As shown, the same scale is used for all curves. This as a rule, makes the power curve inconveniently high, hence it is usually drawn to smaller scale as in fig. 1,345.
How to Obtain the Power Curve.—Since under any phase condition, the power at any instant is equal to the product of the pressure multiplied by the current at that instant, a curve may be easily plotted from the pressure and current curves, giving the instantaneous values of the power through a complete cycle.
In fig. 1,344, from the zero line of the current and pressure curves, draw any ordinate as at F cutting the current curve at G and the pressure curve at G'. The values for current and pressure at this point are from the scale, 2 amperes and 3.7 volts. Since watts = amperes × volts, the ordinate FG is to be multiplied by ordinate FG' that is,
2 × 3.7 = 7.4.
Project up through F the ordinate FG" = 7.4, and this will give one point on the power curve.
Similarly at another point, say M, where the current and pressure are maximum
| MS | × | MS' | = | MS", | that is |
| 3 | × | 5 | = | 15 |
giving S" another point on the curve. Obtaining several points in this way the power curve is then drawn through them as shown.
Fig. 1,345.—Usual method of drawing power curve from the pressure and current curves. A smaller scale is employed for the power curve in order to reduce its height.