Electrical Power Conversion Systems 297
by a load. The smallest reactive power shown (VAR 1 ) results in the volt-
ampere value of VA 1. As reactive power is increased, as shown by the
VAR 2 and VAR 3 values, more volt-amperes (VA 2 and VA 3 ) must be drawn
from the source. This is true since the voltage component of the supplied
volt-amperes remains constant. This example represents the same effect as
a decrease in the power factor, since pf = W/VA, and, as VA increases, the
pf will decrease if W remains constant.
Utility companies usually charge industries for operating at power
factors below a specified level. It is desirable for industries to “correct”
their power factor to avoid such charges and to make more economical
use of electrical energy. Two methods may be used to cause the power
factor to increase: (1) power-factor-corrective capacitors, and (2) three-
phase synchronous motors. Since the effect of capacitive reactance is op-
posite to that of inductive reactance, their reactive effects will counteract
one another. Either power-factor-corrective capacitors, or three-phase
synchronous motors, may be used to add the effect of capacitance to an
AC power line.
An example of power factor correction is shown in Figure 11-3.
We will assume from the example that both true power and inductive
reactive power remain constant at values of 10 kW and 10 kVAR. In
Figure 11-3A, the formulas show that the power factor equals 70 per-
cent. However, if 5-kVAR capacitive reactive power is introduced into
the electrical power system, the net reactive power becomes 5 kVAR
(10-kVAR inductive minus 5-kVAR capacitive), as shown in Figure 11-
3B. With the addition of 5-kVAR capacitive to the system, the power
factor is increased to 89 percent. Now, in Figure 11-3C, if 10-kVAR ca-
pacitive is added to the power system, the total reactive power (kVAR)
becomes zero. The true power is now equal to the apparent power;
therefore, the power factor is 1.0, or 100 percent, which is character-
istic of a purely resistive circuit. The effect of the increased capacitive
reactive power in the system is to increase or “correct” the power fac-
tor and, thus, to reduce the current drawn from the power distribution
lines that supply the loads. In many cases, it is beneficial for industries
to invest in either power-factor-corrective capacitors, or three-phase
synchronous motors, to correct their power factor. Calculations may be
simplified by using the chart of Table 11-1.
Utility companies also attempt to correct the power factor of the
power distribution system. A certain quantity of inductance is present in
most of the power distribution system, including the generator windings,