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19
Power (ac)
19.1 INTRODUCTION
The discussion of power in Chapter 14 included only the average power
delivered to an ac network. We will now examine the total power equation
in a slightly different form and will introduce two additional types
of power: apparent and reactive.
For any system such as in Fig. 19.1, the power delivered to a load at
any instant is defined by the product of the applied voltage and the
resulting current; that is,
p �vi
In this case, since v and i are sinusoidal quantities, let us establish a
general case where
v � Vm sin(qt � v)
and i � Im sin qt
The chosen v and i include all possibilities because, if the load is purely
resistive, v � 0°. If the load is purely inductive or capacitive, v � 90°
or v ��90°, respectively. For a network that is primarily inductive, v
is positive (v leads i), and for a network that is primarily capacitive, v is
negative (i leads v).
p
i
+
v
–
Load
FIG. 19.1
Defining the power delivered to a load.
P q
s