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Electrical Science Volume 3 of 4 | Next
Figure 4 Circuit and Phasor Diagram |
CAPACITANCE
Basic AC Reactive Components
At point d, the capacitor is fully charged, and the current flow is again zero. From points d to
e, the capacitor discharges, and the flow of current is opposite to the voltage. Figure 3 shows
the current leading the applied voltage by 90°. In any purely capacitive circuit, current leads
applied voltage by 90°.
Capacitive Reactance
Capacitive reactance is the opposition by a capacitor or a capacitive circuit to the flow of
current. The current flowing in a capacitive circuit is directly proportional to the capacitance and
to the rate at which the applied voltage is changing. The rate at which the applied voltage is
changing is determined by the frequency of the supply; therefore, if the frequency of the
capacitance of a given circuit is increased, the current flow will increase. It can also be said that
if the frequency or capacitance is increased, the opposition to current flow decreases; therefore,
capacitive reactance, which is the opposition to current flow, is inversely proportional to
frequency and capacitance. Capacitive reactance XC, is measured in ohms, as is inductive
reactance. Equation (8-3) is a mathematical representation for capacitive reactance.
(8-3)
XC
1
2pfC
where
f =
frequency (Hz)
p = ~3.14
C =
capacitance (farads)
Equation (8-4) is the mathematical representation of capacitive reactance when capacitance is
expressed in microfarads (µF).
(8-4)
XC
1,000,000
2pfC
Equation (8-5) is the mathematical representation for the current that flows in a circuit with only
capacitive reactance.
(8-5)
I
E
XC
ES-08
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