Basic AC Reactive Components
CAPACITANCE
CAPACITANCE
There are many natural causes of capacitance in AC power circuits,
such as transmission lines, fluorescent lighting, and computer
monitors.
Normally, these are counteracted by the inductors
previously discussed. However, where capacitors greatly outnumber
inductive devices, we must calculate the amount of capacitance to add
or subtract from an AC circuit by artificial means.
EO 1.5
DEFINE capacitive reactance (XC).
EO 1.6
Given the operating frequency (f) and the value of
capacitance (C), CALCULATE the capacitive reactance
(XC) of a simple AC circuit.
EO 1.7
DESCRIBE the effect on phase relationship between
current (I) and voltage (E) in a capacitive circuit.
EO 1.8
DRAW a simple phasor diagram representing AC
current (I) and voltage (E) in a capacitive circuit.
Capacitors
The variation of an alternating voltage applied to
Figure 3 Voltage, Charge, and Current in
a Capacitor
a capacitor, the charge on the capacitor, and the
current flowing through the capacitor are
represented by Figure 3.
The current flow in a circuit containing
capacitance depends on the rate at which the
voltage changes. The current flow in Figure 3 is
greatest at points a, c, and e. At these points, the
voltage is changing at its maximum rate (i.e.,
passing through zero).
Between points a and b,
the voltage and charge are increasing, and the
current flow is into the capacitor, but decreasing
in value.
At point b, the capacitor is fully
charged, and the current is zero. From points b
to c, the voltage and charge are decreasing as the
capacitor discharges, and its current flows in a
direction opposite to the voltage. From points c
to d, the capacitor begins to charge in the
opposite direction, and the voltage and current are
again in the same direction.
Rev. 0
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