What Is The Role of Capacitor in AC and DC Circuit
What Is The Role of Capacitor in AC and DC Circuit
What Is The Role of Capacitor in AC and DC Circuit
Role of Capacitor in AC Circuits: In an AC circuit, capacitor reverses its charges as the current alternates and produces a lagging voltage (in other words, capacitor
provides
leading
current
in
AC
circuits
and
networks)
Role
and
Performance
of
Capacitor
in
DC
Circuit
In a DC Circuit, the capacitor once charged with the applied voltag e acts as an open switch.
What is Capacitor?
The capacitor is a two terminal electrical device used to store electrical energy in the form of electric field between the two plates. It is also known as a condenser and the
SI unit of its capacitance measure is Farad F, where Farad is a large unit of capacitance, so they are using microfarads (F) or nanofarads (nF) nowadays.
Capacitance (C):
Capacitance is the amount of electric charge moved in the condenser (Capacitor), when one volt power source is attached across its terminal.
Mathematically,
Capacitance Equation:
C=Q/V
Where,
C=Capacitance in Farads (F)
Q=Electrical Charge in Coulombs
V=Voltage in Volts
We will not go in detail because our basic purpose of this discussion is to explain the role and application/uses of capacitors in AC and DC systems. To understand this
basic concept, we have to understand the basic types of capacitor related to our topic (as there are many types of capacitor and we will discuss capacitor types latter in
another post because it is not related to the question)
AC Circuits
7-23-99
Alternating current
Direct current (DC) circuits involve current flowing in one direction. In alternating
current (AC) circuits, instead of a constant voltage supplied by a battery, the
voltage oscillates in a sine wave pattern, varying with time as:
In AC circuits we'll talk a lot about the phase of the current relative to the voltage.
In a circuit which only involves resistors, the current and voltage are in phase with
each other, which means that the peak voltage is reached at the same instant as
peak current. In circuits which have capacitors and inductors (coils) the phase
relationships will be quite different.
Capacitance in an AC circuit
Consider now a circuit which has only a capacitor and an AC power source (such
as a wall outlet). A capacitor is a device for storing charging. It turns out that there
is a 90 phase difference between the current and voltage, with the current reaching
its peak 90 (1/4 cycle) before the voltage reaches its peak. Put another way, the
current leads the voltage by 90 in a purely capacitive circuit.
To understand why this is, we should review some of the relevant equations,
including:
relationship between voltage and charge for a capacitor: CV = Q
Step 4 - After point d, the voltage heads toward zero and the capacitor must
discharge. When the voltage reaches zero it's gone through a full cycle so it's back
to point a again to repeat the cycle.
The larger the capacitance of the capacitor, the more charge has to flow to build up
a particular voltage on the plates, and the higher the current will be. The higher the
frequency of the voltage, the shorter the time available to change the voltage, so
the larger the current has to be. The current, then, increases as the capacitance
increases and as the frequency increases.
Usually this is thought of in terms of the effective resistance of the capacitor,
which is known as the capacitive reactance, measured in ohms. There is an inverse
relationship between current and resistance, so the capacitive reactance is inversely
proportional to the capacitance and the frequency:
A capacitor in an AC circuit exhibits a kind of resistance called capacitive
reactance, measured in ohms. This depends on the frequency of the AC voltage,
and is given by:
Note that V and I are generally the rms values of the voltage and current.
Inductance in an AC circuit
As the voltage from the power source increases from zero, the voltage on the
inductor matches it. With the capacitor, the voltage came from the charge stored on
the capacitor plates (or, equivalently, from the electric field between the plates).
With the inductor, the voltage comes from changing the flux through the coil, or,
equivalently, changing the current through the coil, which changes the magnetic
field in the coil.
To produce a large positive voltage, a large increase in current is required. When
the voltage passes through zero, the current should stop changing just for an
instant. When the voltage is large and negative, the current should be decreasing
quickly. These conditions can all be satisfied by having the current vary like a
negative cosine wave, when the voltage follows a sine wave.
How does the current through the inductor depend on the frequency and the
inductance? If the frequency is raised, there is less time to change the voltage. If
the time interval is reduced, the change in current is also reduced, so the current is
lower. The current is also reduced if the inductance is increased.
As with the capacitor, this is usually put in terms of the effective resistance of the
inductor. This effective resistance is known as the inductive reactance. This is
given by:
where L is the inductance of the coil (this depends on the geometry of the coil and
whether its got a ferromagnetic core). The unit of inductance is the henry.
As with capacitive reactance, the voltage across the inductor is given by: