November 17, 2015

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 voltage acts as an open switch.

Rule of Capacitor in AC and DC Circuit

What is the Role of Capacitor in AC and DC Circuit?
Let’s explain in detail, but we will go back to the basics of capacitor first to discuss the matter.

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 (µF) or nanofarads (nF) nowadays.

How Capacitor Works?

Working and Construction of a capacitor:
Whenever voltage is applied across its terminals, (Also known as charging of a capacitor) current start to flow and continue to travel until the voltage across both the negative and positive (Anode and Cathode) plates become equal to the voltage of the source (Applied Voltage). These two plates are separated by a dielectric material (such as mice, paper, glass, etc. which are insulators), which is used to increase the capacitance of the capacitor.

When we connect a charged capacitor across a small load, it starts to supply the voltage (Stored energy) to that load until the capacitor fully discharges.

Capacitor comes in different shapes and their value is measured in farad (F). Capacitors are used in both AC and DC systems (We will discuss it below).

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).

Polar and Non-Polar Capacitor:
Non Polar Capacitor: (Used in both AC and DC Systems)
The Non Polar capacitors can be used in both AC and DC systems. They can be connected to the power supply in any direction and their capacitance does not effect by the reversal of polarity.

Polar Capacitor: (Only used in DC Circuits and Systems)
This type of capacitor is sensitive about their polarity and can be only used in DC systems and networks. Polar Capacitors don’t work in the AC system, because of the reversal of polarity after each half cycle in AC supply.



Types of Capacitors: Polar and Non Polar Capacitors with Symbols

Role of Capacitors in AC Circuits and System:
The capacitor has lots of applications in AC systems and we will discuss few uses of capacitor in AC networks below:

Transformer less power supply:
Capacitors are used in transformer less power supplies. In such circuits, the capacitor is connected in series with the load because we know that the capacitor and inductor in pure form does not consume power. They just take power in one cycle and deliver it back in the other cycle to the load. In this case, it is used to reduce the voltage with less power wastage.

Split phase induction motors:
The capacitors are also used in induction motor to split a single phase supply into a two phase supply to produce a revolving magnetic field in the rotor to catch that field. This type of capacitor is mostly used in household water pumps, Fans, air conditioner and many devices which need at least two phases to work.

Power Factor Correction and Improvement:
There are lots of advantages of power factor improvement. In a three phase power systems, capacitor bank is used to supply reactive power to the load and hence improve the power factor of the system. Capacitor bank is installed after a precise calculation. Basically, it delivers the reactive power which was previously traveled from the power system, hence it reduces the losses and improves the efficiency of the system.

Role of Capacitors in DC Circuits and system:
Power conditioning:
In DC systems, capacitor is used as a filter (mostly). Its most common use is converting AC to DC power supply in rectification (such as bridge rectifier). When AC power is converted into fluctuating (with ripples i.e. not a steady state with the help of rectifier circuits) DC power (Pulsating DC) in order to smooth and filter out these ripples and fluctuation, DC Polar capacitor is used. Its value is calculated precisely and depends on the system voltage and the demand load current.

Decoupling Capacitor:
Decoupling capacitor is used, where we have to decouple the two electronics circuits. In other words, the noise generated by one circuit is grounded by decoupling capacitor and it does not affect the performance of other circuit.

Coupling Capacitor:
As we know that Capacitor blocks DC and allows AC to flow through it (we will discuss it in the next session that how does it happens). So it is used to separate AC and DC signals (also used in the filter circuits for the same purpose). Its value is calculated in such a way that its reactance is minimized on the basis of frequency, which we want to pass through it. Coupling Capacitor is also used in filters (ripple remover circuits like RC filters) to separate AC and DC signal and removes the ripples from pulsating DC supply voltage to convert it into pure AC voltage after rectification.

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