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Capacitive Reactance: Understanding the Behavior of Capacitors in AC Circuits, Exercises of Electrical Circuit Analysis

Mariano Marcos State University (MMSU)Electrical Circuit Analysis

The concept of capacitive reactance, explaining how capacitors behave in ac circuits and the role they play in storing and releasing energy. It covers the definition of capacitance, the behavior of capacitors when charged and discharged, and the difference between real and reactive power. The document also introduces the concept of capacitive reactance and its relationship to the frequency and capacitance.

Typology: Exercises

2017/2018

Uploaded on 07/14/2018

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Experiment No. EE104L-05
CAPACITIVE REACTANCE
I. OBJECTIVES
1. To study the behavior of the capacitor in ac circuits.
2. To become familiar with the meaning of capacitive reactive power.
II. INSTRUMENTS AND COMPONENTS
Power Supply Module
Capacitance Module
AC Metering Module (Virtual)
AC Metering Module (Virtual)
Single—Phase Wattmeter Module (Virtual)
Connection Leads
III. DISCUSSION
Capacitance may be defined as a measure of the amount of electrical charge which a
device can store in the dielectric (insulation) between two conductors (plates) when a given
voltage is applied. The Basic unit of capacitance is the farad. The farad is used in equations that
include capacitance terms. However, a farad is so large a quantity, which measurements are made
in microfarads (µF) - one millionth of a farad. In electric work the Pico farad (pF) — one
millionth of a (pF) — is a common unit.
If a dc voltage is applied suddenly to a capacitor a large current will flow. This current
will continue to flow at a decreasing rate until the capacitor is charged up (the voltage across the
capacitor equals the source voltage). The current drops to zero as soon as the capacitor voltage
stabilizes (becomes constant — that is, when the capacitor is neither charging nor discharging).
The current can be quite large if the voltage across the capacitor changes quickly. If the source
voltage increases at a rapid rate, a large current will flow into the capacitor to charge it. Under
these conditions the capacitor acts as a load. Conversely, if the source voltage decreases at a rapid
rate, a large current will flow out a momentary source of power; in fact, ‚just like a generator.
A capacitor has the ability to store electric energy by virtue of the electrostatic field
which is set up between its plates. The quantity of energy stored depends upon the capacitance (in
farads) and upon the square of the voltage. When a capacitor is being charged up, it receives and
stores energy, but does not dissipate it. When the unit is subsequently discharged, the stored
energy will be released until the voltage across the capacitor falls to zero.
A capacitor does not dissipate electric energy; it can only store it and then release it. This
is quite different from a resistor which cannot store energy, but can only dissipate it in the form of
heat.
pf2

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Download Capacitive Reactance: Understanding the Behavior of Capacitors in AC Circuits and more Exercises Electrical Circuit Analysis in PDF only on Docsity!

Experiment No. EE104L- CAPACITIVE REACTANCE I. OBJECTIVES

  1. To study the behavior of the capacitor in ac circuits.
  2. To become familiar with the meaning of capacitive reactive power.

II. INSTRUMENTS AND COMPONENTS

Power Supply Module

Capacitance Module

AC Metering Module (Virtual)

AC Metering Module (Virtual)

Single—Phase Wattmeter Module (Virtual)

Connection Leads

III. DISCUSSION

Capacitance may be defined as a measure of the amount of electrical charge which a device can store in the dielectric (insulation) between two conductors (plates) when a given voltage is applied. The Basic unit of capacitance is the farad. The farad is used in equations that include capacitance terms. However, a farad is so large a quantity, which measurements are made in microfarads (μF) - one millionth of a farad. In electric work the Pico farad (pF) — one millionth of a (pF) — is a common unit.

If a dc voltage is applied suddenly to a capacitor a large current will flow. This current will continue to flow at a decreasing rate until the capacitor is charged up (the voltage across the capacitor equals the source voltage). The current drops to zero as soon as the capacitor voltage stabilizes (becomes constant — that is, when the capacitor is neither charging nor discharging). The current can be quite large if the voltage across the capacitor changes quickly. If the source voltage increases at a rapid rate, a large current will flow into the capacitor to charge it. Under these conditions the capacitor acts as a load. Conversely, if the source voltage decreases at a rapid rate, a large current will flow out a momentary source of power; in fact, ‚just like a generator.

A capacitor has the ability to store electric energy by virtue of the electrostatic field which is set up between its plates. The quantity of energy stored depends upon the capacitance (in farads) and upon the square of the voltage. When a capacitor is being charged up, it receives and stores energy, but does not dissipate it. When the unit is subsequently discharged, the stored energy will be released until the voltage across the capacitor falls to zero.

A capacitor does not dissipate electric energy; it can only store it and then release it. This is quite different from a resistor which cannot store energy, but can only dissipate it in the form of heat.

The facts can help us to understand the behavior of a capacitor when it is connected to an ac source of power. The ac voltage is continually increasing, decreasing and reversing its polarity.

When the voltage increases, the capacitor stores energy, and when the voltage decreases, the capacitor must release it. During the ‘storing” period, the capacitor acts as a load on the ac power supply but during the “releasing” period, the capacitor actually returns its energy to the source.

We have the very interesting situation where the capacitor periodically acts as a source of power returning energy to the very supply which gave it its energy in the first place.

In an ac circuit, power flows back and forth between the capacitor and its power source and nothing useful is accomplished, is a wattmeter is placed between the power source and the capacitor of the circuit shown in Fig. 17—l, power will flow from left to right when the capacitor charges up and from right to left when it discharges.

Since no power is dissipated in the capacitor, the wattmeter will indicate zero. (it actually tries to indicate positive when power flows from left to right and negative when the power flow reverses, but the reversal takes place so quickly that the pointer does not have time to respond.)

The real power associated with an ideal capacitor is therefore zero. There will, however, be a voltage drop across the capacitor and current will flow in the circuit. The product of the two is the apparent power. The current leads the voltage by 90 electrical degrees.

The reason the current leads the voltage can be easily seen. When the applied voltage is going through its peak, the voltage for that instant is not changing; hence, the current will be zero. When the voltage is passing through zero it is changing most rapidly, hence, the current is a maximum. Because of this unique condition, the apparent power is also called the reactive power (var). Reactive power associated with capacitors carries a negative sign (—).

Capacitive reactance is the resistance offered to the flow of alternating current by the presence of capacitance in the circuit. It is measured in ohms and is equal to the ratio of E/I.

Reactance also depends upon the frequency and the capacitance in farads and can be expressed mathematically as: