AC Circuits

Electronics engineering involves the use of different circuits each of which is used for various purposes. One such type of circuit is an alternating current circuit, which forms an integral part of the communication system. In this article, we will study what is an AC circuit and what is the categorization of AC circuits.

We will also study the principle of operation of AC circuits and the components that make up these circuits. We will look at some real-life applications of AC circuits to understand their advantages and disadvantages. We will emphasize the difference between AC and DC circuits as well. The article will conclude with some frequently asked questions.

We know that current flows in one direction in DC circuits, but do you know that it is possible to alternate the current direction periodically? AC circuits are the type of circuits used for generating an electric current that changes its direction and magnitude periodically. Here is a formal definition of AC circuits

AC circuits are powered by alternating sources and produce current and voltage that vary in magnitude and direction over time, after which they repeat the cycle.

Let us know about some Terminologies of AC circuits.

• Amplitude: The maximum peak value that the voltage or current can take in one full cycle is known as the amplitude of the alternating quantity.
• Frequency: Frequency is the number of cycles or altercations a quantity can make in one second. It is usually measured in Hertz.
• Time period: The time taken by an alternating quantity like current or voltage to complete one oscillation or cycle is known as the period of that quantity.
• Instantaneous Value: The value of any quantity at that particular instant of time is known as the instantaneous value of that quantity.
• Waveform: Every alternating quantity when measured over a period of time creates a shape known as waveform of that quantity. This waveform is generally plotted on y axis with respect to time.

Here is a list of important formulas

There are different types of AC circuits. Let us take a look at them.

Purely Resistive

This circuit is categorized by low inductance which reduces the overall reactance in comparison to the resistance of the circuit. The resistance is then used to regulate the current flowing in the circuit. Since the inductance is almost negligible, we can call it a non-inductive circuit whose frequency doesn’t change with changing frequency. This is what the purely resistive circuit looks like.

V=V_msin(ωt) ∴ I=V/R ∴ I=V_msin(ωt) /R

Resistor-Inductor Circuit

A resistor-inductor circuit is a type of circuit that has a resistor R and a pure inductor of value L. These can be connected in series with an AC power source. In this circuit, a magnetic field is established in the inductor due to the current flowing in the circuit. The current is periodic and has a time constant known as the inductive time constant. Let us take a look at the Resistive-inductive circuit.

I= V/R where V=V_msin(ωt) and R= √R²+X_L² where X_L=jωL

Resistor-Capacitor Circuit

A resistor-capacitor circuit is a type of circuit that has a resistor R and a pure capacitor of value C. These can be connected in series with an AC power source. In this circuit, an electric field is established in the capacitor due to the charge collected in the capacitor. The current grows or decays exponentially and has a time constant known as the capacitive time constant. Let us take a look at the Resistive-capacitive circuit.

I= V/R where V=V_msin(ωt) and R= √R²+X_c² where X_c=1 /jωC

Resistor Inductor Capacitor circuit

An RLC circuit consists of a resistor, inductor, and a capacitor connected in a series of parallel. The impedance of the inductor and capacitor are frequency dependent, therefore, the output of the series RLC circuit varies with a change in the frequency of the circuit. Therefore, the voltage and current in this circuit exist in phases. Let us take a look at the RLC circuit and the impedance of such a circuit.

I= V/R where V=V_msin(ωt) and R= √R²+(X_L-X_c)² where X_L=jωL and X_c=1/jωC

The principle of working AC circuits is the rotation of a magnet along a set of wires or coils under the effect of the constant magnetic field. An equivalent version of this principle can be understood as if any coil is rotated in a stationary consistent magnetic field, then a current is generated in an AC generator which alternates periodically. Note that this generator is known as an alternator. This principle is applied to designing AC circuits.

AC Circuit Analysis

The term analysis is used for understanding the circuit and calculating the current and voltage components of the circuit. Since we are talking about the AC circuits, the current and voltage values calculated will vary periodically with frequency, and on plotting these values on y axis with time, we will get a sinusoidal function. The voltage generated in the circuit can be calculated by measuring the peak or the rms voltage of the circuit. This voltage can then be used for calculating the current in the circuit if you know the circuit impedance.

AC Circuit Theory

We have seen the principle of AC circuits is the rotation of a magnet along a set of wires or coils under the effect of the constant magnetic field. Let us formally define AC Circuit Theory

The alternating current theory talks about the steady-state behavior of electrical circuits and studies the fluctuations in current and voltage with time.

Fourier series are used for analyzing these alternating signals in the shape of a sinusoid because every signal can be represented using sinusoids.

Let us take a look at the construction of AC circuits.

As we can see, the given circuit is made up of passive components like a resistor, capacitor and an inductor and it also consists of an alternating power source. Let us study these components carefully.

Components

These are the components of AC circuits

Resistor: Resistors are used to limit or regulate the amount of current flowing in the circuit. It is the passive component of the circuit that dissipates heat. The resistance is independent of frequency and can be written as

Resistance= R

Capacitor: A capacitor is a passive device with two terminals that is used for storing the electrical energy generated in the circuit in the form of charges. The impedance of the Capacitor is frequency-dependent and can be written as

X_c=1/jwC

where w=frequency, C=capacitance

Inductor: An inductor is also a two-terminal device that stores electrical energy in the form of a magnetic field. This stored magnetic field depends on the current flowing in the circuit. The impedance of the inductor is frequency dependence and can be written as

X_L=jwL where w=frequency, L=inductance

Let us study the difference between AC and DC circuits.

Let us study some advantages of AC circuits.

• AC circuits have the ability to produce High voltages therefore High power with full efficiency.
• The cost of installing an AC circuit is less as compared to DC circuits making AC a popular choice.
• The power loss in AC circuits is minimal over long distance ranges which is Contrary to DC which has huge power loss with increasing distances
• AC voltage is easier to step up and step down using a transformer. This is not possible in DC circuits.
• The conversion of AC circuits to DC circuits can be done easily depending on the need but conversion from DC to AC is difficult.

Let us study some disadvantages of AC circuits.

• Due to the alternating nature of current in AC circuits, these circuits can’t be used for charging different appliances.
• The skin effect is prominent in AC circuits which reduces the actual amount of current flowing in the circuit.
• Components of AC circuits are prone to vibrations due to their frequency dependence therefore they are short-lived.
• Voltage regulation in AC circuits is difficult due to the high voltage drop.
• Working with AC circuits is very dangerous due to the high voltage generated therefore High supervision is required with proper precautions.

Let us study some applications of AC circuits.

• The power fed to households is AC in nature and uses AC circuits to ensure power is available at large distances.
• Electric motors in power plants operate on AC circuits. These motors are used to generate electrical energy from mechanical energy.
• AC circuits are used for generating power to drive electric vehicles therefore AC circuits have an important role in transportation.
• Majority of appliances like lights, fans, microwave ovens, washing machines, and kettles need alternating current to operate therefore they make use of AC circuits.
• AC circuits form a core part of the syllabus in institutions since the concepts of AC help to form the base of electronics engineering.

We have seen how AC circuits form an important part of the electronics industry. Their ability to carry power over long distances without any major loss is something that is used in daily life for power transfer. We have also understood how AC circuits differ from DC circuits which can be used for circuit selection. We are now well aware of the workings of AC circuits and what components make up these circuits. Despite the advantages offered, there are some disadvantages associated with these circuits that have been discussed in the article.

What is the difference between AC circuits and DC circuits?

In DC circuits, the current flows in one direction with 0 free whereas in AC circuits the current and voltage vary periodically in magnitude and direction.

What is meant by a passive component and what are the passive components in AC circuits?

A passive component as opposed to an active component, consumes the electrical energy of the circuit and stores it in some form. The resistor, capacitor, and inductor are passive components of the AC circuit.

What is meant by reactive power in an AC circuit?

Reactive power is present due to reactive load in the circuit. This is the power out of the total power that remains unused and flows back and forth in the circuit.