Do you ever wonder what kind of current flows in the power lines of our household? Is it the same current that is generated in electronic devices with DC battery supply or is it something different? In this article, we will discuss a different current that alternates i.e. varies in magnitude and direction. This type of current is called alternating current.

Alternating current is generally seen flowing in power lines(telephone cables, office lines) and normal household electricity that comes from a wall outlet. It is basically used for industrial, chemical, and domestic power usage where it is transmitted over a long distance. The most common frequency for AC power in many parts of the world is 50 or 60 hertz (Hz), meaning the direction of the current changes 50 or 60 times per second. On the contrary, we have DC current which flows in one direction and doesn’t change polarity.

In this article, we will see all the terminologies related to AC current, the method used for generating AC current, and the difference between AC and DC current. We will also discuss the phasors and the application of AC currents. The later part of this article will talk about AC circuit analysis.

What is AC?

Alternating Current (AC) is a type of electrical current, in which the direction of the current switches back and forth at regular intervals. Since it is a periodic current it can be represented by any periodic function of finite magnitude. It is the most common type of electrical power used in homes and businesses, and it is typically generated by power stations and distributed through power lines.

An Example of AC

Primary Terminologies of Alternating Current

• Amplitude: It is the highest positive or negative value obtained by an alternating quantity in a single full cycle.
• Alternating: alternating indicates varying. Quantities that change polarity or direction, respectively, over time.
• Instantaneous value: It is the value of voltage or current at any particular instant of time.
• Frequency: It is defined as the number of cycles per second made by an alternating quantity.
• Time Period: The time taken in seconds by a quantity(usually voltage or current) to complete one cycle is called time period.
• Waveform: It is a shape created by plotting the instantaneous values of an alternating variable like voltage and current along the y-axis and the time or angle along the x-axis.

AC Waveform

A waveform is a graphical representation of the variation of AC current with respect to time in an electrical circuit. The shape of waveform can vary depending upon the source. Generally AC current waveform is of sinusoidal pattern. The value of current rises from zero to a peak value and then attains zero from the peak value. The wave then changes its direction and repeats the same.

Some Possible AC Waveforms

The waveform in an AC circuit depends on the requirements of the equipment involved. Sine waves are preferred for general power distribution because they are less likely to cause electrical interference in the system. Square and triangular waves are used in applications where rapid switching or specific waveform characteristics are necessary due to the shape which aids in faster switching action.

Phasor Diagram

The term ‘phase’ indicates a distinct state or process. Phasor diagram is a great mathematical instrument to study our circuit and understand the relationship between the values in terms of magnitude and phases . We can study the dependency or relation between two waves at same frequency, these waves are generally a function representing current and voltages of circuit. For an AC circuit with frequency ω , we can plot the phasor with a magnitude and show them moving anti-clockwise with frequency ω about the origin.

Phasor Diagram

A phasor generally has two parts, a magnitude and a phase angle.

• Magnitude: This demonstrates the peak value of the AC voltage or current. This simply gives us the information about amplitude of wave.
• Phase Angle: The phase angle is a representation of the angular displacement of wave with respect to a certain value. The general unit used for measuring phase angle can be degree or radian.

Working Principles of Alternating Current

In order to produce alternating current , an alternator is used which is also known as a generator . This can be produced by many methods but the most usable and best method is using a coil AC generator or alternator. This setup consist of two pole magnets and a single loop of wire around it in rectangular shape . So basically this works on the principle of Faraday’s law of electromagnetic induction , according to which in a magnetic field when a coil rotates so it produces the magnetic flux change in that field so emf produced which results in induced current. The magnetic flux can not be remain same so it varies based on the rotation of coil present in magnetic field. We also can say that induced current also depends on these factors :

• number of turns in the armature
• strength of magnetic field
• area of cross section of coil
• speed of the rotation

The electric current passes through the galvanometer , which varies between negative and positive values which shows the alternating current in the process which is flowing through it .

The frequency of alternating current is determined by number of rotations of coil in one second and speed of rotation coil , which can be shown by the given formula:

v= w/2π

So basically we learned from this thing that the generator produces electricity through rotation. The mechanical energy is supplied to AC generator through various turbines and engines and in the output we get the electric power.

Steps to Generate Alternating Current

Step 1: Get An Appropriate Generator

To generate AC, you need a device that can produce a changing magnetic field. This changing magnetic field induces a voltage in a conductor, leading to the generation of AC current. The most common device for this purpose is an alternator or generator.

Step 2: Get a Mechanical Energy Source

In order for our generator to work it needs some mechanical energy to drive it. In large power plants steam/water turbines are used to store this energy while we can use diesel engines for small generators.

Driving Source

Step 3: Completing the Connections

Connect the mechanical energy source to the generator. This involves connecting a shaft from the mechanical energy source to the rotor of the generator.

Mechanical Source with Generator

Step 4: Creating a Magnetic Field

Inside the generator, there is a rotor (rotating part). The rotor usually has strong magnets or field windings that produce a magnetic field.

Magnetic Flux Direction

Step 5: Induce Voltage in Stator

Due to the changing magnetic field in the rotor, a voltage is induced in the stator windings(coils) . This voltage is what generates AC current. The generator contains coils of wire that are connected to the output terminals of the generator.

Induced EMF

Step 6: Voltage Regulation

Once the voltage has been induced, it needs to be regulated. This can be achieved through control systems that adjust the speed of the energy source or the field current of the generator.

Step 7: Built an Output Connection

The final circuit is connected to the output terminals of the generator. These terminals can be used to access the AC current generated.

Terminals of Generator

How is AC Current Measured?

The two methods used to measure AC current:

• Using Multimeter: In order to measure AC from multimeter ,connect the multimeter in series with the circuit. Set the multimeter in the approximate range where you expect your current value will lie. Ensure that the power source is turned on and measure the reading given by multimeter. This method is direct method by making connection with current-carrying circuit.
• Utilizing Flux Generated by Circuit : This method employs the use of current transformers which have primary and secondary windings on a magnetic core. The ac current in primary coil generates a magnetic field in the core which induces current in secondary coil. A term ‘step down ratio is defined as the ratio of number of turns in primary and secondary windings. A low level of current is easily detected using certain instrumentation.

Calculate Impedances

For each component (resistor, capacitor, and inductor), calculate its impedance (Z) based on the component’s value and the frequency of the AC source.

For Resistors

Z_R = R

R →resistance

For Capacitors

Z_C = 1 / (jωC)

j→ imaginary unit

ω→ angular frequency (2πf)

C→ capacitance

For Inductors

Z_L = jωL

L→ inductance.

Calculate Current

Use Ohm’s Law for AC circuits, which relates voltage (V), current (I), and impedance (Z): V = IZ. For each component, calculate the current by rearranging the formula: I = V / Z.

In AC circuits with inductors and capacitors, consider the phase relationships between voltage and current for each component. These relationships can be leading or lagging, depending on the component type.

Use Ohm’s Law to find the total current in the circuit:

I_Total = V / Z_Total.

Calculate Power

For resistors, calculate power dissipation using P = I² * R, where P is power, I is the RMS current, and R is the resistance.

For inductors and capacitors, calculate reactive power, which may be leading or lagging, based on the phase relationships.

This is a generalized method to analyze the AC circuit.

Characteristics of Alternating Current

There are certain characteristics of AC stated below, these are characteristics to describe AC quantities:

1. The magnitude of current in AC circuit is variable. This means that the amplitude of current can be represented as a function.
2. The value and phase of current are periodic. This indicates that the value of quantities repeats after a certain interval of time.
3. Alternating Currents can be transferred over large range of distances without any significant power loss.
4. Generally AC current is represented using a sin wave but it can be triangular, rectangular or any other finite periodic function.
5. Alternating currents generally require less maintenance cost than DC current.

Types of AC Circuits

There are three types of AC Circuits :

• Purely Reactive
• Purely Capacitive
• Purely Inductive

Purely Reactive

A purely resistive circuit has a very negligible amount of inductance such that the reactance offered by such circuits is very small when compared to the resistance even at normal frequency.

Reactive Circuit

V=V_msin(ωt)

The current flowing through the purely resistive circuit can be derived as

I=VR

=V_mRsin(ωt)

I=I_msin(ωt)

Purely Capacitive

The circuit containing only a pure capacitor of capacitance C farads is known as a Pure Capacitor Circuit. The capacitor works as a storage device, and it gets charged when the supply in ON and gets discharged when the supply is OFF.

Capacitive Circuit

The equation of the alternating voltage that is applied across the circuit is

V =V_msinωt (1)

At any point in time, let the charge of the capacitor be

q =CV (2)

The current flowing through the purely capacitive circuit is given by:

i =dq/dt

Substituting (2) in the above equation,

i =d(CV) (3)

i =d (CV_msinωt)/dt

=CV_{m ˟} d(sinωt)/dt

i =ωC_mcosωt=V_{m ˟} ωC ⨯ sin(ωt+π2)

i =V_mX_csin[ωt+π/2] (4)

where is X_c=1/ωC

Current will be maximum when sin(ωt+π/2)=1 . Hence,

I_m =V_mX_c

On substituting this in (4) we get,

i =Imsin(ωt+π2)

Purely Inductive

The circuit which contains only inductance (L) and not any other quantities like resistance and capacitance in the circuit is called a Pure inductive circuit. In this type of circuit, the current lags behind the voltage by an angle of 90 degrees.

Inductive Circuit

The equation of the alternating voltage that is applied across the circuit is

V=V_msinωt (1)

This results in current flow along with an induced emf given by

e=−L(di/dt)

Since the induced emf is equal and opposite to the applied voltage,

V=−(−L⨯(di/dt))

V_msinωt=L(di/dt)

dI=V_mLsinωtdt (2)

Integrating both sides of (2) we get,

I =V_mωLsin(ωt−π2)

=V_mX_Lsin(ωt−π2) (3)

where X_L is the opposition offered by pure inductance to the flow of alternating current and is called inductive reactance.

The maximum current will be when sin(ωt−π2)=1

I_m=V_mX_L (4)

Substituting (3) in (4) we get,

I=I_msin(ωt−π/2)

Analysis of AC Circuit

The term ‘Analysis’ stands for detailed examination of something so circuit analysis means carefully breaking down our complex circuit into small parts which makes it easier for us to understand the functioning of our circuit.

In order to analyze the circuit we need to find the currents and voltages values in our circuit. We can employ simple methods to compute these values. We can use ohm’s law to calculate the current in a simple circuit ( voltage by the resistance). The circuit data like angular frequency, and the time are used to compute the ac current.

For a general circuit, the RMS voltage (V_RMS) and the frequency (f) of the AC voltage source are provided i.e. given.

Difference Between AC & DC