Alternating Currents

Introduction

The flow of electrical charges through a conductor from higher to lower potential is known as a current. There are two types of currents.

1.  Direct current- DC
2.  Alternating current- AC

The current whose direction does not change with time through a load is known as direct current (DC), whereas the current whose direction changes periodically through a load is known as alternating current (AC). The voltage through AC is known as alternating voltage. AC is significantly used as it can be transmitted quickly over long distances without much loss. The AC voltage is preferred as it can be easily and efficiently converted from one voltage to another using a transformer. 

Alternating Current

Alternating current is defined as the current that changes its magnitude and polarity or reverses its direction at regular time intervals.

The current takes the form of a sinusoidal wave when diagrammatically or graphically represented. The changing polarity after every half cycle is represented as maximum positive and negative values, above and below the axis, respectively. 

The charged electrical particles in an alternating current start from the axis, zero, reach the maximum positive and then return to zero, thus completing one positive cycle. These particles further fall down below the axis to the maximum negative and then return to zero, thus completing one negative cycle. 

A phasor diagram is used to represent the phase relationship between voltage and current in an AC circuit. It is a vector, which rotates around the origin with angular speed Omega, ω. Magnitudes of phasors, V0, represent the peak value or the amplitude of voltage (V); I0 represents the peak value or the amplitude of the current (I).

Production of an Alternating Current 

An alternating current can be produced using various devices under the umbrella category of alternators described below.

1. AC generator that follows Faraday’s principle of electromagnetic induction and converts mechanical energy into electrical energy
2. Turbine-based AC generation in power plants
3. Conversion of DC into AC using an inverter

An AC generator responsible for converting mechanical energy into electrical energy consists of the following parts.

1. Coil mounted on a rotor shaft, wherein the axis of rotation of this coil is perpendicular to the direction of the magnetic field.
2. A pair of magnets

When the coil is mechanically rotated in the magnetic field by using external force, the rotation causes a change in magnetic flux through the coil; hence, the electromagnetic force (EMF) is induced in the coil. The ends of the coil are connected to an external circuit using slip rings and brushes to make the rotation easier. 

 The changing flux is represented by

Φ(t) = B⋅A(t) = BAcos(ωt)

Here,

B = magnetic field

A = area of cross section

ω = angular speed of the coil

t = time

Φ = magnetic flux

 E = -N Δϕ/Δ(t)

E = induced EMF

N= number of turns in the rotating coil

E = −NBAωsin(ωt)

The above equations represent the maximum value of EMF that can be produced in the generator.

The mechanical energy required for rotation of the coil in the magnetic field for commercial manufacture of AC is provided by water falling from a height that rotates the turbine for the armature coil. The electric current is induced by the interaction between charged particles and magnetic fields, which convert the kinetic energy of the turbine into the kinetic energy of electrons. 

Applications of Alternating Current 

Almost every daily use device uses AC as DC is costlier, has a risk of producing fire, and cannot be converted from high to low or low to high voltages using transformers. Hence, AC is preferred over DC. 

• AC is majorly used for the industrial purpose for transportation and electricity production
•  in household devices such as refrigerators, dishwashers, garbage disposals and toasters
• AC is used in power houses
• AC is used to power electric motors, responsible for converting electrical energy to mechanical energy

AC Waveform 

An AC waveform is a periodic waveform. It is also known as a bidirectional waveform. This time-varying sinusoidal waves are described as sine waves. One wave consists of a maximum positive cycle and a minimum positive cycle. This wave is called AC due to its constant change in polarity during every half cycle, from positive to negative and vice versa.

Characteristics of the waves of AC

1. Time period (T): The time required by a waveform to repeat itself from the beginning is called its time period. It is measured in seconds and is also known as the periodic time of the waveform for the pulse width for square AC waveforms.
2.  Frequency (f): The number of times a waveform repeats itself within a time period of one second is called frequency. The unit to measure frequency is Hertz (Hz) and frequency is reciprocal of time period. It can be represented as f = 1/T.
3. Attitude (A): The magnitude or intensity of the signal waveform measured in volts is called its amplitude. In simpler language, the distance from the excess to the maximum or minimum positive value of the wave is called amplitude. 

Conclusion 

AC is the most widely used form of current. It is safer than DC due to transfer over longer distances and helps maintain electric power. The frequency of alternating current can be controlled. The electrons in the electric charges that  repeatedly generate alternating current change their direction.