There are two types of power supply: DC and AC. DC supply has constant magnitude with respect to time and hence called a Direct Current.
In contrast, in AC supply the magnitude and direction change with time, hence called Alternating Current. Of both the supply, more than 90% of the electrical energy used is AC in nature because of its advantages over DC.
The change in magnitude and direction of AC supply is measured in terms of cycles. A cycle consists of two half-cycles, namely positive cycle and negative cycle.
The current increases in magnitude, in one particular direction, attains a maximum in a positive cycle. From there, it starts decreasing, passing through zero it increases in opposite direction and attains a maximum in a negative cycle. Then it starts increasing and it continues to flow similarly, as shown below.
Important terms in AC waveform
In the waveform, the maximum value attained by an alternating quantity(voltage or current) either during the positive half cycle or negative half cycle is known as amplitude. It is generally represented as Em or Vm for voltage and Im for current.
A set of positive and negative half-cycles constitute a complete cycle.
The time taken by an alternating quantity to complete one complete cycle is known as the time period. It is denoted by T seconds. Hence, each cycle repeats for every T seconds.
The number of cycles per second of an alternating quantity is called frequency(f) and its unit is expressed in cycles/second or Hertz(Hz). Frequency is equal to the reciprocal of time period and is given by,
Types of AC waveform
A quantity that varies with time in its magnitude and direction is termed as an alternating quantity. Such quantity is drawn by means of waveforms, called AC waveforms.
There are different types of AC waveforms in practice such as sine waveform, square form, triangular waveform and trapezoidal waveform. These waveforms are shown below.
Of all the AC waveforms, the sinusoidal waveform is preferred over the other types of waveform. It is because of its added advantages.
- Sinusoidal waves have less amount of distortion when transmitting over the linear circuit.
- The sinusoidal waveforms retain their shape even after differentiation and integration.
- The mathematical calculation of sinusoidal function is easy comparing it to other waveforms.
How AC is generated?
The AC voltage is generated in a machine called AC Generator or Alternator or Synchronous Generator. There are different generating stations like a thermal power plant, hydro power plant, nuclear power plant, etc.
In these power plants, the energy produced by thermal, hydro and nuclear is used to run the turbine, which acts as a prime-mover for the generator. The rotor of the generator rotates with the prime-mover and generates electric voltage.
The principle used for the generation of AC voltage is Faraday’s Law of Electromagnetic induction. The generated voltage will have three phases, so-called Three Phase AC voltage.
Based on the number of phases in the generated voltage, the AC voltage can be classified into single-phase AC voltage and Three-phase AC voltage.
What are the Advantages of AC waveform?
- AC supply can be generated, transmitted, distributed and utilized in electrical utilities.
- It is easy to generate high AC voltage with high-speed alternators. The construction and cost of which is very low compared to DC machines.
- AC voltages can be easily increased or decreased by using a transformer, which cannot be done easily with DC.
- The amount of copper required is less for transmitting the AC voltage from one place to another place.
- Transmission of high voltages and less current, over long distances, reduces the copper loss.