Alternating current, (AC) is current that changes direction cyclically, passing first in one direction then in the other through a circuit. Such alternating currents are produced by generators and other such voltage sources whose polarities alternate between a positive direction and a negative direction rather than being fixed in a constant direction as with DC sources. By convention,
Alternating currents are called AC currents
Alternating voltages are called AC voltages
The most common AC source is the commercial AC power system that supplies energy to your home.
The variation of an AC voltage or an AC current over time is called a waveform. Since these waveforms vary with time, AC supplies are designated by lowercase letters v(t) for voltage, and i(t) for current instead of uppercase letters V and I for DC values. Note that the subscript (t) represents time.
There are many different types and shapes of waveforms but the most fundamental is the sine wave (also called sinusoid). The sine wave or sinusoidal AC waveform is the voltage and current waveform shape at the wall socket outlets in your home.
What is Periodic Waveform
The waveform starts at zero, increases to a positive maximum (called the peak), and then decreases to zero, changes polarity, increases to a negative maximum, then returns again back to zero. One complete variation between the same points on the waveform is referred to as a cycle. Since the waveform repeats itself at regular intervals over time, it is called a periodic waveform.
The calculation of Average, R.M.S, Form Factor and Crest Factor can be use with any type of periodic waveform including Triangular, Square, Sawtoothed or any other irregular or complex voltage or current waveform shape. For a pure sinusoidal waveform the effective or R.M.S. value will always be equal to 1/√2 x Amax which is equal to 0.707 x Amax and this relationship also holds true for RMS values of current. The RMS value for a sinusoidal waveform is always greater than its Average value.
What is cycle?
Each repetition of a set of positive and negative instantaneous values of the alternating waveform is called a cycle.
What is Time Period?
The sine wave function is periodic in time. This means that the instantaneous value at time t will be exactly the same at a later time. The time taken by the alternating waveform to complete one full cycle is known as its time period (also called wavelength in radio), denoted by T seconds.
The number of cycles per second of a waveform is defined as its frequency.
Then we can say that frequency is the reciprocal of the time period as ƒ = 1/T and has the unit of inverse seconds, s-1
In the SI system, the unit of frequency is the hertz ( Hz ) and by definition:
1 Hertz ( Hz ) = 1 cycle per second
Angular frequency ( ω ) is the waveforms frequency expressed in electrical radians per second. As one cycle of an alternating waveform corresponds to 2p radians, the angular frequency can therefore be expressed as:
2p cycles/sec. Angular frequency has the units of radians/second. The relationship between frequency, ƒ and angular frequency, ω is:
ω = 2πƒ rad/sec. or ω = 2π/T radians/sec
The advantage of using alternating voltages and currents for electronic power supplies is that they can be raised and lowered with the help of a device called a transformer. In DC circuits, raising and lowering voltages is not so easy because transformers cannot be used with direct current