What is Form Factor? Form Factor Formula & its Derivation

What is Form Factor?

The Form Factor is a very important parameter for knowing the perfectness of the waveform. Any periodic waveform has varying magnitude with time. The waveform attains maximum or peak value at a point in time. The direct current has its maximum amplitude at all the times i,e. It has a constant magnitude that does not change with time. Contrary to this, the instantaneous value of AC varies with time and attains maximum or peak value; again, the magnitude decreases. Before understanding what the Form Factor is, let us understand the crest factor of AC and its RMS  and average value. 

Crest Factor

The crest factor of an AC current or voltage waveform is the ratio of its peak value to the root mean square(RMS) value. The crest factor of an AC voltage or current can be found by measuring its peak value and root mean square value. The root mean square value of an AC current is equal to the heating effect of the AC current and is equal to the heating effect of the DC current. The root mean square value of a perfect sinusoidal AC voltage or current has a certain relationship with its peak or maximum value.

The RMS value of an AC voltage is given below.

Mathematical derivation of AC RMS Value

derivation of ac rms value

Mathematical Derivation of Crest Factor 

Mathematical derivation of crest factor

What is the form factor of AC waveform?

The magnitude of the alternating current varies with time, both in the positive and negative half of the complete cycle. Form Factor shows how distorted the waveform is. The RMS value of AC is equal to the direct current, which, when passed through the resistor for a given period of time, will produce the same heating effect as produced by alternating current when passed through the same resistor for the same time. As derived above, the RMS value of the ac waveform is equal to Vp/√2. Where, Vp is the peak value.

The average value of AC is equal to the steady current, which transfers the same amount of charge in a given time as transferred by an AC across the same circuit in the same circuit. The derivation of the average value of AC is given below.

mathematical derivation of average value of ac

Form Factor Formula

Mathematically, we can express the form factor as;

form factor formula

Form Factor Formula Derivation

The form factor of the ac waveform is equal to the ratio of the rms value to the average value.

Derivation of form factor

Form Factor of various Waveforms

The form factors of various AC wave shapes are given below.

form factor of various waveform

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