AC FUNDAMENTALS – PART – 02 – AVERAGE AND RMS VALUE OF A SINE WAVEFORM AND FORM AND PEAK FACTOR

Heat produced by an alternating current = i^2Rdt

Heat produced by an direct current                 = I^2RT

The r.m.s. voltage is denoted by capital letters without any suffix.

Irms  = 0.07Im                 Vrms = 0.07Vm

Iavg.  = 0.637Im               Vavg. = 0.637 Vm

Peak to Peak Vp = 2Vm

[Vm is the maximum voltage and Im is the maximum current]

IMPORTANCE OF RMS VALUE

Alternating voltage and current is always specified in terms of RMS.

The ammeter and voltmeter records only RMS values.

RMS value of ac wave is always greater than the average value.

If a motor draws 5A, it means the alternating current flowing to the circuit has the same heating effect as 5A dc.

FORM FACTOR = Kf = RMS value / Average value

PEAK FACTOR = Kp = Maximum value / RMS value

IMPORTANCE OF FORM FACTOR AND PEAK FACTOR

Form factor and peak factor give an indication of the shape of the waveform.

Form factor gives the measure of the “peakiness” of the waveform.

Peak factor indicates the maximum voltage being applied to various parts of the apparatus.

The voltage applied across the cable or capacitor, the breakdown of insulation will depend upon the maximum voltage.

The insulation must be able to withstand the maximum voltage rather than the RMS value of voltage.