Page 160 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

In the case of AC there are two half cycles which are exactly opposite to each other therefore the average value
over a complete cycle is zero. The average value over a half cycle is obtained as per above figure is
I =(i + i + i +.............+ i ) / n
3
n
1
2
av
RMS value [effective value]
The effective value of an alternating current is that value which will produce the same heating effect as specific
value of a steady directcurrent.For determining RMS value of an alternating current its instantaneous values are
taken at several points of time during a cycle. These values are then squared , added and then averaged . The
square root of this average value gives RMS value of the AC (or emf)
V = 0.707 V m
rms
I = 0.707 I m
rms
Form Factor

It is the ratio of effective (rms)value to average value of half cycle
Form factor = Irms / Iav
= (Im X .707)/Im X 0.637
= 0.707/0.637
Form factor = 1.11

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CITS : Power - Electrician & Wireman - Lesson 26-29