In case of the sinusoidal waveform of the current
from an electric company, there are the following relations;
Average value (I ave.) = 0.637 x Peak value (Im)
R.M.S. value (I) = 0.707 x Peak value (Im)
R.M.S. value (I) = 1.111 x Average value (I ave.)
When a measuring devise uses a R.M.S. value measurement, it directly
display the measured result. When it uses a average measurement, it
multiples 1.111 to the measured result. This is equivalent to a R.M.S.
value. Therefore, the results by a R.M.S. measurement method and a
average value measurement method is same as long as the current waveform
is clearly sinusoidal.
In case of distorted waveform, the above equation is not true, then:
R.M.S. (I) = 1.111 x Average value (I ave.)
Consequently, the results of an R.M.S. value measurement are different
from that of an average value measurement. |
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The
display of the current meter on our control panel differs from that of a
digital indicating tester. The current meter receives the signal from a
CT transmitter. Why does the difference occur?
We
found the control panel uses a CT transmitter of the R.M.S. operation. A
possible cause for the difference is that distorted waveform, such as thryrister-controlled
waveform interfered the sinusoidal waveform of the current by some causes.
Most CT transmitters of R.M.S. operation is designed for the sinusoidal
wave form. In addition, the digital indicating tester was displaying the
average value. These may be the causes of the difference. 