Page 451 - Instrumentation Reference Book 3E

434 Electrical measurements

rectifier converts the a.c. signal into a unidirec- For a sinusoid I(f) = i. sin wt the rms yalue is
tional signal through the moving-coil instrument ild2 and its mean absolute value is 2Zh. The
which then responds to the average d.c. current Form Factor for a sinusoid is thus 20.11. Rectifier
through it. Such instruments measure the mean instruments indicate 1.1 1. Imab. For waveforms
absolute value of the waveform and are cali- which are not sinusoidal rectifier instruments will
brated to indicate the rms value of the wave on provide an indication which will have an error of
the assumption that it is a sinusoid.
For a periodic current waveform Z(t) through
the instrument the mean absolute value, Imab, is
given by
Figure 20.12 shows several waveforms with their
Form Factors and the errors of indication which
occur if they are measured with mean absolute
and its rms value is given by value measuring-rms scaled instruments. This
Form Factor error also occurs in the measure-
ment of a.c. current and voltage using digital
voltmeters which employ rectification for the
conversion from a.c. to d.c.
where Tis the period of the wave. As a current-measuring device the diodes
The Form Factor (FF) for the current wave- should be selected for their current-carrying cap-
form is defined as ability. The non-linear characteristics of the
diodes make range extension using shunts
impractical, and therefore it is necessary to use
rectifier instruments with current transformers
(see section 20.2.3). The forward diode drop
places a lower limit on the voltage which can be

~__
Percentage error in
Wavesham Form Factor measurement using
mean sensing-rms
indicating
instruments

1.11 0

r"y' 1'57 -29.3

1.15 -3.96

1 +11.1

Figure 20.12 Waveform Form Factors and errors of indication for rectifier instruments.