Arbiter Systems 931A Application Note

Using Clamp-On Current Transformers (CTs)
Model 931A Power Analyzer
Model 930A Three-Phase Power Analyzer
Model 929A Three-Phase Power Meter
Introduction
This application note deals with extending the
current measuring range of the Arbiter Models
931A, 930A and the Model 929A power
analyzers. It also covers techniques to greatly
improve the accuracy, and essentially calibrate
out the measurement inaccuracies associated
with the use of clamp-on current transformers
(CTs).
All three Arbiter power analyzers are state-of-
the art instruments offering unprecedented
accuracy in the measurement of parameters
related to AC Power Generation, Transmission,
and Distribution. Frequently, however, direct
measurements are not possible since the
parameter being measured is beyond the range
of the instrument, or the circuit cannot be
disconnected. One such electrical parameter is
current. Since each of these instruments is
limited to measure up to 20 Amps directly, an-
other method is needed to extend the measure-
ment range beyond 20 Amps.
Employing the use of a clamp-on current trans-
former, with a known ratio (e.g. 100:1), can
effectively extend the current measuring range
of these Arbiter products, and not interrupt the
circuits being tested. One of the negative
aspects of using a clamp-on current transformer
is the measurement error due to accuracy and
phase shift introduced by the clamp-on CT.
Since these measurement errors can be
accounted for by using the scaling feature on
these power analyzers, using a clamp-on CT
should no longer be a concern.
Arbiter Systems, Inc. 1324 Vendels Circle, Suite 121 Paso Robles, CA 93446
Tel +1 805 237 3831 800 321 3831 Fax +1 805 238 5717 E-Mail sales@arbiter.com
with Arbiter Power Analyzers
APPLICATION NOTE 113
Lastly, this application note will review a very
simple technique in calibrating the CT for use
in the field or in the lab.
Theory of Operation
An AC clamp-on current probe (CT) is a type
of current transformer. A transformer is formed
essentially from two coils wound on a common
iron core (Figure 1). A current I1 applied
through the coil B1 induces through the
common core a current I2 in coil B2. The
current I2 in B2 is determined by the ratio N1 x
I1 = N2 x I2, where N1 and N2 are the number
of turns in each coil.
I1
B1
Iron Core
Figure 1
The same principle is applied to a current
probe, which is simply an articulated magnetic
core (Figure 2). The articulated core holds the
coil B2, and clamps over the conductor being
measured. If B1 is the conductor being
measured, then N1 is the number of turns of
that conductor, which is normally equal to one.
When the current probe is clamped around the
conductor it provides an output proportional to
the number of turns of B2, such that
I2 (probe output) = (N1 / N2) x I1 where N1 =
1, or the probe output = I1 / N2. Without a CT
I2
B2
Z
1