isolated current voltage transducers

Air-Core Technologies
measured. The integrator circuit (Fig. 45) is kept as close
to the transducer head as possible to reduce stray inductive
pickup that would give rise to errors.
Finally, the comments made previously (§ 5.1) regarding the
instantaneous voltage collected by the coils of an air-core
technology are again applicable.
5.3.2 Characteristics and Features
Figure 43: PRiME TM sensor multi-layers printed wiring board
Figure 44: PRiME TM – two coils concentric loops
constant DC current will not produce a voltage and
therefore cannot be measured. However, as with LEM~flex,
it will respond to pulsed currents.
The output signal V TOTAL
(Fig. 45), used for the current
measurement is:
V TOTAL
= V INNER
– V OUTER
/ λ = V INNER
• [1 - (V OUTER
/ V INNER
)/ λ]
For an external current source, the essence of the
technology is that the ratio of the induced voltage in each
loop of sensor PWBs (V OUTER
/ V INNER
) is almost constant,
irrespective of the magnitude or position of this external
source. As a result, the external field effects are cancelled if
the correct proportion of the voltage induced in the outer
loop (V OUTER
) is substracted from that in the inner loop
(V INNER
). Making λ = V OUTER
/ V INNER
will force V TOTAL
= 0 or, in
other words, an external current source will not produce any
signal at the output.
To measure the current from the conductor placed within the
aperture, an optimized transducer design leads to a typical
V OUTER
/ V INNER
ratio of λ / 2, again irrespective of the
conductor position. As a result, the usable signal for the
measurement is close to V TOTAL
= V INNER
/ 2, meaning that
approximately half of the signal collected by the inner
concentric chain of sensing PWB coils is used for the
measurement.
In short, this novel arrangement provides one-half of the total
possible signal for measurement while effectively canceling
the unwanted influence of signals from external current
sources.
As with the LEM~flex, the collected signal is proportional to
the derivative of the current and it is necessary to integrate
the voltage induced in the transducer head to obtain both
amplitude and phase information for the current being
Since PRiME TM utilizes an air-core coil as the sensing
element, there is no magnetic hysteresis, saturation, or nonlinearity,
as is present in a current transformer with a
magnetic core.
There is no theoretical maximum limit to the measuring
range, but the typical dynamic range is 1000:1,
corresponding to the ratio of the maximum to minimum
measurable current with a given transducer. The accuracy is
typically specified as a percentage of reading, above 10 %
of the nominal rated current, leading to a highly accurate
solution when the current is only a fraction of the nominal
current. The accuracy is generally better than 0.8 % of
reading and the gain variation due to temperature is low,
typically 0.01 %/K.
The output voltage is directly proportional to the measured
current and provides accurate phase information. The level
of sensitivity depends on the required measuring range and
supply voltage.
The aperture sizes for existing transducers range from
20 mm to 160 mm in diameter, in both ring and split format.
There is no theoretical limit to the size of the aperture. With
split versions, installation and measurement can be
performed without mechanical or electrical interruption of the
current carrying conductor, while also ensuring galvanic
isolation.
The bandwidth of PRiME TM has both a high frequency and a
low frequency cut off. The high frequency limit is dependent
on the resonant frequency of the sensors while the low
frequency limit is a function of the integrator design.
Products are designed for a given bandwidth, typically 5 to
100 kHz, but an upper limit in the MHz range appears to be
feasible.
V inner
V outer
R 1
R 1
λ
Figure 45: PRiME TM – processing of the coil signals
-
+
C f
V total
37