isolated current voltage transducers

Hall Effect Technologies
Demagnetization current
> 5 cycles
> 30 cycles
• core losses become significant at high frequencies and it
is essential to limit the current amplitude at these
frequencies to acceptable levels (dependent on ambient
and maximum transducer temperatures); this implies not
only limiting the maximum frequency of the fundamental
current, but also harmonic content, since even a low
amplitude signal may create unacceptable losses at high
frequencies.
3.1.7.2 Core loss rule-of-thumbs
Iron losses calculation are complex and, as a „rule-of-thumb“
judgment, it is possible to consider that the iron losses are
minimized if the product "N • I • f" is kept as small as
possible, where:
Figure 5: Degauss cycle current
Alternatively, a partial demagnetization of the core is possible
by providing an appropriate signal in the opposite polarity of
the magnetization. The difficulty is determining the exact
amplitude and duration to obtain a satisfactory result. With a
well-defined application it may be feasible to determine the
required value empirically and apply this correction as
necessary.
3.1.7 Bandwidth and core losses
The bandwidth limitation of open loop transducers is mainly
due to two factors:
• limitations of the processing electronics
• magnetic core heating due to core losses, a combination of
eddy current and hysteresis losses
3.1.7.1 Core losses
The magnetic material and core design as well as the current
amplitude versus frequency spectra define the level of core
losses:
• eddy current losses are proportional to the square of three
different parameters: the peak flux density in the core, the
frequency of induction and the lamination sheet thickness
of the core
• hysteresis losses are proportional to frequency, core
volume and the square of peak flux density
For LEM transducers, this leads to the following conclusions:
• these losses are directly proportional to the square of the
flux density, which is directly related to the primary ampereturns,
implying core losses are theoretically proportional to
the square of primary ampere-turns if no magnetic
saturation occurs. When increasing sensitivity by using
multiple primary turns, core losses are increased by the
square of the turns
N = number of internal or external primary turns
I = primary current or amplitude of a current harmonic
f = frequency of the primary current or current harmonic
As a result, when one of the three factors is increased (i.e.
the current), the iron losses are increased unless at least
one of the two other factor is decreased (i.e. the frequency of
the measured current and/or the number of primary turns).
While this formula implies that the core losses will increase
with an increase of any of these parameters, it is not
intended to say that acceptable core losses are realized if
the product of the three parameters is kept constant. For
example, it is wrong to say that one can operate at twice the
frequency if the Ampere-turns are cut in half.
At a given frequency, it is nevertheless correct to assume
that keeping constant the "N • I" product implies similar iron
losses, even the probable change on the primary conductor
magnetic coupling may affect the iron losses value.
To conclude, trouble-free operation of a current transducer
requires limiting the temperature rise to avoid overheating
the internal components. Parameters affecting temperature
rise go beyond core losses and include the primary busbar
resistive losses, the losses of the electronics and the various
thermal resistances. In particular, to keep losses constant
requires to decrease the transducer primary current while
the working frequency increases.
3.1.7.3 A core loss example
Considering the LEM open loop HY 10-P transducer, where
the primary is integrated into the transducer and the number
of primary turns cannot be changed, tests show the following
maximum working conditions:
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