current sensors using magnetic materials

current sensors using magnetic materials
Journal of Optoelectronics and Advanced Materials Vol. 6, No. 2, June 2004, p. 587 - 592
INVITED PAPER
CURRENT SENSORS USING MAGNETIC MATERIALS
P. Ripka*
Czech Technical University, Technicka 2, 166 27 Prague, Czech Republic
Precise contactless DC and AC magnetic sensors are required by car industry, chemical
industry, for measurement of power and many other applications. The emphasis is given on
current sensors based on magnetic materials, but other methods are also mentioned for
comparison. Discussed are the factors influencing precision and geometrical selectivity.
(Received April 26, 2004; accepted June 3, 2004)
Keywords: Current sensors, Current clamps, Current transformers, Current comparators
1. Introduction
The current measurement using shunt resistor is in some cases impractical or impossible: for
large currents the shunts are heavy and they cause voltage drop and dissipate heat. They are not
insulated and the conductor should be disconnected for mounting. Contactless current sensors may
be used for remote conductors at high potentials, underground cables etc., but they are usually more
expensive. This paper is based on overview of magnetic sensors given in [1].
A wide range of AC and DC contactless current sensors is produced by LEM, F.W. Bell,
VAC, Honeywell, Telcon and other manufacturers. Overview of traditional non-contact current
sensors is given in [2].
Besides fulfilling the requirements common to magnetic field sensors, such as linearity,
offset and sensitivity stability, and low perming, contactless current sensors should be geometrically
selective – i.e. sensitive to measured currents, and resistant against interferences from other currents
and external fields. The easiest way how to guarantee this, is to use a closed magnetic circuit with a
measured conductor inside. This is used in current transformers, fluxgate current sensors and most of
the Hall current sensors. If this is not possible (e.g. when the yoke is too large, which usually is the
case when measuring large currents), gradient techniques can be used. Simple integrated current
sensors use folded conductor and gradient field sensor which suppresses response from distance
sources, which give low gradient. For large currents the current bar should be kept straight and
circular arrays of typically four to eight sensors are being used [3]. Averaging of the sensor output
increases sensitivity to the conductor between them and decreases sensitivity to the conductors
outside. Instead of simple averaging of the sensor outputs, higher-order algorithms can give more
effective rejection for closely located false currents [4].
2. Instrument current transformers
Current transformers have a primary winding with few turns (or a single conductor through
the core opening) and a secondary winding, which should be ideally short-circuits, but in reality it
has some small burden. The core is usually ring-shaped, either wound of high permeability tape (for
precise, low-frequency devices) or made from ferrite (for high-frequency devices). The current
transformer amplitude and phase errors depend on the core material and size, winding geometry, and
amplitude and frequency of the measured current and also on the value of the burden (which is
*
Corresponding author: [email protected]
588
P. Ripka
sometimes in the form of current-to-voltage converter).
Nanocrystalline materials are very promising for use in instrument current transformers.
Relative current ratio error of the transformer with core from Nanocrystalline Vitroperm 500 F is
very small and phase error is almost constant even for low values of primary current [5]. This is
thanks to the material small loss angle and constant permeability over wide field amplitude range.
Another advantage of this material is its larger saturation induction than commonly used Permalloy,
which allows significant reduction of the core dimensions.
Electronically enhanced two-stage current transformers show accuracy improvement by two
orders of magnitude: with a low burden the resulting error is below 10 ppm [6]. These devices can
also indicate the remanence of the transformer core or DC component in primary current which may
degrade the performance of classical current transformer; they may have lower number of turns,
which avoids problems with parasitic capacitances and allows use the device at higher frequencies;
and finally the volume of the core may be reduced.
3. Rogowski coil
Circular Rogowski coils (also called di/dt coils) may be used to measure AC or transient
currents. The device is extremely linear, as it has air core. It is sensitive to di/dt, so that the output
voltage should be integrated. Precise Rogowski coils have both magnetic and electrostatic shielding
to suppress interference. Rogowski coil with digital integrator is being used for power meters: using
AD 7759 signal processor with built-in sigma-delta A/D converters gives 0.1% error from the
measured value in 1000:1 dynamic range [7]. Rogowski coil with integrator can also be used to
measure changes in DC current: however, the limiting factor here is the offset drift of the integrator.
4. Current comparators
Current comparators are described in details in a book written by their “fathers” Miljanic
and Moore [8].
AC current comparator is three-winding device on ring (torroidal) core. If the primary and
secondary currents are balanced, i.e. N1I1 = N2I2, the core flux and also the voltage induced into the
detection winding is zero. The main application of AC current comparator is calibration of
instrument current transformers and other metrological tasks, such as null indicator in AC bridges.
Practical devices are large and complex: they are compensated by additional windings and they have
several active and passive shielding to reduce errors. AC comparators have errors below 1 ppm in
amplitude and 3⋅10-6 deg in phase.
DC current comparators are based on fluxgate effect. They are usually feedbackcompensated, the core consists of two detection torroids excited in opposite directions.
The basic schematic diagram of DC current comparator is shown in Fig 1. Also DC current
comparators have errors below 1 ppm..
Small-size AC/DC current comparator with amorphous cores was described in [9] and [10].
The device is excited in resonant mode by short 16 A p-p current pulses. The range is 200 A from
DC up to 3 kHz. The device can work in three modes: as passive DC comparator, passive AC
current transformer and active feedback-compensated AC/DC comparator. While the passive DC
ratio error of the magnetic circuit was below 0.3 %, in the transformer mode the maximum
amplitude error in the whole frequency range was 0.4% and the phase error was 0.5 deg. In the
active comparator mode the amplitude and phase error was 0.2% and 0.2 deg, respectively.
Fluxgate DC current sensors or “DC transformers” are similar to DC comparators but of
a much simpler design. The accuracy of a typical commercial 40 A module is 0.5 % , linearity 0.1
%, current temperature drift <30µA (-250C..700C). First fluxgate current sensor in PCB (printed
circuit board) technology was described by Gijs et al. [11]. Their sensor had a single winding of 36
turns over toroidal core made of amorphous magnetic foil. They reached 10 mV/A sensitivity and
ranges up to 5 A. Prototype of a fluxgate current sensor with electroplated core in PCB technology
was described in [12]. PCB current sensors have low cross-section of ferromagnetic core and high
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Current sensors using magnetic materials
resistance of the coils. Thus they can not be effectively tuned neither in the excitation circuit, nor at
the output. External saturable inductor can be used in order to lower the excitation power [13]. The
open-loop linear range is 1 A, but using a 40-turn winding also for the feedback can improve the
linearity and increase the range to 10 A. Similar miniature current sensors can also be made in thinfilm technology: sensor based on 2.6 mm diameter saturable ferromagnetic ring was described in
[14]. Because of their low offset drift, fluxgate-based “DC current transformers” are superior to the
current sensors having Hall sensor in the airgap.
Nex N1
N
c
Magnetic
shielding
Detectio
n ring
I
N
I
f
G
ref
2
I
PSD
R
Out
Fig. 1. DC Current comparator (from [9]).
Hall current sensors. Traditional current sensors are based on the Hall element in the airgap
of a magnetic yoke. To improve the linearity, the measured current may be compensated. However,
open-loop meters are preferred for battery-operated devices because of smaller size and weight and
mainly lower power consumption than more precise feedback-compensated sensors. One of the best
feedback-compensated small-range Hall current sensors is LTS 25-NP manufactured by LEM. The
device has 25 A range and DC to 200 kHz bandwidth, error is 0.02 % and sensitivity TC is 50
ppm/K. The main problem of these sensors is their limited zero stability given by the Hall sensor
offset: typical offset drift of a 50 A sensor is 600 µA in the (00 C...70 0C) range. This parameter is
20-times worse than that of fluxgate-type current sensor modules. Even when using magnetic yoke,
Hall current sensors are sensitive to external magnetic fields and close currents due to the magnetic
leakage associated with the airgap (necessary to accommodate the sensor). Another DC error is
caused by the hysteresis of the magnetic core – only few Hall current meters have AC
demagnetization circuit to erase perming after overrange DC current.
Low-cost current sensor based on highly sensitive Hall sensor with integrated flux
concentrators is described in [15]. The sensor has only simple ferromagnetic circuit and it has 1%
accuracy in the ± 12 A range. The device is made in PCB-based technology, which allows easy
batch processing.
Yoke-less current transducer with six Hall-probes around the rectangular bus bar achieved
0.5% linearity and 0.2% temperature stability in the 100 kA range [16]. The same 100 kA return
current in 50 cm distance was suppressed by the factor of 100.
Magnetoresistive current sensor shown in Fig. 2 is based on an AMR bridge, which is
made insensitive to an external field, but sensitive to measured current through the primary bus bar
[17]. The measured current is compensated by feedback current through compensation conductor.
Typical application is galvanically isolated current sensing in PWM regulated brushless motor.
These sensors are manufactured by F.W. Bell and Sensitech with ranges from 5 to 50 A. Achieved
linearity is 0.1 % , temperature coefficient of sensitivity is 100 ppm/K, offset drift in the (–45 0 C to
+85 0C ) range is 1.4% FS.
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P. Ripka
Similar sensor with GMR detector was developed by Siemens [18]. While linearization of
the AMR sensors is made by using barber poles, these GMR sensors should be biased by permanent
magnet, which is a source of instability. Spin-valve bridge current sensor was described in [19].
These sensors are likely to exhibit large change of characteristic when subjected to over-currents.
Magnetooptical current sensors are suitable for high-voltage high-current applications, but
the reported errors are more than 1% even after temperature compensation [20], [21].
Magnetooptical current clamps were described in [22]. They do not use optical fiber, but bulk-optic
glass. Achieved accuracy was 1% for 50 Hz AC current in a 1000 A range; the sensitivity was
4.45x10-5 rad /A, which is double the Verdet constant of the SF-6 glass. By using bulk flint glass
optical detector in the 20 mm wide airgap of ferromagnetic yoke the noise level of 1.6 mA/√Hz
@280 Hz was achieved. However such a large airgap should significantly reduce the geometrical
selectivity [23].
GMI current sensor was reported in [24]. Amorphous Co67Fe4 Cr7Si8B14 strip was annealed
to have 230% GMI at 20 MHz. The schematic diagram of the current sensor is shown in Fig 3. The
strip was wound around the measured conductor and DC biased by external coil to achieve linear
response. The first prototype has only two turns of the GMI core. The open-loop linearity was 2% in
the +/- 2A range. Linearity and stability of the sensitivity can be improved by using of the feedback.
The weak point of this type of sensors is poor DC offset stability: the change of the sensitivity with
the temperature is 197 ppm/ºC and the GMI offset drift Z/ T = 25.35 m /ºC. With current
sensitivity of 0.24 /A the offset drift recalculated to the input is 105 mA/ºC. This offset drift can be
suppressed by differential configuration. When the number of turns is increased from 2 to 200, the
sensitivity will be increased by the factor of 100. The impedance of such sensor would be about 2.5
k . The open-loop sensitivity tempco would remain the same, but it can be easily reduced to about
50 ppm/ºC by using the feedback. The feedback would also improve the linearity.
Ibias=
Idet~
Vout
~
Imeas =
Imeas
Fig. 2. Magnetoresistive current sensor – after [17].
Fig. 3. GMI current sensor.
Current clamps: AC and DC. Current clamps usually consist of an openable magnetic
circuit, which ensures that the reading is not dependent on the actual position of the clamped
conductor and the device is insensitive to unclamped conductors.
AC current clamps are usually based on current transformers with openable core. The
measured conductor forms a primary winding, secondary winding is terminated by a small resistor,
or connected to current-to voltage converter. Very accurate clamp current transformers use
electronic compensation of the magnetization current and achieve error of 0.05% from the measured
value in 1 % FS to 100 % FS [25]. High-current AC and AC/DC openable current transformer
clamps [26] and low-current multistage clamp-on current transformer with ratio errors below 50
ppm [27] were developed by So and Bennet.
Some of the available DC current clamps based on Hall sensor may have 10 mA
resolution, but the maximum achieved accuracy is 30 mA, even if they are of the compensated type.
Replacing the Hall sensor with magnetoresistor brought no significant improvement: although they
Current sensors using magnetic materials
591
are more sensitive and stable, magnetoresistors require larger airgap (about 2 mm minimum), which
degrades the sensor linearity and geometrical selectivity (i.e. the reading is dependent on the position
of the clamped conductor and the sensor is sensitive to close unclamped currents). Precise DC/AC
current clamps based on shielded fluxgate sensor were described in [28]. The device has rectangular
ferrite core consisting of two symmetrical L-shaped halves. The main advantage of fluxgate current
clamps is that they need no airgap in their magnetic circuit. Permalloy shielding serves for
decreasing the effect of the residual airgap at the clamp joint. Single winding serves for the
excitation (by 1 kHz squarewave voltage), sensing (second harmonics in the excitation current) and
feedback. The sensor linearity and hysteresis error is less than 0.3% of the 40 A full-scale. The noise
is 10 µA p-p, long-term zero stability is 1 mA. The main advantage is high suppression of the
external currents: 40 A current 10 mm from the sensor causes error of only 3 mA – this error linearly
decreases with increasing distance.
Very simple AC/DC current clamps can be improvised from AC commercially available
current clamps, which are excited by external AC generator into the fluxgate mode [29]. In order to
minimize the effect of the source impedance and AC interference injected into the measured circuit,
two antiserially connected clamps were used [30]. Two AC oscilloscope current probes Iwatsu CP
502 supplied by 700 Hz/290 mA p-p sinewave into serially connected secondary windings tuned by
parallel capacitor gave 11.6 mV/A sensitivity in the voltage-output mode and 300 mA/A sensitivity
in the current-output mode (per 1 turn of added detection winding).
5. Magnetometric measurement of hidden currents
While the field of the small distant current loop has dipole character, i.e. the field is
decreasing with 1/r3, where r is a distance, field from the long straight conductor decreases with 1/r.
Underground electric conductors can be located and their current can be remotely monitored by
measuring the magnetic field in several points supposing that the back conductor is distant. In the
case that the cable contains both forward and returns currents, its detection is possible, but the
current value cannot be precisely measured. This technique was used for location of underwater
optical cables which contain also metallic conductor delivering a DC current of about 1 A to supply
the repeaters. The field distribution was measured by two three-axial fluxgate magnetometers. The
cables were detected from 40 m distance and their position determined with 0.1 m accuracy from 4
m distance [31].
The magnetometer method is also used to measure the currents in constructions such as
bridges and in pipelines. Changing natural magnetic fields may induce large currents in long
conductors: 70 A current was observed to flow in Alaska Oil Pipeline [32].
6. Other methods
Current meters using NMR and ESR have been reported [33]. Resolution of 10-13 A is
achievable with SQUID, but this device has very limited dynamic range. Current weights measure
Lorenz force – on the same principle work some published microelectromechanical devices.
7. Conclusions
Traditional DC and AC contactless current sensors are available for ranges from mA to kA
with precision from 3% (uncompensated Hall current sensors) to 0.1 % (compensated Hall devices
and magnetic amplifiers). Higher precision is easily achievable with Current comparators. Very
promising are current sensors based on AMR effect and di/dt sensors.
592
P. Ripka
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