Method and device for measuring the difference in magnetic...

Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Magnetic saturation

Reexamination Certificate

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C324S127000

Reexamination Certificate

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06310470

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a method and a device for measuring electric currents in n conductors where n is a natural number and n≧2.
It is generally known to measure electric currents using a magnetoresistive sensor, in which the output signal is proportional to an electric current to be measured, as described in DE-PS 43 00 605 C2. The magnetoresistive sensor can be used to carry out a potential-free measurement of the current strength of an electric current such that the magnetic field caused by the electric current and the magnetic field gradients are measured with the sensor.
When polyphase currents are to be measured, Hall transducers are used. The disadvantage of Hall transducers is that, due to the physically prescribed magnetic field sensitivity of the Hall elements, an iron core (laminated cores or ferrites) has to be used to guide the flux and concentrate the field. This results in a relatively large overall volume which is an obstacle for integration of the device into planar design technology, such as hybrid circuits. In addition, the Hall elements have to be mounted in an air gap in the flux concentrator, which increases the expenditure for maintaining the air gaps and creepage paths, if the measuring conductor cannot be adequately insulated with respect to the iron core.
According to another known technique described in EP 597 404 A2, operations are carried out with fewer sensors than the number of currents to be determined. For example, the currents of a three-conductor system are sensed with two sensors. According to this technique, it is not the individual magnetic field of the electric conductor which is used for measurement purposes (page 2, lines 50 to 52) but rather the total magnetic field which is produced in a multiconductor system through superimposition of the individual magnetic fields. Determining the field strengths by means of vectors requires the angles of the arrangement to be known. As a result, a complex calculation is necessary which is undesirable.
Furthermore, in the known technique there is a need for sensors which supply an electrical measurement signal which is proportional to the vectorial magnetic field strength.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and device for measuring electric currents in at least two conductors, by using a technically simple and compact design of the measuring arrangement and by determining the desired values by means of a simple calculation.
In accordance with the invention, magnetoresistive sensors are used which are designed as gradiometers to sense the difference in the magnetic field strengths produced by the conductor currents, such that when there are n conductors, n−1 scalar variables are obtained. According to the invention, the desired values are obtained by means of a simple calculation.
In accordance with an embodiment of the invention, the measurement of electric currents in n conductors is carried out with only n−1 magnetoresistive sensors (n≧2). Contrary to the conventional measurement with Hall sensors; according to the invention, no flux concentrators are needed, since the magnetic field of sensitivity of the magnetoresistive technology is greater than that of the Hall effect approximately by a factor of 20. Furthermore, the magnetoresistive sensors have a very compact design, as compared to the current sensors based on the Hall effect, which use an iron core for guiding the flux and concentrating the field. Such a core is not required in the case of magnetoresistive sensors. Furthermore, in the case of magnetoresistive sensors, the conductor for field compensation is integrated into the sensor chip as a microsystem, thereby resulting in a very compact design. Furthermore, the technology which is based on magnetoresistive current sensors does not require any auxiliary energy at high measurement potential, such as is required, for example, when measuring currents with separated potentials at shunt resistors.
In another embodiment of the invention flux concentrators are used. However, the compact design according to the invention is retained, in contrast to the conventional art, since the arrangement of the flux concentrators enables the magnetoresistive sensors to be arranged more closely one next to the other. As a result, it is ensured that the compact design achieved by using the magnetoresistive sensors is retained.
In magnetoresistive sensor technology, the measurement of currents is also reduced to the measurement of the magnetic currents of conductors through which current is flowing.
According to the invention, electric currents can be measured in at least two conductors with a simple and compact design. For example, a polyphase current measurement can be carried out at the motor output of a power inverter. The present method can also be applied, for example, in electronic power actuators for electric drives, such as in frequency converters or in servo controllers.
By virtue of the fact that electric currents are measured and determined in n conductors with n−1 magnetoresistive sensors, in a further embodiment of the invention, the present method and device can be used for fault detection. By determining the current strength of the individual currents, excess current and/or fault current can be detected, thereby protecting appropriate modules for drive applications.
Furthermore, according to the invention, other variables, such as voltage and power levels, can be derived from the current measurement.
In the case of a short circuit, reliable deactivation of an output stage requires reaction times to be shorter than ten microseconds. This protection can be ensured with the device according to the invention.
An overload protection can be carried out, for example, by monitoring the integral ∫i
2
dt or ∫idt or via a temperature sensor.
For a polyphase current, according to the invention, two magnetoresistive sensors are necessary to determine the current strength of the three phase currents.
The output signal supplied by the magnetoresistive sensors is a signal which is proportional to the electric current to be measured. Depending on the direction in which the current is flowing in the current conductors, the sensors measure the difference in the magnetic fields of the current conductors or the magnetic fields are summed.
If a measurement of a polyphase current is carried out and the three phase currents have a positive current direction, the two magnetoresistive sensors provided in the measurement of the polyphase current measure the difference in the strength of the magnetic fields of, in each case, two of the three current conductors and in each case, output directly as an output signal a voltage which is in proportion to the difference between the respective current strength of the phase currents. By forming the equations for this and using the rule that the sum of all the current strengths at any time is equal to zero, appropriate relationships can be produced, with the result that desired conclusions can be derived regarding the three conductors by using two sensors.
In the measuring arrangement according to the invention, with flux concentrators, in which arrangement the conductors extend through ferrite cores, the following advantages occur:
(a) decoupling of field strength and field profile from the conductor geometry is obtained;
(b) an increase in primary conductor cross section, since the flux concentration is no longer performed by making the conductor tracks narrower; as a result of the enlarged primary conductor cross section, the power loss is lower; as a result of this, implementation as an SMD (Surface Mounted Device) module is provided;
(c) magnetic shielding by means of the ferrite core, and less Eddy current is produced in a metal substrate serving, if appropriate, as a circuit carrier and/or cooling surface mounted under the measuring arrangement, for example in an IMS (Insulated Metal Substrate) or DCB (Direct Copper Bonding) substrate, or in other adjace

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