WO1994027157A1

WO1994027157A1 - Current measurement transducer based on magnetic flux density measurement

Info

Publication number: WO1994027157A1
Application number: PCT/FI1994/000184
Authority: WO
Inventors: Jussi Kuosa; Mikael NÅHLS
Original assignee: Abb Strömberg Kojeet Oy
Priority date: 1993-05-11
Filing date: 1994-05-10
Publication date: 1994-11-24
Also published as: AU6651094A; FI932122A; FI932122A0

Abstract

The invention is related to a current transducer based on the measurement of magnetic flux density and a method for calibrating the same. The transducer comprises a frame structure (5, 8), adapted to said frame structure (5, 8), an essentially ring-shaped magnetic circuit (2) having the annular shape gapped to form an air gap (4) and concentrate the magnetic flux to the air gap (4), said ring-shaped magnetic circuit (2) incorporating an aperture (10) through which the current conductor (6, 7) to be measured can be routed, a Hall-effect device (1) insertable in said air gap, and an electromagnetic shield structure (3) surrounding the frame structure to the end of preventing external interference fields from affecting the Hall-effect device. According to the invention the electromagnetic shield structure (3) is made of a material essentially similar to that of the magnetic circuit (2), and both the magnetic circuit (2) and the body structure (5) are so shaped as to be capable of accommodating either a straight current conductor (6), or alternatively, a winding structure (7) with which current measurement sensitivity can be multiplied.

Description

CURRENT MEASUREMENT TRANSDUCER BASED ON MAGNETIC FLUX DENSITY MEASUREMENT.

The present invention is related to an electric current measurement transducer in accordance with the preamble of claim 1 based on magnetic flux density measurement.

The invention also concerns a method for calibrating the transducer.

CH patent application 00518/86-9 describes a three-legged magnetic circuit of electrical steel using a Hall-effect device for measuring electric current. The disclosed structure is mechanically complicated and thus costly to produce. Implementation of 3-phase current measurement has not been specifically taken into account in the embodiment.

EP patent application 90,904,843.1 describes a comb-shaped magnetic circuit of electrical steel for measuring three-phase electric current. Due to mutual coupling between the different phases, compensation of external disturbances and calibration in single-phase use are complicated in the embodiment disclosed in cited publication.

It is an object of the present invention to overcome the drawbacks of the above- described prior-art techniques and to provide an entirely novel type of electric current measurement transducer based on magnetic flux density measurement.

The invention is based on designing the electromagnetic shielding of the transducer to be essentially of the same material as the magnetic circuit of the transducer itself, and that both the magnetic circuit and the enclosing structure are shaped so as to permit the accommodation of either a straight conductor or a coiled conductor for multiplication of the transducer current sensitivity. More specifically, the transducer according to the invention is characterized by what is stated in the characterizing part of claim 1.

Furthermore, the method according to the invention is characterized by what is stated in the characterizing part of claim 7.

The invention offers significant benefits.

The disclosed structure is small and simple. The structure is basically designed for three-phase use, while it can also be used in single-phase applications. The manufacture of the transducer according to the invention is easy and quick. Further, a single magnetic circuit provides two basic measurement ranges, and the basic ranges can be further expanded to larger currents by means of current transformers. The magnetic circuit and the transducing Hall-effect device can have the same basic construction for all measurement ranges. The measurement range is wide, extending approx. 10 times above the nominal current of the transducer. One of the measurement basic ranges is designed for the secondary current of a standard current transformer, whereby the other basic range is dimensioned for a tenfold current. Owing to the advantageous design of the magnetic circuit, tight tracking tolerances are achieved for the measurement values of the different phase legs. The tuning of the transducers will be simplified as the magnetic circuit elements are stamped using a single tool, whereby improved air gap precision is attained and the linearity deviations which are typical of magnetic circuits are reduced. The electromagnetic shield can be fabricated using a single tool only, which lowers the manufacturing costs of the transducer. The transducer is encased within a Faraday shield permitting its use under conditions of high electromagnetic interference levels. The transducer is also suited for measurement of inverter currents (transient-containing currents). The cost reduction offered herein is significant with regard to a conventional cuπent transformer implementation. The structure according to the invention achieves a 1 kN insulation level in a small volume. In the following a comparison of the cost reduction achievable by means of the invention with regard to a conventional current measurement techniques is given. The comparison covers current ranges from 0.2 A to 100 A, and the costs are calculated as an average cost of the current measurement elements required for cited current range in each implementation technique, respectively.

Measurement technique Minimum cost Maximum cost [FIM/pc] [FIM/pc]

In-house manufactured 3 -phase 854 854 current transformer

3 -phase current transformer from 680 680 vendor A

3 -phase current transformer from 300 1030 vendor B

3 -phase cuπent transducer 28 93 according to the invention

In the following, the invention will be examined in more detail by means of exemplifying embodiments with reference to the attached drawings, in which:

Figure 1 is an exploded perspective view of a current measurement transducer according to the invention designed for a high current range;

Figure 2 is an exploded perspective view of another current measurement transducer according to the invention designed for a low current range;

Figure 3 is a perspective view of the transducer shown in Fig. 2 ready-assembled; Figure 4 is a perspective view of the transducer shown in Fig. 1 ready-assembled and complemented with an electromagnetic shield;

Figure 5 is a perspective view of the magnetic circuit of a transducer according to the invention;

Figure 6 is a perspective view of the shield design for a transducer according to the invention;

Figure 7 is a sectional perspective view of the shield design shown in Fig. 6 encasing one magnetic circuit; and

Figure 8 is a sectional perspective view of the transducer structure shown in Fig. 3.

The terms with reference numerals used later in the text are given in the following list:

magnetic circuit 2 magnetic circuit air gap 4 left-side plastic end piece 5 right-side plastic end piece 8 current bar 6 winding 7 electromagnetic shield 3 magnetic circuit opening 10 magnetic circuit aperture 1 for winding 7 magnetic circuit lower rivet 14 magnetic circuit upper rivet 15 upper rivet corner projection 12 upper corner cut 13 housing 20 housing cover 21 recesses 22 for conductor termination lugs recess 23 for electronics fixing clips 24 fixing notches 25 cavities 30 for Hall-effect devices length A of housing 20 width C of housing 20 height B of housing 20.

With reference to Fig. 1, the current measurement transducer according to the invention comprises electric-steel magnetic circuits 2, one for each phase, current busbars 6 inserted through said magnetic circuits 2, and a support structure enclosing said magnetic circuits, whereby said support structure to the end of easier assembly comprises two parts, a left-side plastic end piece 5 and a right- side plastic end piece 8. The current busbars 6 are of copper or other high- conductivity material. The elements of the magnetic circuits 2 are brought to mutual alignment with each other with the help of the plastic end pieces 5 and 8. A further purpose of the plastic end pieces is to provide corrosion protection and insulate the windings 7 from the magnetic circuit 2. Additionally, the plastic end pieces 5 and 8 perform the centering alignment of the magnetic circuits 2 within the electromagnetic shield so that the sensor devices measuring the magnetic flux density are easy to insert into the air gap 4 of the magnetic circuit 2. The plastic end pieces 5 and 8 can be made from, e.g., polycarbonate. A suitable commercial grade is manufactured by, e.g., Bayer AG, and coded as CYBE 1D4 in the ABB Strόmberg material database.

The magnetic flux density evoked in the magnetic circuit 2 by the current passing through the current busbars 6 is measured by means of a Hall-effect device (not shown) placed into the air gap 4 of the magnetic circuit 2. 7157

With reference to Fig. 2, the current busbars shown in Fig. 1 can be replaced at lower current with a wire of smaller cross section wound into a winding 7 inserted in the opening of the magnetic circuit 2, whereby the smaller current in the conductor of the winding invokes in the magnetic circuit a magnetic flux density proportional to the number of turns in the winding.

With reference to Fig. 3, the structure illustrated in Fig. 2 is shown ready assembled.

With further reference to Fig. 4, the structure illustrated in Fig. 1 is shown ready assembled and encased with an electromagnetic shield 3. The electromagnetic shield 3 is advantageously of the same material as the magnetic circuit 2 itself. The shield 3 serves for excluding interference caused by external low-frequency disturbance fields on the magnetic flux density in the air gap 4 of the measuring magnetic circuit 2. The electromagnetic field imposed by the external interference source is shunted by the shield 3 of a ferromagnetic material enclosing the magnetic circuit 2, whereby the disturbing effect of the external field is minimized. In a practical design the current busbars 6 are isolated from the electromagnetic shield 3 by means of insulation tubing (not shown).

The structure of the electromagnetic shield 3 is designed for a maximum shielding effect within machining and production constraints. The shield can be fabricated using a single tool only, whereby a fast and low-cost production cycle is attained. The shield design is optimized to comprise maximally large contiguous surfaces and small gaps to minimize the effect of the external disturbance fields.

The shield 3 can be machined using a combination press tool which cuts, punches and bends the shield into final shape in a single workphase.

A suitable material for the shield 3 is, e.g., a commercial grade electrical steel manufactured by Bochum, and coded as AHLY 10070 in the ABB Strόmberg material database. To its characteristics, this material is essentially similar to the electrical steel grade used in the fabrication of the magnetic circuit 2.

Advantageous shielding efficacy measurement results were obtained by placing the shield 3 to a distance from the upper surface of the magnetic circuit 2 which was approximately equal to the air gap width of the magnetic circuit.

With reference to Fig. 5, the magnetic circuit 2 shown in the earlier diagrams is described below in greater detail. The function of the magnetic circuit 2 is to con- centrate the flux evoked by the cuπent conductor as homogeneously as possible in the air gap 4 of the magnetic circuit. The width of the air gap 4 determines the flux density in the magnetic circuit gap, said flux density being proportional to the current passing through the conductor.

Using structures and materials selected on the basis of laboratory tests, the construction of the magnetic circuit 2 is optimized for the exemplifying design values given below. Theoretically, the flux density peak value in the air gap is dependent on the effective cuπent passing through the conductor according to the following formula:

The magnetic circuit 2 is basically ring-shaped, whereby an aperture 10 remains in the center of the magnetic circuit for the current conductor. The aperture formed by the ring-shaped magnetic circuit includes an air gap 4, whose width in the exemplifying case is 2.5 mm. The magnetic circuit 2 is made from separate laminations of electrical sheet steel stacked together by means of rivets 14 and 15. In the exemplifying case the magnetic circuit 2 comprises 16 laminations of 27157

0.5 mm thick electrical sheet steel. The magnetic circuit 2 has an external height of 19 mm and an external width of 18 mm. For the smaller current range the lower edge of the magnetic circuit is provided with a notch 11 to accommodate the winding 7. The turns of the winding 7 are wound about the isthmus remaining between the notch 11 and the aperture 10. At the upper rivet 15, the magnetic circuit 2 has an upper projection 12, while the other upper corner of the magnetic circuit 2 is provided with a cut 13 to accommodate the upper projection 12 of the adjacent magnetic circuit. In fact, the magnetic circuit 2 can be considered to comprise two opposing hammers, whereby the air gap 4 is formed between the impact surfaces of the hammers. On the basis of measurements it has been found that the best linearity is attained by means of such a magnetic circuit in which the cross sections of the hammer-like upper ends are square. However, to optimize the design for the confined space, the left-side hammer-like end of the magnetic circuit has been narrowed by the upper corner cut 13, whereby only the right-side hammer-like end in the exemplifying embodiment has a cross section optimal in terms of linearity requirements.

The structure of the magnetic circuit 2 is characterized in being made from separate laminations of electrical steel which are riveted together to form a tightly packed magnetic circuit 2 using rivets of a ferromagnetic material or any other suitable material (e.g., brass). The structure of the magnetic circuit 2 has been designed so as to make it suitable for single-phase measurement applications as well. Owing to the advantageous design of the magnetic circuit, the mutual distances between the current conductors are minimized. By virtue of the above- described design principles, a compact structure has been achieved for the three- phase magnetic circuit 2.

The design of the magnetic circuit 2 offers two different current measurement basic ranges with a single core structure alone. The smaller of the current ranges is implemented with windings, while the larger of the current ranges uses a current busbar or a straight conductor. According to one primary idea of the invention, the 7157

smaller of the current basic ranges is designed equal to the nominal secondary current of a current transformer, whereby additional measurement ranges besides the two basic ranges are easy to scale by using current transformers.

The structure and production method of the magnetic circuit 2 have been carefully taken into account to reduce the manufacturing tolerance effects of the air gap 4 on the distribution of the transducer sensitivity. As the magnetic circuit is manufactured for three-phase current measurement transducers, the sensitivity differences between the different phase legs have been minimized by using a single stamping tool only, whereby the stamping tolerances will be equal for all phase legs. The manufacturing method applied attains a tolerance of approx. ±0.05 mm.

The sensitivity of the transducer has been found to drop when the thickness of the electrical steel laminations 2 is selected larger. On the basis of laboratory tests, the steel grade selected for the transducer has been compatible with cost, quality and machinability requirements.

The core must be free from inter-lamination air gaps between the laminations, which is prevented by bonding the laminations tightly together with rivets. Any separation between the laminations of the core cause nonlinearity of the magnetic circuit function and reduce the transducer sensitivity.

Minimization of the flux path length in the magnetic circuit 2 has been attempted by dimensioning the cross section of the current conductor, or alternatively, the current busbar smallest possible and placing the Hall-effect device as close to the cuπent conductor as feasible.

The magnetic circuit 2 can be fabricated using only a single machine, which is adapted to stamp and rivet the magnetic circuit core into a ready-assembled entity.

During the first workphase the electric sheet steel laminations for the magnetic 7157

10

circuit are stamped, after which the machine inserts them into the riveting jig and bonds the stack of laminations with rivets. The machine capacity is in excess of 10 magnetic circuit cores per minute. The laminations for the magnetic circuit core are stamped using one and only one stamping tool.

Suitable material for the magnetic circuit core 2 is, e.g., a commercial grade electrical steel manufactured by Bochum, and coded as AHLY 5040 in the ABB Strόmberg material database.

With reference to Fig. 6, the transducer construction can be enclosed in a housing

20 having recesses 22 for the terminations of the external conductors. The transducer structure, of which only the shield 3 is visible in the diagram, is located in the mid-part of the housing 20. The housing 20 is provided with fixing clips 24 suited for mating with the fixing notches 25 of the cover part 21, thus locking the cover. The cover 21 for the housing further has a recess for an electronics card employed for conditioning the current-proportional voltage output signal from the measuring sensor and calibrating the entire transducer (comprised by the magnetic circuit 2 and the measuring sensor). In the embodiment described here, the Hall- effect devices used as the measuring sensors are mounted on said electronics card. The external dimensions A, B and C of the housing for the embodiment are: length A 105 mm, height B 33 mm and width C 65 mm.

With reference to Fig. 7, the Hall-effect devices are accommodated by a cavity 30 designed at the air gap 4 of the magnetic circuit core 2 and extending through the plastic end piece 5, the shield 3 and the cover 21 of the housing. Thus the cavity

30 provides a contiguous space reaching through the entire structure from the air gap 4 of the magnetic circuit core 2 and through the cover 21 of the housing up to the recess for the electronics card. With reference to the sectional diagram of Fig. 8, the location of the windings 7 relative to the magnetic circuit core 2 is shown. Accordingly, the turns of the windings 7 are wound about the plastic end pieces 5 and 8 enclosing the core 2.

A practical structure and dimensioning calculations for a transducer according to the invention are as follows:

If the calculations are made for a single phase only with the additional assumption that the stray fluxes and other similar secondary effects do not impair the accuracy of the calculations, the magnetic flux density can be obtained from the same basic formula given above:

The selected values are compatible with the dimensions of the above-described exemplifying embodiment.

For a nominal current measurement range of 0.2 - 2.5 A:

Initial data:

Magnetic flux sensor used is Honeywell X91973-SS

I_n = motor nominal current = 2.5 A I_{n max} ⁼ motor maximum start cuπent = 7- 2.5 A = 17.5 A

Current conductor must be thermally rated to carry 17.5 A for approx. 25 s l_δ - magnetic circuit air gap width 7157

12

Bo_δ ⁼ specified measurement range of sensor measuring magnetic flux density in the air gap = ±0.1 T.

1. The ratio between the cuπent measurement basic ranges is selected as 10, whereby the second basic range will be 2 - 25 A.

2. As the ratio between the current measurement basic ranges is 10, the winding for the lower basic range must have 10 turns to attain the same flux density in both basic ranges.

The magnetic circuit air gap width is computed:

As the Hall-effect device has a sensitivity of 25 mN/mT with a measurement range of 100 mT, the maximum effective value of sensor output voltage is 2500 mV. Using these values, the sensitivity of the sensor can be computed:

Sens= ^2500mV=142.857 -143— at the range 0. 2A - 17.5A

17.5A A

Then, the nominal current range is 2 - 25 A.

Coπespondingly, the maximum cuπent of the higher cuπent range is 175 A and the sensitivity is:

Sens=^250QmV=14.2857_*14.3— at the range 2A - 175A 175A A 7157

13

The cuπent conductor must be thermally rated to carry 175 A for approx. 25 s.

Depending on the production tolerances of the sensors used to measure the magnetic flux density, the sensors must be calibrated to the desired sensitivity values by means of active or passive components on the electronics card. As the linearity as well as the sensitivity of the transducers according to tests entirely follows the characteristic curve of the sensor employed, the transducer can be used for extremely accurate measurements if the sensor nonlinearity is taken into account by the design of the transducer electronics card. Thus, the sensor nonlinearity can be compensated for by programming means, or alternatively, using a nonlinear amplifier.

In the embodiment described above, the transducers were calibrated so that their nominal sensitivities (as mN/A) coπesponded to the average sensitivity of transducers at the nominal cuπent.

The computational principles and formulas employed according to the invention are also valid for magnetic circuits with different dimensions. For the validity of the calculations, however, it is essential that the basic form of the magnetic circuit is maintained unchanged.

Instead of the Hall-effect device, the magnetic flux density can be measured using any other sensor responding to the magnetic flux density, such as a small coil.

Claims

2715714Claims:

1. A cuπent transducer based on the measurement of magnetic flux density, said transducer comprising

- a frame structure (5, 8),

- adapted to said a frame structure (5, 8), an essentially ring-shaped magnetic circuit (2) having the annual shape gapped to form an air gap (4) and concentrate the magnetic flux to the air gap (4), said ring- shaped magnetic circuit (2) incorporating an aperture (10) through which the current conductor (6, 7) to be measured can be routed,

- a magnetic flux sensor insertable in said air gap, and

- an electromagnetic shield structure (3) surrounding the frame structure (5, 8) to the end of preventing external interference fields from affecting the magnetic flux sensor,

c h a r a c t e r i z e d in that

- the electromagnetic shield structure (3) is made of a material essentially similar to that of the magnetic circuit (2), and

- both the magnetic circuit (2) and the body structure (5, 8) are so shaped as to be capable of accommodating either a straight current conductor (6) or a winding structure (7) as well with which cuπent measurement sensitivity can be multiplied. 15

2. A current transducer as defined in claim 1 using a Hall-effect device as the magnetic flux sensor, characterized in that the magnetic circuit (2) is shaped so as to comprise two opposing hammers having their lower ends joined together, whereby the air gap (4) of the magnetic circuit (2) is formed between the impact surfaces of the hammers.

3. A cuπent transducer as defined in claim l or2, characterized in that both the magnetic circuit (2) and the frame structure (5, 8) incorporate a notch (11) suited to accommodate the turns of a winding (7) wound about the magnetic circuit (2).

4. A cuπent transducer as defined in claim 1, characterized in that at least one of the hammer-like upper ends of the magnetic circuit (2) at the air gap (4) has a square cross section.

5. A current transducer as defined in claim 1, characterized in that the distance of the electromagnetic shield (3) from the upper surface of the magnetic circuit (2) at the magnetic circuit air gap (2) is approximately equal to the width of the air gap (4).

6. A current transducer as defined in claim 1, characterized in that the magnetic circuit (2) is made from laminations bonded together by riveting.

7. A method for calibrating a cuπent transducer based on magnetic flux measurement, characterized in that the nominal sensitivities (mV/A) of the current transducers are adjusted to the value of the average sensitivity of the transducers at the transducer nominal current.