Magnetic potentiometer as an aid in testing and analyzing magnetic devicesD. McDonald and E. Steingroever Citation: Journal of Applied Physics 49, 1791 (1978); doi: 10.1063/1.324868 View online: http://dx.doi.org/10.1063/1.324868 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/49/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A versatile magnetic refrigeration test device Rev. Sci. Instrum. 79, 093906 (2008); 10.1063/1.2981692 The Potentiometer Phys. Teach. 43, 232 (2005); 10.1119/1.1888084 Development of a magnetic CAM for the computer aided tap test system AIP Conf. Proc. 557, 1966 (2001); 10.1063/1.1373993 Magnetic attachment device for insertion and removal of hearing aid J. Acoust. Soc. Am. 85, 525 (1989); 10.1121/1.397640 Potentiometers Rev. Sci. Instrum. 17, 356 (1946); 10.1063/1.1770397

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MAGNETIC POTENTIOMETER AS ru~ AID IN TESTING AND ru~ALYZING MAGNETIC DEVICES

D. McDonald O.S. Walker Co.,Inc.,Worcester,Mass. 01606

and E. Steingroever

Magnet-Physik, Koln, Germany

ABSTRACT

The steel portions of a properly designed, constant romf magnetic device should be operated at the highest values of flux density consistent with the requirement that these steel portions consume a minimum of available romf. The customary procedure to determine if the above criterion has been met is an indirect one. This procedure is to measure the flux density B in the several steel branches of the magnetic circuit and then calculate from B-H curves of this steel the romf required to drive these levels of B through these branches. Although techniques for directly measuring the romf consumed by any branch of a magnetic circuit have been available for many years, these romf's are usually obtained by indirect procedure noted above. Both the instrumentation and the techniques for obtaining the desired romf values by direct measurement are described.

RESULTS

Means for measuring the magnetic potential of constant romf magnetic systems have been available since the late 1800's. However, the magnetic potentiometer has been given very little attention even though a magnetic potentiometer can very readily provide data useful in evaluating the performance of constant romf magnetic circuits. In many instances, such data may be consider­ably more difficult to obtain using conven­tional flux loop techniques and the conclusions derived from flux loop data also may be less accurate.

Dr. E. Steingroever has recently perfected a modified version of the original magnetic potentiometer invented by A.P. Chattock Lll in 1887. It has been employed quite effectively in Europe, and at O.S. Walker Co., Inc. we have started to use this magnetic potentiometer to make a number of important measurements. We have found it to yield information concerning the performance of magnetic devices and materials that was previously either not available or very difficult to obtain.

- A Steingroever version of a magnetic potentiometer is schematically represented in Figure 1. The flux turns linkages of the potentiometer are shown in this figure. The magnetic potentiometer measures the magnetic potential difference between points A and B of the potentiometer independent of how the magnetic field varies betweens these two ends of the potentiometer.

Appendix A explains how the magnetic potentiometer is used to measure the magnetic potential of a magnetic device. The differ­ence in the quantities measured by (1) the magnetic potentiometer and (2) a flux loop or a Hall probe is also covered. A simple bar

magnet is used as an illustration for these

two different measurements and for the technique employed to use the magnetic potentiometer. Calibration techniques are also discussed.

In the design of any magnetic device onE key objective is to make the most effective use of the available magnetomotive force, rrmf, and the iron in the magnetic circuit. A criterion of this effectiveness is how large a percentage of the available romf is developed across the working gap and the load. The magnetic potentiometer can be a powerful tool in measuring this percentage.

Magnetic leakage and magnetic constrict-­ions in the iron of the magnetic circuit are two major factors that influence the percentage of the available romf that is across the working gap and load. Magnetic leakage may become the preponderant factor in high romf devices like lifting magnets. The following examples illustrate how the magnetic potentiometer aids in establishing this percentage.

Figure 2 shows the magnetic circuit of a moving coil meter and some of the magnetic potential readings taken with the magnetic potentiometer at several locations in the magnetic circuit. The large potential difference between location (a) and location (b) as compared to the potential difference between (b) and (d) indicates that the bolt hole between (a) and (b) reduces the magnetic cross section greatly. In addition, the potential difference between the tips of the pole face (c) or (e) and the center of the pole face (d) indicates that the iron paths leading to these tips are too narrow.

Figure 3 shows a sectional view of the magnetic circuit of a loudspeaker along with magnetic potentials at several locations. The difference in potential at positions (b) and (c) reduces the potential that can be developed across the working gap. If the diameter of the central portion of the magnetic circuit is increased as shown by the broken lines in the sectional view, the potential loss between (b) and (c) can be reduced and a larger potential difference developed across (a) and (b).

The magnetic potential measurements made on the meter and loudspeaker were made very easily and rapidly. Identifying the detrimental potential drops from these measurements is no task at all.

However, if flux loops were used in Figures 2 and 3 instead of a magnetic potentiometer, it would be much more difficult to calculate the detrimental potential drops. The word calculate is used because with flux loop measurements fluxes are the quantities measured and the desired quantity, potential drops, must be calculated by reference to B-H curveS for the iron used. Assumptions as to the actual distribution of B must also be made in order to calculate B from the flux measurements. For example, measurements made to establish the potential drops at (a) - (b) - and (c) - (d) of Figure 2 require some flux mapping to properly identify B beoause of the varying iron cross sections between these points. Attempts to measure the potential drops with a Hall probe would yeild completely erroneous results as described in Appendix A.

1791 J. Appl. Phys. 49(3). March 1978 0021-8979/78/4903-1791$01.10 © 1978 American Institute of Physics 1791

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The two previous applications of the magnetic potentiometer were ones where magnetic leakage wasn't as important as iron bottlenecks. Figure 4 is a sectional view of a bipolar lifting magnet which is a high romf electromagnet where the leakage flu>: flowins through the air influences the ITagnetic design as much as the flux flowin9 through the load. The several flux paths (broken lines) as ~lell as the resulting magnetic potentials are sho'i'm in this figure. The magnetic potentials are shown in ampere turns rather than oersted centimeters to make it easier to compare them to the source rrmf which is always described in terms of ampere turns for electromagnets.

With a low magnetic density load like a bundle of tubing, one ~'ould expect tl".e iron core to be operating at a relath'ely 10 .. : flux density. HO~lever, the potential difference (a) - (b) across the core is significantly less than the available coil romf. This loss of potential indicates that the core is operatin,?, at a relatively high flux density that requires considerable rr®f to conduct the flux generated.

The loss of rrmf across the core reduces the nwf available across the load (c) - (d). With a low density load like a bundle of tubing, a great deal of romf is required to obtain high values of B in the load. Thus, the lifting capacity of this magnet can be significantly increased by increasing the core cross section which will lower the flux density in the core and reduce the romf loss across the core.

The above three illustrations shm·/ the relative ease by which very useful performance data can be obtained for maqnetic circuits with the magnetic potentiometer. Such data can be used to maximize the performance of the, magnetic circuit.

The magnetic potentiometer can also be used very effectively to measure the romf developed across a sample in a hyster­esisgraph. In many cases this measurement is more accurate than that obtained using either a Hall probe or a flux loop placed in the air adjacent to the sample.

APPENDIX

To obtain data with a magnetic potentiometer its output leads must be connected to an integrating type fluxmeter. The fluxmeter will provide an output proportional to the line integral of H taken between End A and End B of the potentiometer of Figure 5 when (1) the potentiometer is moved from position #1 to position #2 or (2) the potentiometer remains at pOSition #1 and the bar magnet is moved to position #2. Position #2 is far enough away from the bar magnet that is magnetic field is negligible at position #2. (If the bar magnet were replaced with an electromagnet, neither the potentiometer nor the magnet would have to be moved; instead the excitation of the electromagnet would be interrupted. )

The line integral noted above is by definition proportional to the romf difference of the magnetic field between End A and ~nd B of the potentiometer. To be the most useful, this romf difference should be

1792 J. Appl. Phys., Vol. 49, No.3, March 1978

between the field at the position of interest on the magnet and free space. This can be achieved if (1) the length of the magnetic potentiometer is great enough that End B in Figure 5 is essentially outside the magmetic field of the magnet under test and (2) there are no other masnetic sources or their field in this region.

A flux loop or Hall probe alone cannot measure the I1"'agnets' potential. If a loop or probe .. Jas placed next to End A of the potentiometer of Figure 5 it would only measure the value of H at the end of the bar magnet. To obtain the potential or romf of the bar magnet above its surroundings, all the different values of H starting at the end of the bar magnet and traveling beyond the magnets' field must be measured and summed. The single flux loop or Hall probe placed at the end of the bar magnet doesn't do this. However, the rragnetic potentiometer does this because it is composed of a continuous series of flux loops covering the distance from the end of the magnet to essentially the limit of the magnets' field.

The potentiometer can be calibrated very easily using a solenoid at least 1 1/2 times as long as the potentiometer.

REFERENCES

1. A.P. Chattock, Phil. Mag. 24, 94, 1887

A X ---.- B

@ ICC:X: X X ::c :xr-rx::rr T'

Flux Turn Linkages= na u

1\ H dx

x

" :-; H dx i' x

romf between the A End and the B End of potentiometer

n = turns/cm of potentiometer

a = cross sectional area of potenti­ometer

Fig. 1. Flux turn linkages of magnetic potentiometer

Magnetism & Magnetic Materials-1977 1792

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(d) 6()()

(b) 6l.o

(a. )

PERM;>..NENT M"~ET

Fig. 2. The magnetic circuit of a moving coil meter. mmf's given in gilberts.

PERM~NENT MAG-NET

Fig. 3. Sectional view of loudspeaker magnetic circuit. mmf's given in gilberts.

1793 J. Appl. Phys., Vol. 49, No.3, March 1978

-- -" /'

( \ I -----

1 - -/' " \ \ ,Z COIL 20,000 '" / / \ ( \ / \ I AMPERE TURNS , / / '" \ I

/ / / '" '" \ '-.... -...... "" "- / <.//' /' '- ~~~

--­/' ~300 (C) (

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Fig. 4. magnet.

Sectional view of bipolar lifting mmf's given in ampere turns

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POSITION */

Fig. 5. Potentiometer being used to measure the potential of a bar magnet.

Magnetism & Magnetic Materials-1977 1793

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