REALIZATION OF A VARIABLE MAGNETIC FIELD SOURCE WITH PERMANENT MAGNETS

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REALIZATION OF A VARIABLE MAGNETIC FIELD SOURCE WITH PERMANENT MAGNETS

He Zhenying, Wang Yi, He Lilu

To cite this version:

He Zhenying, Wang Yi, He Lilu. REALIZATION OF A VARIABLE MAGNETIC FIELD SOURCE WITH PERMANENT MAGNETS. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-329-C1-332.

�10.1051/jphyscol:1984167�. �jpa-00223723�

R E A L I Z A T I O N OF A V A R I A B L E MAGNETIC F I E L D SOURCE W I T H PERMANENT MAGNETS

He Z h e n y i n g , Wang Y i and He L i l u *

Kweilin I n s t i t u t e of Electrical AppZiances, Ministry o f Machine Building Industry, P.R. o f China

* ~ e i jing I n s t i t u t e of Automation for the Machine Bui Zding Industry, Ministry of Machine Building Industry, P.R. of China

Re'sum6

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Abstract

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fineness of adjustment both better than 10-4, can be obtained in the air-gap.

For quite a long time already, permanent-magnets have been used as sources of constant or periodically variable magnetic-fields, and the technique is quite well-known. They are widely used in the instrument, electrical and electronic industries. With the development of per- manent-magnet materials, the use of permanent-magnets as sources of magnetic-field has grown from small devices to large-size equipments, replacing in many cases electromagnets. But electromagnets are still widely used for the production of continuously adjustable magnetic-

fields

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We have used better quality Alnico permanent-magnets in the realiza- tion of a continuously adjustable magnetic-field source, to replace electromagnets as standard magnetic-field in measurements. This re- placement offers the following advantages : (1) the saving of a highly stzbilized power supply; (2) saving in energy consumption; (3) saving in copper; (4) compactness and light weight; and (5) easiness of main- tenance. In the following sections we are going to present the con- struction, principle of operation and performances of this device.

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We have built several different devices of this type, for use in pre- cision magnetic measurements and NMR spectrometry, Since all these instruments require magnetic-field with high stability and homogeneity hence the structure of their magnetic-circuits is identical, and of the screened type "cylinder" symmetrical structure as shown schemati- cally in Fig. 1. The principal difference compared to conventional permanent-magnets is the existence of a winding around each permanent magnet as well as the adjunction of adjustable magnetic shunts on both sides of the pole-shoes, when the adjustment range is small, the mag- netic shunts can be fitted directly over the pole-shoes.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984167

C1-330 JOURNAL DE _PHYSIQUE

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Fig. 1

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In a magnetic circuit with permanent-magnet, the magnetic induction provided in the air-gap by the permanent-magnet depends upon the fol- lowing two factors : Firstly, the magnetic indcction of the permanent- magnet Bm, which provides the magnetic circuit with a total flux of

@m = Bm-Sm, where Sm is the cross-section of the permanent-magnet.

Secondly, the ratio of the total flux @m to the magnetic flux through the air-gap @g, i.e. K = h/gg,(K is called the factor of flux-leakage ) the rest of the flux leaking through other paths.

In magnetic oircuits with permanent-magnet, the practice is generally to magnetize the permanent-magnet up to saturation, so that the mag- netic circuit is functioning along the curve of demagnetization, and its magnetic induction is at the height of the intersection between the air-gap line and the curve of demagnetization (cf. Fig. 2). The slope of the a'.r-gap line of the magnetic circuit is given by :

tga =

Lm

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Sm Lm _Sm ( ~ g

+

+

where : Lrn is the length of the permanent-magnet,

Go is the total permeance of the magnetic circuit, Gg is the permeance of the air-gap,

Ge is the leakage permeance of the pole-face, and

G ' m is the permpance of the permanent-magnet Gm transfered to the pole tip.

Fig. 3 is a schematic of the various permeances.

Fig. 2

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Since : K = G O / G ~ ,

hence we have : Bg = @g/Sg = Brn*~m/~g.K.

From the above two formulae, it can be seen that a and K are func- tions of the structure. After the choice of the permanent-magnet, the

There are two ways of varying the air-gap field-strength through structural variations : one is by varying the width of the air-gap;

when the air-gap varies,Gg and Ge vary, so that Bg is varied. But varying the air-gap can be detrimental to the field homogeneity. When the air-gap length lg is larger than 0.4 D (D is the diameter of the pole-face), there is no more useful homogenous region in the air-gap magnetic-field. Hence this practice is undesirable in standard mag- netic-field. The second way is to change the position of the magnetic s h u ~ t , this varies mainly Ge. Since the magnetic shunting should not be too important, the range of adjustment of Ge is rather limited.

Through mechanical adjustment however, the rate of adjustment can be very fine, this is indeed a good means of fine adjustment.

For large amplitude adjustments, one can only rely upon changing Bm by varying the external magnetic-field. This is the reason why we have fitted a field winding around the permanent-magnet. Apart from being used to magnetize the permanent-magnet (to saturation), this coil is also used to demagnetize it to a certain extend. When a cur- rent Ia, whose direction is opposite to that of the magnetizing cur- rent, is passed through that coil, a demagnetizing field Ha is pro- duced. The magnetic induction of the permanent-magnet will descend along its curve of demagnetization to point Ba. After the shutoff of this current, the magnetic induction will rise along the curve of re- c o i l u to its intersection point with the air-gap line (i.e. B'm cf.

Fig. 27 The induction in the air-gap will then be : Bg = B'm

srn/sg

Hence by applying an inverse current of a certain value according to need, one can obtain an appropriate value of the magnetic-field. By this means, it is not only possible to adjust the ma,gnetic-field in a wide range, but even inverse it. It is however very difficult for the field thus obtained to beaccurate in value according to the require- ment. So, in order to obtain an accurate value of the magnetic-field,

one must use this means in conjunction with a magnetic shunt. Hence we have fitted a pair of magnetic shunts to our device.

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For the magnetization of the magnet, an impulse power supply is used as a matter of course, since the magnetization winding can then be made small, and a saving in energyconsumptioncan be achieved. For de- magnetization, we have tried d.c.

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Fig. 4

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C1-332 JOURNAL DE PHYSIQUE

In order to achieve simplicity and compactness, our power supply used a thyristor and adopted a random triggering technique. By controlling its triggering (switching-on),weuse half a period of the 50 Hz a.c.

current as discharge current. Two types of triggering circuit are shown in Fig. 4 and Fig. 5 respectively.

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Magnetic-field sources of this rating has already been widely used in NMR spectrometers of under 90 MHz. Since the building of permanent- magnets with a total magnetic induction of under 2 T and a field homo- geneity in the order of 10-5 to 1 0 " ~ has already been mastered by a few major manufacturers of NMR spectrom~-ters . For precision magnetic measurements, a field homogeneity and adjustability both better than

10-4 are enough. What one is more concerned about, is the field stabi- lity in time after demagnetization. We have done some short time ob- servations in accordance with our specific uses of the magnetic-field source, i.e. observation of field variation within 45 minutes after demagnetization. Because 45minutesis long enough to proceed with any single experiment. The results are as follows.

When the magnet is magnetized up to saturation, the field is unstable, it can demagnetize by about 0.07% within 15 minutes. If we demagnetize the permanent-magnet for a value of between 3% to 85%, then the field can be stabilize within about 0.005% for a duration of 45 minutes. The data of one series of tests are given below :

Relative variation of the air-gap induction after demagnetization Demagnetization (%)

Variation after 15 min. ($) Variation after 45 min. ($1

The temperature coefficient of this magnetic-field source is about -0.018%/O~,so that by simply placing the magnetic system in a thermo- static enclosure with a temperature control accuracy of 2 0 , l OC, we can then realize an adjustable precision magnetic-field source with homo- geneity, stability and fineness of adjustment all better than 10-4.

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From the above data it can be seen that with a permanent- magnet sys- tem, a relatively simple impulse power source and an NMR magnetometer, one can constitute a standard magnetic-field generator with adjustable field-strength, for precision magnetic measurements, and thereby re- place the commonly used electromagnet.

0 -0.031 -0.068 Demagnetization ($1

Variation after 15 min. (%) Variation after 45 min. ( 5 )

2.9 -0.002 -0.005 20.2

-0.002 -0.003

3.9 -0.001 -0.003 26.5

-0.003 -0.004

5.5 -0.007 -0.006 35.3

-0.005 -0.005

8.8 -0.001 -0.004 51 .O

+0.002 +0.002

68.6 +3.001 +0.002