abatement of electromagnetic noise of vaccum interrupters

7
IEEE Transactions on Power Apparatus and Systems, Vol. PAS-100, No. 4, April 1981 ABATEMENT OF ELECTROMAGNETIC NOISE OF VACUUM INTERRUPTERS YOSHIYUKI INNAMI, HltFUMI YANAGISAWA*, EIICHI FUJII and MOTONORI KANAZASHI, Senior Member IEEE Development Division, Meidensha Electric Mfg. Co., Ltd. Tokyo, Japan *Engineering Section VI Department, GEMVAC Co., Ltd. ABSTRACT - When a 3000A alternating current of commercial frequency (50 or 60Hz) flows to the central conductor in a vacuum interrupter (VI) a magnetostrictive noise of about 77dB(A) is generated on account of a magnetostrictive vibration of the metallic seal rings. From environmental considerations, the elimination of the VI noise is a pressing problem to be solved. In the newly developed low noise VI reported here, magnets have been bonded to the metallic seal rings. They produce a powerful magnetic field to saturate always the mag- netic flux density in the ring so as to reduce or almost suppress a mag- netostriction change. Repeating many experiments by this method, we have found an optimum condition and developed a 38dB(A) low noise vacuum interrupter at 3kA, 12kV. INTRODUCTION As a demand for electric power has increased at large cities in recent years, the electric power distribution- eqiipment has become compact and has been rationalized. With the development and intro- duction of large-capacity distri'bution equipment, the electromagnetic noise'has caused a serious problem. The noise of vacuum interrupt'er (VI) used in the breaking section of the vac'uum circuit breaker (VCB) is one of the above problems. As regards power transformers, the subject for noise reduction has been widely studied. In 1967, a large-scale joint research was carried out about transformer noises as joint studies of the electric power, iron and steel manufacturing, and electric machinery fields in England to search the relation between noises and various characteristics of mate- rials employed for transformer core [ 1] As regards VI, however, we don't know any papers published on this problem until now, it may be, because VI involves certain diffi- culties to be solved. As shown in Fig. 1, the VI consists of copper lead rods, electrodes, non-magnetic steel bellows, shields, flanges and glass or ceramic insu- lating tubs. The metallic seal rings for maintaing vacuum are used to connect glass with metallic parts. They are made of ferromagnetic material having a coefficient of thermal expansion equal to that of glass. Therefore, they are distorted on account of magnetostrictive phenomenon caused by alternating magnetic fields. It gives rise to a serious vibration noise when the current reaches or exceeds 3000A (RMS). In general, the internal residual strain in silicon steel sheets can be reduced by mechanical stress [21, surface treatment [3] and heat treatment [41, resulting in the reduction of the magnetostriction, and some papers have been published on these problems. Unlike these conventional methods, we employed a strong rare-earth cobalt magnet to increase the magnetic resistance of the seal rings forming the magne- tic circuit and eliminate effects of an alternating magnetic field caused by the flowing current as far as possible. Thus, we successfully reduced or eliminated vibration noises due to a magnetostrictive phenomenon. 80 SM 698-1 A paper recommended and approved by the Th;EE Switchgear Committee of the IEEE Power Engineer- ing Society for presentation at the IEEE PES Summer Meeting, Minneapolis, Minnesota, July 13-18, 1980. Manuscript submitted December 24, 1979; made available for printing May 12, 1980. Fig. 1 Sectional View of Vacuum Interrupter (VI). In this study, we measured applied magnetic fields, vibration accel- erations, maximum magnetic flux density changes and the relation between noises and frequency analysis, and then, compared the theory and the experimental results. BASIC STRUCTURE OF VI Fig. 1 indicates the basic structure of VI. The VI is composed of an insulated vessel made of hard glass and metal flanges at both ends, a fixed electrode mounted to one flange, a movable electrode mounted to the other flange via a metallic bellows and a non-magnetic metallic shield arranged around these electrodes inside the glass cylinder. The interior of the vacuum vessel is always kept in high vacuum. The shields protect the inner surface of the insulated cylinder from a contamination due to the metallic vapor produced by breaking a current and also improve the breaking performance by acting as an effective condensing and re-combination surface of charged substances. Each metallic seal ring, mounted between the glass and the non-mag- netic flange or shields is made of a sealing alloy with the coefficient of thermal expansion equal to that of the glass and it is an object of the discussion in the present paper. It is a ferromagnetic substance having a metallic composition of 29% Ni-17% Co-Fe (ASTM F15-68). Its thick- ness is about 0.7mm. According 'to F. Brailsford's calculation, the magnetic flux almost penetrates into the interior of an ordinary electric iron sheet of 0.8mm thick when the sheet is excited at AC 50Hz [5]. Both electrodes are making contact with each other mechanically under theic steady-state condition, and a large current of exceeding 3000A (1 2kV) flows through these electrodes. The lead rods and elec- trodes are slightly heated by Joule's heat and the heat is transferred to the seal ring. Therefore, the measuring temperature range in the present experiments was maintained to ±10C by cooling. In these conditions the measured magnetostriction X and the thermal expansion coefficient a are 8.5 x 10-6 (in the field corresponding to 3000A, 50Hz) and 4.8 x 10-6, respectively. Both quantities are almost equal to each other. This will not develop any adverse -effects because the measurement in the experiments is based on the change in a short period. PRINCIPLE AND EXPERIMENTAL METHOD (1) Principle We first presumed that the noises in question were generated by the magnetostrictive vibration of the metallic seal rings. When a magnetic 1981 IEEE 1911

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IEEE Transactions on Power Apparatus and Systems, Vol. PAS-100, No. 4, April 1981

ABATEMENT OF ELECTROMAGNETIC NOISE OF VACUUM INTERRUPTERS

YOSHIYUKI INNAMI, HltFUMI YANAGISAWA*, EIICHI FUJIIand MOTONORI KANAZASHI, Senior Member IEEE

Development Division, Meidensha Electric Mfg. Co., Ltd.Tokyo, Japan

*Engineering Section VI Department, GEMVAC Co., Ltd.

ABSTRACT - When a 3000A alternating current of commercialfrequency (50 or 60Hz) flows to the central conductor in a vacuum

interrupter (VI) a magnetostrictive noise of about 77dB(A) is generatedon account of a magnetostrictive vibration of the metallic seal rings.From environmental considerations, the elimination of the VI noise isa pressing problem to be solved. In the newly developed low noise VIreported here, magnets have been bonded to the metallic seal rings.They produce a powerful magnetic field to saturate always the mag-netic flux density in the ring so as to reduce or almost suppress a mag-netostriction change. Repeating many experiments by this method, we

have found an optimum condition and developed a 38dB(A) low noisevacuum interrupter at 3kA, 12kV.

INTRODUCTION

As a demand for electric power has increased at large cities inrecent years, the electric power distribution- eqiipment has becomecompact and has been rationalized. With the development and intro-duction of large-capacity distri'bution equipment, the electromagneticnoise'has caused a serious problem. The noise of vacuum interrupt'er(VI) used in the breaking section of the vac'uum circuit breaker (VCB)is one of the above problems.

As regards power transformers, the subject for noise reductionhas been widely studied. In 1967, a large-scale joint research was carriedout about transformer noises as joint studies of the electric power, iron

and steel manufacturing, and electric machinery fields in England tosearch the relation between noises and various characteristics of mate-rials employed for transformer core [ 1]

As regards VI, however, we don't know any papers published on

this problem until now, it may be, because VI involves certain diffi-culties to be solved.

As shown in Fig. 1, the VI consists of copper lead rods, electrodes,non-magnetic steel bellows, shields, flanges and glass or ceramic insu-lating tubs. The metallic seal rings for maintaing vacuum are used toconnect glass with metallic parts. They are made of ferromagneticmaterial having a coefficient of thermal expansion equal to that ofglass. Therefore, they are distorted on account of magnetostrictivephenomenon caused by alternating magnetic fields. It gives rise to a

serious vibration noise when the current reaches or exceeds 3000A(RMS).

In general, the internal residual strain in silicon steel sheets can bereduced by mechanical stress [21, surface treatment [3] and heattreatment [41, resulting in the reduction of the magnetostriction, andsome papers have been published on these problems. Unlike theseconventional methods, we employed a strong rare-earth cobalt magnetto increase the magnetic resistance of the seal rings forming the magne-tic circuit and eliminate effects of an alternating magnetic field causedby the flowing current as far as possible. Thus, we successfully reducedor eliminated vibration noises due to a magnetostrictive phenomenon.

80 SM 698-1 A paper recommended and approved by theTh;EE Switchgear Committee of the IEEE Power Engineer-ing Society for presentation at the IEEE PES SummerMeeting, Minneapolis, Minnesota, July 13-18, 1980.Manuscript submitted December 24, 1979; madeavailable for printing May 12, 1980.

Fig. 1 Sectional View of Vacuum Interrupter (VI).

In this study, we measured applied magnetic fields, vibration accel-erations, maximum magnetic flux density changes and the relationbetween noises and frequency analysis, and then, compared the theoryand the experimental results.

BASIC STRUCTURE OF VI

Fig. 1 indicates the basic structure of VI. The VI is composed of an

insulated vessel made of hard glass and metal flanges at both ends, a

fixed electrode mounted to one flange, a movable electrode mounted tothe other flange via a metallic bellows and a non-magnetic metallicshield arranged around these electrodes inside the glass cylinder. Theinterior of the vacuum vessel is always kept in high vacuum.

The shields protect the inner surface of the insulated cylinder froma contamination due to the metallic vapor produced by breaking a

current and also improve the breaking performance by acting as an

effective condensing and re-combination surface of charged substances.Each metallic seal ring, mounted between the glass and the non-mag-netic flange or shields is made of a sealing alloy with the coefficient ofthermal expansion equal to that of the glass and it is an object of thediscussion in the present paper. It is a ferromagnetic substance havinga metallic composition of 29% Ni-17% Co-Fe (ASTM F15-68). Its thick-ness is about 0.7mm. According 'to F. Brailsford's calculation, themagnetic flux almost penetrates into the interior of an ordinary electriciron sheet of 0.8mm thick when the sheet is excited at AC 50Hz [5].

Both electrodes are making contact with each other mechanicallyunder theic steady-state condition, and a large current of exceeding3000A (1 2kV) flows through these electrodes. The lead rods and elec-trodes are slightly heated by Joule's heat and the heat is transferred tothe seal ring. Therefore, the measuring temperature range in the presentexperiments was maintained to ±10C by cooling. In these conditionsthe measured magnetostriction X and the thermal expansion coefficienta are 8.5 x 10-6 (in the field corresponding to 3000A, 50Hz) and4.8 x 10-6, respectively. Both quantities are almost equal to each other.This will not develop any adverse -effects because the measurement inthe experiments is based on the change in a short period.

PRINCIPLE AND EXPERIMENTAL METHOD

(1) PrincipleWe first presumed that the noises in question were generated by the

magnetostrictive vibration of the metallic seal rings. When a magnetic

1981 IEEE

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IV. When a magnetic field of Hd was applied by magnet.

Fig. 2 BH Hysteresis Curves of Seal Rings Placed in AlternatingMagnetic Field (±Ha).

substance is magnetized in a magnetic field H, the change in length dueto magnetostriction occurs. Its strain can be expressed by the variationratio of length as X = IAQ/9 1.Magnetostriction X increases with increasing H until the magnetizationreaches the saturation.

Fig. 2 shows the change in the BH curve when the DC magneticfield is superposed on the alternating magnetic field in the ring. Whenthe current of commercial frequency flows through the electrodes, analternating magnetic field from - Ha to Ha as shown in BH curve ofFig.-2-1 is applied to the rings in the VI without magnet, and A8,appears as a maximum magnetic flux density change. However, when aconstant DC magnetic field Hb is always applied by mounting a magnet,the BH curve shifts as shown in Fig. 2-11. The maximum magnetic fluxdensity change reduces from AB, to AB2 at the same time. The appliedDC magnetic field increases through Fig. 2-III to Fig. 2-IV. The maxi-mum magnetic flux density change becomes AB4, and it is almrost zeroas shown in Fig. 2-IV. This means that the change of magnetostriction Xis greatly reduced,that is,the electromagnetic noise is almost eliminated.

The design of the closed magnetic circuit is indicated in Fig. 3. Themagnet for producing DC magnetic field is made of a strong samarium-cobalt magnets because of the limitation in size. Two, three and fourpairs of magnets were mounted on the metallic seal rings at equallydivided positions of the circle of the rings.

(2) Magnetization characteristicWe broken the VI glass first to detach the flange with seal ring only,

Fig. 5 Sectional View of Magnets and Search Coils Mounted on

Seal Ring.

and mounted a lead rod at the center of the flange circle. A polyvinylformal coated wire 0.?5mmo was threaded into the flange hole, andturned around the ring by 5 turns as B coil. On the other hand, a H coilwas mounted at the same position as that of the seal ring from the

central conductor. These search coils were connected to the self-made

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A, B and C. The A characteristic is such a one that is close to the sense

of human ears in a quiet environment, and generally employed as noisecharacteristic. The C characteristic shows an almost flat frequency char-acteristic. Therefore, the All Pass analysis was made by the A charac-teristic, while the frequency analysis was made by the C characteristic.These measuring values were shown by a sound pressure level (dB)value.

A precision condenser microphone which is not easily affected bythe magnetic inductance was employed as a noise sensor, and the centerof the neighbouring magnets on the outer side of the closed magneticcircuit of the seal ring was specified as the measuring position. A 1/3octave band-pass filter, automatic analysis drive unit, and level recorderwere employed as the frequency analyzer. The sound pressure level wasshown by compensating the background noise (A characteristic: 43dB)produced from the power supply and cable. Since it was difficult todetect the sound pressure level of improved VI at a measuring pointseparated from the sound source by lm, partial data were measured at0.1m point.

Fig. 6 Vibrating Acceleration Measurement.

Fig. 7 Measurement of Sound Pressure Level and FaequencyAnalysis of Noises.

AC magnetic transducers to measure the change of the BH curves of theseal ring on a CRT synchroscope.

These two transducers were separated from VI and power cable atleast 5m during measurements. The magnetic field at this site was lowerthan the detection limit of a gauss meter using hall element probe grad-uated with sensitivity of 0.4 G/Div. Therefore, the measurements in thisexperiment are considered to be not affected by the magnetic field.

The magnets were mounted, as shown in Fig. 5, in such a mannerthat the polarity of magnetic poles of neighbouring magnets wasopposite in each other. Accordingly, the magnetic flux direction in themetal rings is reversed at the mounting positions of magnets.

(3) Vibrating accelerationThe vibrating acceleration was observed by using an actual VI.

Measured values of the magnetostriction largely varies with mass ofsensor, in qeneral. Accordingly, an acceleration type vibration pick-upof a 0.4g weight and a preamplifier were employed. The measuring limitis 1 x 10-3ms2, and the measurable frequency range is from 0.2Hz to100kHz. In this experiments, frequencies higher than 30kHz were cutoff.

The magnetic sensitivity of a piezoelectric accelerometer is verylow, and an effect of the applied magnetic field in the direction of themost sensitive axis is the order of 1 - 30 ms-2/T. Thus, the effect of themagnetic field on this meter is considered to be negligible. The outoutvoltage of the amplifier was measured by a digital voltmeter, and itseffective value was converted into a peak value. The acceleration typevibration pick-up was mounted not in the magnetostrictive direction,but perpendicular to the ring face. It is because this direction is that ofthe transmission of the acoustic wave of noise.

(4) Measurement of sound pressure level and frequency analysis ofnoises.The sound pressure level was also measured by using actual VI. The

noise meter presents results of three kinds of frequency characteristics

RESULTS AND DISCUSSION

(1) Magnetostrictive noiseIn order to confirm that the magnetostriction of the seal ring is a

noise source, a current of commercial frequency (50Hz) was applied tothe VI without magnet and the maximum magnetic flux density change,sound pressure level of noise at 0.Um apart from VI, and vibrating accel-eration of the ring were measured, Fig. 8 shows the results. From thesefigures, we can clearly understand that the magnetic field at the sealring increased as the current increased, and the vibrating accelerationrised with increased the maximum magnetic flux density change, caus-ing the sound pressure level of noise to increase, as a result.

When the current was higher than 2000A, the rise of the noisebecame alleviation, and this may be considered to be caused by sucha phenomenon that the magnetic flux density in the seal rings ap-proaches the saturation point as observed in the BH hysteresis curve asshown in Fig. 8. This noise is not only generated from the alternatingly-magnetized ring but mainly generated from glass, and shield of the VIstructure by the vibromotive force which is transmitted from the rings.This was confirmed by the fact that the noise from single ring was verylow 48 dB(A) compared with that from whole VI structure. However,since the frequency spectrum of the noise from VI, the vibration accel-eration of the ring in the VI and the magnetostriction of the single ringare the same, it is concluded that the noise is generated by the mag-netostrictive vibration.

(2) Magnetic characteristicAs the number of the pairs of magnets increased, the maximum

magnetic flux density change in the outer side of the closed magneticcircuit decreased as shown in Fig. 9. Since this value was measured bythe AC magnetic transducer, the DC components were not taken intoaccount apparently. Therefore, it is considered that the BH curves areshifted toward the first quadrant [6]. The magnetic flux is almost

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Fig. 10 Relation between the Number of Pairs of Magnets and theMaximum Magnetic Flux Density Change. (Current =

3000A, f = 50Hz)

11. Outside the closed magnetic cir- V. Outside the closed magnetic cir-

cuit with a pair of magnets cuit with four pairs of magnets

mounted. mounted.

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m. Outside the closed magnetic cir- VI. Inside the closed magnetic fieldcuit with two pairs of magnets with a pair of magnets mounted.mounted.

Fig. 9 Example of Measurements of AC B-H Curve When a DCMagnetic Field was Applied. (Current = 2000A, f = 50Hz,Y = 3520 G/div, X = 37 Oe/div)

saturated inside the closed magnetic circuit, and the maximum magnet-ic flux density change is very little. Accordingly, the change in lengthdue to magnetostriction scarcely occurs in this portion of the seal ring.

Fig. 10 shows the maximum magnetic flux density change causedby the alternating magnetic field inside and outside the closed magneticcircuit of the seal ring, when the number of magnets is increased. Forthe first time, we considered that the maximum magnetic flux densitychange shows a little variation at respective positions of the seal ring,because the magnetic permeability (p) of the ring is very high as com-

pared with that of air. However, it varies considerably, as illustrated.When the number of the pairs of magnets was increased, the maximummagnetic flux density change is reduced no more inside the closed mag-

netic circuit, while the magnetic flux density change outside the circuitvaries more or less. This may be considered to be caused by the residualinternal magnetic flux flowing into the external side.

In addition, the reason why the maximum magnetic tlux censitychange inside the closed magnetic circuit is not reduced from a certainvalue, even if the magnets are increased, may be considered that actualleakage factor is larger than the presumed value. As a matter of course,it was confirmed that the peak-to-peak value of the magnetic fieldintensity at the seal ring positions remains unchanged, irrespective ofthe number of magnets, if the applied current (AC) is kept constant.

Number of pairs of magnets or iron pieces

Fig. 11 Relation between the Number of Pairs of Magnets or IronPieces and Vibrating Acceleration.

From the above facts, it was also confirmed that the average valueof maximum magnetic flux density change of the seal ring decreases,as the number of mounting magnets increases.

(3) Vibrating accelerationThe acceleration type vibration pick-up was mounted on the outer

surface of the seal ring at the center of the neighbouring magnets out-side the closed magnetic circuit. Fig. 11 shows measuring results. Undera current of 3000A for example, the acceleration became very smallwith two pairs of magnets corresponding to the decrease of maximummagnetic flux density change shown in Fig. 10. However, this resultmay be considered that the vibrating acceleration induced by the mag-netostriction was reduced not only by a magnetic effect but also bya mechanical stress effect to a certain extent. This assumption was

examined by bonding iron pieces having the same size as that of themagnets to the yoke and the seal rings by means of a bonding agent.This result is shown by a dotted line in Fig. 11. The vibrating accelera-tion was reduced by increasing the number of pairs of iron pieces at allapplied currents. However, this effect was proved to be very small, as

compared with the magnetic effect.

(4) Sound pressure level of noiseThe effect of reducing the sound pressure level by mounting mag-

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Fig. 12 Effect of Magnets and Iron Pieces to Sound PressureLevel. (Current = 3000A, f = 50Hz)

nets is shown in Fgi. 12. This figure indicates the results when theapplied current is 3000A. If the current is 1000A and 2000A, thenoise level was equal to that of the background noise, and could not bemeasured when more than 2 or 3 pairs of magnets were mounted, re-

40

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20 50 100 200 500 1000 2000 5000 10000

Frequency (Hz)

Fig. 13-I Frequency Analysis of the Background Noise. (Current= 3000A, f = 50Hz, Distance = 10cm)

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Frequency (Hz)

Fig. 13-11 Frequency Analysis of the Non-treated VI. (Current=3000A, f = 50Hz, Distance = 1 Ocm)

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spectively. A sound pressure level of 77dB (A) measured by applying3000A (50Hz) at O.1m apart from VI was reduced to 38dB (A) bymounting four pairs of magnets. In this case the maximum magneticflux density change was reduced by about 23000G and 8000G, insideand outside the closed magnetic circuit, respectively.

The dotted line indicates measured values when iron pieces wereemployed instead of magnets in the same manner as in the previousparagraph. It was confirmed that the noise level was slightly reduced inthe same way as in the case of the vibrating acceleration level.

(5) Frequency analysis of noiseFig. 13-I indicates the frequency spectrum of the noise from cable

and transformer when the cable was shorted at;the position of VI and3000A (50Hz) was applied, as the background noise in this experi-ment. A high sound pressure level is shown at about 50Hz, and its high-er harmonics are also confirmed. Fig. 13-11 shows the measured resultsof the without magnet VI. This figure shows a very high sound pressurelevel at 500-3000Hz and 5000-9000Hz, as compared with the back-ground noise. This result is very serious because the hearing characteris-tics of the human ear is most sensitive at the range of 1000-4000Hz.

However, it was revealed from the result of Fig. 13-E obtained byusing iron pieces, that the entire sound pressure level is reduced byabout 1OdB(C). Fig. 13-IV shows, on the other hand, that this value isgreatly reduced when mounting magnets and it becomes almost equalto the sound pressure level of the background noise spectrum shown inFig. 13-I. Thus, the noise with the spectrum higher than 500Hz in Fig.13-11 is a noise from VI, and this noise could be attenuated almost com-pletely by nearly saturating the magnetic flux density in the metallicseal rings by mounting magnets.

(6) Electrical characteristicThe current breaking test and withstand voltage test were con-

ducfed to examine the influence of the leakage magnetic flux to these

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Fig. 13-11 Frequency Analysis of the VI with Four Pairs of IronPieces. (Current = 3000A, f = 50Hz, Distance = 10cm)

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2020 50 100 200 500 1000 2000 5000 10000Frequency (Hz)

Fig. 13-IV Frequency Analysis of the VI with Four Pairs of Mag-nets. (Current = 3000A, f = 50Hz, Distance = 10cm)

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performances. These test results revealed that no unfavorable influenceappeared even when four pairs of magnets are mounted. This may beconsidered to be caused by such a fact that the magnetic flux (about1430G) of the electrode section, produced by the rated breakingcurrent (36kA), for example, is very large as compared with the leakagemagnetic flux of less than 1G.

CONCLUSION

In order to reduce the noise of vacuum interrupter (VI), we effec-tively applied the saturation effect of the magnetic flux density in VI'sindispensable ferromagnetic part: metallic seal ring. The large magneticfield was applied by magnets mounted on the ring. By this method thenoise level was successfully reduced from 77dB(A) to 38dB(A). Theresults of experiments made to confirm this effect are as follows:(1) From the measurement of the maximum magnetic flux densitychange in the ring, its vibrating acceleration and sound pressure level ofnoises, the noise of VI was proved to be caused by the magnetostrictivevibration of the metallic seal ring.(2) From the result of frequency analysis of noises, it was revealed thatthe noise includes high harmonic components (500-10000Hz).(3) When the pairs of magnets were mounted on the ring and the num-ber of the pairs was increased, the magnetic flux in the ring approachedthe saturation. This resulted in the decrease of the average value ofmaximum magnetic flux density change in the ring caused by alternat-ing magnetic field, accordingly the decrease of the vibrating accelera-tion of the ring and the sound pressure level of the noise.(4) The leakage magnetic flux from magnets caused any unfavorablyeffects on neither current breaking ability nor withstand voltage of VI.(5) It was confirmed that the sound pressure level of noise, or, higherharmonic components can be also reduced by a mechanical stress to acertain extent.

ACKNOWLEDGEMENT

The author wishes to express his heartfelt thanks to Dr. MasaakiImamura, assistance of Engineering Faculty of Gifu University andMr. lkuo Hirano, assistant to manager, Engineering Department ofTransformer Factor of Meidensha for their valuable discussions on thispaper, and Mr. Toshimasa Awano, Development Division of Meidenshafor his kind cooperation in the experiments.

REFERENCES

[1] IEE Conference Publication No. 33, Magnetic Materials and TheirApplications.

[2] lEE Conf. Publ. 33, P. 962 (1967).[3] COLIN HOLT and J.A. ROBEY, "The AC Magnetostriction of

3.25-Percent Grain-Oriented Silicon-Iron under Combined Longi-tudinal and Normal Compressive Stress," IEEE Trans. MAG-5, No.3, September, 1969, p384-390.

[4] W.R. GEORGE, C. HOLT, and J.E. THOMPSON, "Magnetostric-tion in Grain-Oriented Silicon-Iron," Proc. IEE, Pt. A, 109-43,1962, P. 101-108.

[5] F. Brailsford, Magnetic Materials: 1951 P. 16.[6] Y. Sakaki, "Experimental Study on the Flux Control Character-

istic of Tape-Wound Cores under a DC Diametric Magnetic Field,"IEEJ, vol. 99-c, No. 10, Oct. 1979, p. 243-247.

Yoshiyuki Innami was born in Tochigi Pre-fecture, Japan, on November 8, 1947. Hegraduated from the Nihon University in1970. From 1970 to 1972 he was assistanceof the Science Faculty of Nihon University.In 1972, he joined the Research Laboratoryof Meidensha Electric Mfg. Co., Ltd., wherehe have been engaged in researches on RareEarth Cobalt magnets, he is now the assist-ant to manager of Material Laboratory. Mr.Innami is now the member of the Institute

of Electrical Engineers of Japan.

Hifumi Yanagisawa was born in Nagano, Ja-pan on May 27, 1937. He graduated fromthe Kyoto University in 1962. In 1962 hejoined the Research Laboratory of Meiden-sha Electric Mfg. Co., Ltd., Tokyo, where hehad been engaged in research on Magneto-hydro dynamics, Nuclear, etc. In 1970 hechanged his employment to GEMVAC Co.,Ltd., Tokyo, where he is engaged in researchand development on Vacuum Interrupter, heis now the Manager of Engineering Section.Mr. Yanagisawa is now the member of the

Institute of Electrical Engineers of Japan.

Eiichi Fujii was born in Kobe, Japan in May28, 1950. He graduated from the HimejiInstitute of Technology in 1973. In 1973, hejoined the development division of Meiden-sha, Electric Mfg. Co., Ltd., where he havebeen engaged in researches on Rare EarthCobalt magnets.

Motonori Kanazashi (SM'73) was born inTokyo, Japan on February 5, 1927. He re-ceived the B.Sc. and the Ph.D. degrees inchemistry from the University of Tokyo,Tokyo, Japan, in 1948 and 1962, respective-ly.

From 1948 to 1973, he had been em-ployed at the Electrotechnical Laboratory ofthe Ministry of International Trade and In-dustry, Tokyo, where he had been engagedin research in synthesis of silicones and de-terioration of insulating materials by heat,discharge, etc. In 1973, he changed his em-

ployment to Meidensha Electric Mfg. Co., Ltd., Tokyo, where he is nowthe Manager of Insulation Department.

Dr. Kanazashi is a member of the Institute of Electrical Engineersof Japan, the Chemical Society of Japan, and the American ChemicalSociety. In 1980, he has been chairman, The Tokyo Chapter of IEEESociety on Electrical Insulation.

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DiscussionD. R. Kurtz, (General Electric Co., Philadelphia, PA): The authorshave contributed an interesting analysis of electromagnetic noise andhave applied it in an approach to reducing noise in their vacuum inter-rupter. We agree with the authors that noise is an important considera-tion in the design of vacuum interrupters as it is with any alternatingcurrent device, and we have made similar studies. Our analysis is ingeneral agreement with the authors; however, we find that quietvacuum interrupters can be designed for 3000 amperes continuous cur-rent without the use of magnets bonded to the sealing metal or auxiliarydevices. Figure I shows such an interrupter. The authors indicate thatnoise is generated in their vacuum interrupter by the glass and the metalshields because of transmission of the magnetostriction vibration of theNiCoFe sealing ring to these parts. Have the authors made any exper-iments to reduce this transmission or to change the natural frequency ofthe parts generating noise? Have any experiments been made with asealing ring thickness greater than 0.7 mm?

Figure I Vacuum Interrupter for 3000 ampere continuous current

Manuscript received November 6, 1980.

Y. Innami, H. Yanagihara, E. Fujil, and M. Kanazashi The authors ex-press their appreciation to Mr. D. R. Kurtz for his interest in the paperand valuable discussions thereon. As suggested by the discussor, thereare some ways to reduce electromagnetic noise of vacuum interrupters.They are: to reduce the transmission of the magnetostriction vibrationof the seal ring to glass envelopes and the metal shields, to change the

natural vibration frequency of the parts, and to use seal rings withdecreased width and increased thickness.

Figure 1. Vacuum Interrupter for 3000 A continuous current

Figure 1 shows one of our vacuum interrupter. Comparing this withdiscussor's Figure 1, apparent difference of width of seal ring is found.This difference is considered to be caused by the difference in the form-ing method of glass envelope. Discussor's is supposed to be made bycasting, then to have a thicker wall. Ours is hand-made and has a thin-ner wall. We understand that thick glass envelope makes it possible touse narrower and thicker seal ring and result in a large reduction of thenoise level. Our vacuum interrupters have already an experience of 13years and number of 150,000. Our method described in the paper has amerit that it is applicable to small numbers of special purpose vacuuminterrupters without any change in the materials and manufacturingsystems.We have made no experiment to reduce the transmission of the

magnetostriction vibration. As found in the paper, only a single sealring causes the noise of 48 dB (A). Therefore, this reduction of thetransmission is considered to be not so effective to the noise reduction.We have made also no experiment concerning the noise level of thevacuum interrupter with a seal ring thickness greater than 0.7 mm. Wequite agree with the effects of thicker seal ring as described above. Ourexperience concerning the guarantee of the vacuum grade in theenvelope, however, suggest that the thickness of seal ring has direct in-fluence to the residual stress in the glass envelope.

Manuscript received September 29, 1980.