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198 IEEE TRANSACTIONS ON COMPONENTS, HYBRIDS, AND MANUFACTURING TECHNOLOGY, VOL. 15 , NO. 2, APRIL 1992

Switching Performances of Hollow Contact Rivets Peter Zunko, Member, ZEEE, and Martin Bizjak

Abstract-A possibility of reducing contact material require- ments in relays by application of hollow contact rivets was investigated. In the existing type of relays, standard massive contacts were replaced by specially prepared hollow contacts, shaped in the form of shells made of thin plates of the contact material. The results of contact performances of hollow and massive contact rivets, applied in the same type of relays for tests were compared. Evaluation of bouncing, temperature rise on contacts, arc duration at break, switching capacity, and electric contact life were performed according to IEC standards. Good or even better performances of hollow contacts were obtained. After tests some microscopical examinations of contact surfaces were accomplished which indicates erosion characteristics at higher electrical loads.

Keywords-Hollow contact rivets, switching capacity, electric contact life, contact bouncing, arc duration, contact erosion.

I. INTRODUCTION ASSIVE contact rivets are commonly used for relay M contacts when switching currents in ampere range at

voltages of several 10 V. Some disadvantages of standard contact rivets can be indicated, such as relatively high mass, and therefore, cost of contact pieces, bouncing behavior at make, and formation of pips and craters on contact surfaces due to the existence of intensive electric erosion modes at dc loads causing premature total failures [l].

Deterioration of contacts due to switching phenomena is concentrated mainly on contact surfaces, therefore, only rela- tively thin surface layer of contact pieces is affected by them. The majority of contact material remains unused at the end of contact life. The choice of bimetal contact rivets enables a more economical use of contact material, having a contact material layer of appropriate thickness over a nonprecious base material; but the reduction of cost is minimal due to a higher technological level of manufacturing contact rivets. Other dis- advantages cannot be avoided by application of bimetal rivets.

An alternative to bimetal rivets is application of hollow contact rivets described in this paper. Instead of massive contact rivets, test contacts made of thin shell of contact material are prepared and applied in commercially available relays for measurements.

11. PREPARATION OF HOLLOW CONTACT SAMPLES

Hollow contact samples were stamped from a sheet of 0.1-

Manuscript received October 7, 1991; revised January 17, 1992. This paper was presented at the 37th IEEE Holm Conference on Electrical Contacts, Chicago, IL, October 7-, 1991.

The authors are with the Faculty of Electrical and Computer Engineering, Department of Electric Power Engineering, University of Ljubljana, 61000 Ljubljana, R Slovenia, Yugoslavia.

IEEE Log Number 9107097.

Fig. 1. Single-side and double-side hollow contact element.

1 Fig. 2. Altemative to double-side hollow contact assembly.

mm thick plate of fine Ag and inserted onto contact springs of commercially manufactured relays for testing purposes. After the insertion, the final configuration of hollow contacts was obtained by pressing to achieve the desired contact dimensions. Some constructional solutions for the preparation of hollow contact rivets are shown in Figs. 1 and 2.

Applied standard massive contact rivets are made of fine Ag, covered by 0.5-pm galvanic Au, as used in commercially manufactured relays. Test relays with this type of contacts were applied for comparison tests.

The cross section of the hollow contact sample inserted on contact spring is shown in Fig. 3. The cross-sectional view of a standard contact rivet is also shown for comparison in Fig. 4.

Both types of contacts were tested in relays of the same commercial type, as indicated in this section. The moving contact spring is driven by relay armature, which is controlled by the actuating coil. The contact gap in the open position is nominally 0.5 mm.

111. TESTING PROCEDURE Extensive tests were performed in order to obtain complete

characteristics of hollow contacts and comparative results for

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TUNKO AND BIZJAK: PERFORMANCES OF HOLLOW CONTACT RIVETS

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Fig. 3. Cross section of hollow contact sample.

Fig. 4. Cross section of standard massive contact.

standard massive contacts as well. The test schedule was as follows:

switching capacity test; contact voltage fall measurements; contact bouncing measurements; arc duration at break; electric contact life.

After life tests, some typical samples of tested contacts were investigated by scanning electron microscopy. The contact erosion patterns and effects causing contact failures were indicated.

A. Switching Capacity Switching capacity for standard massive contacts and hol-

low contacts was determined according to IEC standards [2 ] . Two types of switching capacity were determined:

rated switching capacity; limit switching capacity.

In accordance with test requirements, the capacity at make followed by the break operation is considered to be the switching capacity of contacts. Tests were performed at a frequency of contact operations of 1 Hz. The test circuit with ohmic load and stabilized dc voltage source was applied. Various open contact voltages as an additional test parameter were realized by a variation of source voltage.

Switching capacity is determined by load current value at which the relay is able to perform the specified number of switching operations without failure. To obtain the limit capacity, 10 successive operations at specified conditions are required, according to IEC standard [3]. Rated capacity is de- fined by the manufacturer of relays, therefore, 100 successive

Fig. 5. Open contact voltage versus load current characteristics of switching capacity of standard massive contacts ( N = 10: limit capacity, N = 100: rated capacity).

switching operations without failure at specified load current and open contact voltage were required.

Prior to the tests, 20 samples of hollow contacts and the same number of massive contacts were prepared for the assembly of the test relays. Switching capacity tests were performed at four different load currents:

3.5 A, 5.0 A; 7.0 A; 9.0 A.

Tests were performed at specified load current value and four different source voltages differing from each other by some volts were used as additional test conditions at each load current. In this way a number of successive switching operations until the first total failure was determined at four source voltage values and at the specified load current. From test values the relationship between the applied voltage (U,) and the number of operations until total failure (N) at the same load current was obtained by linear regression analysis. By applying the obtained linear function U, versus N, it is possible, with estimated uncertainty, to evaluate U, by causing the first total failure after performing the predetermined N operations. Results of source voltage U, at N = 10 and N = 100 for various load currents, plotted on a graph, represent voltage current characteristics of switching capacity, for N = 10 as limit capacity and for N = 100 as rated capacity. Voltage-current characteristics of switching capacity for massive contacts are shown in Fig. 5 and for hollow con- tacts in Fig. 6. The range of uncertainty for each characteristic was determined by linear regression analysis indicated by the shadowed area, which represents the area of 68% probability.

As can be indicated by the comparison of switching capacity for both types of contacts, the results are not very different from each other considering the difference in the mass of hollow and massive contacts in which the heat released by

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Fig. 6. Switching capacity of hollow contacts; characteristics are obtained at the same conditions as for Fig. 5.

the switching operation is absorbed.

B. Voltage Fall Measurement on Closed Contacts

B of contact spot [4]. It can be roughly estimated by Contact voltage fall is closely related to the overtemperature

B = 3200 U,. (1)

Voltage fall measurements were not performed directly on contacts but on contact spring connections of sample relays in order to maintain undisturbed contact conditions and thermal equilibrium. A certain contribution of voltage fall on contact springs is, therefore, expected in measured results. Thus the obtained results were applied for comparison between massive and hollow contacts.

Results of voltage fall on contact connections (Ucn) for two different load currents (I*) are obtained for massive and hollow contacts. They are listed in the following table:

I b 0.5 A 1.0 A hollow UCLl 16.2 mV 35.3 mV contacts std dev. 2.3 mV 3.5 mV massive U, 14.2 mV 32.8 mV contacts std dev. 1.3 mV 2.8 mV

Voltage fall differences between samples of massive and hollow contacts are considered small relative to their absolute values unless the contact spring resistance of approximately 10 mil is neglected.

C. Bouncing of Contacts Contact bouncing phenomena can detrimentally affect con-

tact surfaces due to enhanced erosion on make and welding of contacts [5]. In the period between two successive bounces,

an arc can appear after each bouncing transition. Due to the released thermal energy the contact erosion is enhanced and the probability of contact welding increased. The reduction of bouncing can increase contact life and switching capacity.

Bounce dynamics is related to oscillatory characteristics of contact rivets-contact spring system and other phenomena related to contact closure. A rough estimation can be made of how the reduction of contact mass influences bouncing intensity. By assuming that a contact system is an ideal spring oscillator having the contact spring of negligible mass, we can assess the movement of contacts between two bounces as an oscillation with a period equal to the double period between two successive bounce events. Two oscillators of equal spring length and different masses m, and mh have different oscillating periods T, and Th related to:

Let the time interval between two successive bounces be At, for the contact system with mass m, and Ath for mh. Following the applied approximation, the following equation can be used:

If the mass of the massive contact rivet is assumed as m, and the mass of hollow contact as mh, respectively, then according to (2) and (3) the value of fraction T,./Th is obtained:

If we consider moments of inertia of oscillating contact system instead of contact masses, the calculated fraction should be less than the obtained value.

Measurement of bounce times were made using a CRT oscilloscope. From the measured results it follows that the time interval between the first two bounces for massive contacts (At , , , ) and for hollow contacts (At1,h) on the average has the following value:

At,,, = 73.7 f 1 3 . 6 , ~ At1,h = 42.4 f 5 . 7 0 , ~ ~ ~

and the fraction of bounce time intervals is on the average:

By comparing both calculated values, we can assume that (2) and (3) can give us a qualitatively good estimate of bounce times.

From a simplified model of contact oscillations it can be expected that time decrement of bounce amplitude is expected to be greater for hollow contact than for massive ones.

ZUNKO AND BIZIAK: PERFORMANCES OF HOLLOW CONTACT RIVETS 201

Damping of bounce oscillations is caused by partial absorbtion of kinetic energy in contacts at their impact, by air resistance due to the velocity of contact spring blade, etc. The dissipation of oscillator energy into ambient air is proportional to the velocity of movement. We can expect greater damping effects at oscillations with higher frequencies. According to (2), a higher frequency is related to the lower mass of an oscillator, therefore, more intensive damping of bounce amplitude is expected on samples with hollow contacts and also lower number of bounce events compared to massive rivets.

Results obtained by CRT oscillograms indicate that the number of bounce transitions for massive contacts is 8 and for hollow contacts, 4. Bouncing at hollow contacts is reduced by factor 2.

D. Duration of Arc at Break

Arc duration was investigated by the CRT oscilloscope. The arc voltage at break dc currents and inductive loads were measured on new contacts of both types. Inductive loads were realized by various relay actuating coils. The open contact voltage was set by a dc source ranging from 12 to 110 V. By following arc voltage variation in time, the time interval from contact opening to arc quenching was determined. Duration of the breaking arc was found to be approximately 1 ms at 110 V, 100 ps at 24 V, and 50 ps at 12 V.

Arc burning time measured on numerous new samples of hollow and massive contacts indicates longer arc duration on

E. Tests of Electric Contact Life

Electric contact life was tested at dc voltages and ohmic loads, and the frequency of operation was 10 Hz. The applied test frequency was higher then normally used for relay testing. Its value was chosen principally for faster comparison of the life test results for hollow and massive contacts. Following the declared performances of commercial types of relays having standard massive contact rivets which were applied for test purposes, electric life of contacts was evaluated for maximal rated conditions of 100-V dc and 2 A at a maximal power of 35 W. Life tests were performed at various test circuit conditions and at the constant switching power of 35 W. The criterion for failure was a contact resistivity of 0.5 R or any other type of failure in normal operation.

Test results at various source voltages and load currents are presented in the following table as the number of contact operations without failure. Standard deviation of evaluated contact life is also estimated from the test results.

TEST PARAMETERS NO. OF OPERATIONS source load massive hollow voltage current contacts contacts 17.5 V 2.0 A 829 800 1 244 900

* 151 971 24 V 1.5 A 4 130 633 1 383 500

e135 594

e 62 642

e 2 084 074 100 v 0.35 A =. 7 * 1n7 7 * 1n7

samples with massive contacts. Arc time for samples with

relative to massive at various ODen contact hollow contacts is found to be reduced by factors 0.6 to 0.7, By ‘Omparing life Of standard massive ‘Ontact rivets, it can

be concluded that electric contact life of hollow contacts is

There is no correlation between arc t;me and open concact voltage, therefore, a reduction factor of an average value of 0.65 is considered independent of the source voltage. Iv. ELECTRIC EROSION OF HOLLOW CONTACTS Effects causing shorter arc burning time on hollow contacts are not fully understood. Presumably, the difference of the contact masses could not influence arc duration in the contact gap, but rather various conditions on contact surfaces. As described in Section 11, massive contact surfaces are covered by thin galvanic Au overlay. This type of contact is normally used for manufacturing of commercial relays, therefore, con- tact samples of the same type are applied for measurements and testing in a new and unused state. The arc voltage is approximately 150 V at the moment of contact separation; its value differs insignificantly for hollow and massive contacts. Arc voltage increases slowly by arc burning time, but is faster on hollow contacts then on massive ones. At the moment of arc extinction, the arc voltage on hollow contacts is approximately 50 V higher then on massive contacts at the moment of arc extinction. Due to the higher arc voltage, arc burning time on hollow contacts should be consequently shorter. This qualitative explanation is presumably valid for reduction of arc time on hollow contacts. After some 10 switching operations, no essential difference in arc time between both types of contacts are indicated. Therefore, it can be concluded that some kind of “contact activation” by galvanic residues on the surface of massive contacts results in difference of average arc time.

Erosion of contacts due to switching of load currents causes deep erosion patterns on contact surfaces after long time contact service. It can be clearly visible by a microscope, especially after the life test procedure. Life tested contact samples were, therefore, investigated by a SEM microscope. Since both types of contacts exhibit cathodic erosion patterns it was more interesting to investigate the propagation of erosion phenomena on hollow contact rivets. Fig. 7 shows erosion effects on cathodic and Fig. 8 on anodic hollow contact rivets after 467 000 make and break operations at ohmic load, 200-V dc, and at load current of 0.5 A. The same phenomena with less detrimental consequence are shown in Fig. 9 (cathodic contact) and Fig. 10 (anodic contact) on hollow contacts after 436 000 operations at 80-V dc and 0.8 A, other test conditions being the same.

It can be assumed that at the beginning, the erosion starts at the center of the contact rivet and penetrates from the contact surface into the underlying layers until the whole thickness of the contact shell has been burned out. The erosion process then proceeds from the edge of the eroded hole and increases its diameter. The formation of high erosional pips and craters is prevented in this way, and consequently, also early failures due to contact interlocking.

202 IEEE TRANSACTIONS ON COMPONENTS, HYBRIDS, AND MANUFACTURING TECHNOLOGY, VOL. 1 5 , NO. 2, APRIL 1992

Fig. 7. Eroded surface of cathodic hollow contact after 467 000 switching operations at 200-V dc and 0.5 A.

Fig. 8. Eroded surface of anodic hollow contact mated to contact shown in Fig. 7.

V. CONCLUSIONS

By comparing switching performances of standard massive and hollow contact rivets, some advantages of hollow con- tact applications in commercial available relay types were established.

At significantly reduced contact material requirements, the switching performances of test relays were not reduced over the accepted limit. Potentially, the most critical test of the switching capacity, considering the low content of contact material, indicates an insignificant reduction of switching capacity of relays with hollow contact rivets. Electric life is also considered to meet the requirements. The temperature rise of closed contacts estimated from voltage fall measurements is only slightly lower on massive contacts. By applying hollow contacts, bouncing is appreciably reduced. Pip formation on contact surfaces due to electric erosion is prevented by hollow

Fig. 9. Eroded surface of cathodic hollow contact after 436 000 switching operations at 80-V dc and 0.8 A.

Fig. 10. Eroded surface of anodic hollow contact mated to contact shown in Fig. 9.

contact rivets in the form of a shell. The manufacturing process of hollow contact parts requires

more advanced technology than the standard technology of production of massive contact rivets and classical contact parts, referring to the manufacturing of contacts and the assembly of contact springs.

REF ER EN c E s C.-L. Meyer, Werkstoffefir Elektrische Kontakte. Grafenau, Germany: Expert-Verlag, 1980, paragraph 12. “Electrical relays-Contact performances of electrical relays,’’ IEC Publication 2554&20, 1974. “All-or-nothing electrical relays,” IEC Standard Publication 255-1-00, 1975. R. Holm, Electric Contact Handbook,. New York: Springer, 1958, pp.

B. Gengenbach, R. Michal, and G. Horn, “ Erosion characteristics of silver based contact materials in DC contactors,” in Proc. 30th Holm Conf on Electrical Contacts, Chicago, 1984, pp. 201-207.

65-70.

h N K O AND BIZIAK: PERFORMANCES OF HOLLOW CONTACT RIVETS 203

Peter Bunko (M'89) received the M.Sc. degree in 1974 and the D.Sc. degree in 1978 from the University of Ljubljana, Yugoslavia.

From 1978 to 1985, he was an Assistant Professor and from 1985 to the present a Professor with the Faculty of Electrical and Computer Engineering, and Head of the Department of Electrical Power Systems and Devices, University of Ljubljana. He is also currently a member of the advisory group of the State Department of Energy and a Research Fellow of the Institut Joief Stefan. His research interests

include transformation and transmission equipment, transient analysis, and switching devices.

Dr. Tunko is a member of LEC Belgium.

Martin Biqjak received the Dipl. Phys and the M.Sc. degrees from the Faculty of Science and Technology, University of Ljubljana, Yugoslavia, in 1973 and 1988, respectively.

He was involved with research with the Research and Development Department, IS-. Currently, he is a Research Engineer with the Department of Power Systems and Devices, Faculty of Electrical and Computer Engineering, University of Ljubljana. Since 1984, his research interests have included electric contact phenomena and electric arc char- acteristics.