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Long-life contacts for unidirectional currents of 1¿20 amperes
Abstract
After an introduction to vibrator-contact problems it is shown that platinum contacts exhibit reverse and zero migration in two sets of circumstances, namely at 1¿2 volts and at 10¿20 amp, a small arcquench inductance having a critical influence. A long, though not unlimited, vibrator-contact life was obtainable either from platinum contacts with appropriate circuit conditions or from contacts of dissimilar metals, but an adequate life on heavy duty was obtained only with tungsten contacts in an atmosphere free of oxygen to a high degree. The trend of methods for producing exceptionally long contact life is reviewed.
The investigation described is a continuation of the work on transfer between platinum contacts published by the author in a previous paper. An account is given of the manner in which the metal transfer between platinum contacts breaking a 6-volt circuit varies with current and circuit inductance. Curves of the net transfer are given for currents and inductances in the ranges 1.8¿7.6 amp and 0.06¿117 ¿H respectively. The net transfer consists of up to four different types of transfer superimposed one on another, namely ¿bridge, ¿ ¿short arc,¿ ¿long arc¿ and ¿reversed short arc¿ transfers. The existence of the fourth type of transfer had been observed previously by Dr. A. L. Allen for currents of 10 and 20 amp, but was not known to occur at currents as low as 2 amp. It is shown that the build-up of pips on the contacts varies in steepness with the nature of the transfer. Some suggestions are given as to the mechanism of the different types of transfer, but at present a complete explanation cannot be given. Consideration is given to the practical application of the results, and it is shown that under some conditions the life of platinum contacts could be improved by controlling the effective circuit inductance at break to a value of about 0.6¿H.
The modes of operation of the various circuits employed in vibratory converters are briefly described and a more detailed analysis is then made. Formulae are derived for use in design and in most instances these are presented graphically. The paper includes calculation of the transformer flux density, the buffer capacitance, and the no-load (constant) losses, together with load characteristics for a.c. outputs with resistive or inductive load, and d.c. outputs employing full-wave, half-wave or voltage-doubling rectifying circuits. The ripple voltage across d.c. outputs is calculated and its harmonic content analysed. Formulae for input current, current in the transformer windings, and copper loss are also included, and are utilized to compare the power ratings for a given size of transformer operating in any of the various circuits. The paper deals mainly with conventional circuits for low or medium power, up to about 50 watts; however, one section analyses some special circuits for d.c. outputs of high power in which the current pulses in the transformer are so shaped that the vibrator contacts open at virtually no load.
The arc duration, material transfer, contact morphology, surface composition, and contact resistance of dissimilar contact combinations of palladium and silver were experimentally examined with breaking inductive load from 0.5 A to 4.0 A for 6×104 times of operations in air at atmospheric pressure. The test results have led to the conclusion that the polarity significantly affects the arc erosion and electrical contact resistance behaviours. By comparing with the previous results, it is known that the cathode material plays a main role in arc erosion, such as mass transfer, arc duration, and erosion pattern. The contact resistance is, however, dominated by the anode material due to surface contamination, especially the anode oxidation in late gaseous phase. Also, evidence is found in this work that the material transfer between two electrodes greatly influences the arc erosion and contact resistance behaviours.
Until the middle of the 20th century the electric arc was known
solely as a steady, self sustaining gas discharge phenomenon requiring
certain minimum values of current and voltage, depending on the
electrode material and the gas environment. Special investigations on
material transfer in contacts switching low direct currents, however,
revealed step by step that there were certain till then unknown unstable
arc modes appearing in extremely short electrode gaps only but requiring
minimum current and voltage values distinctly lower than those of the
classical steady arc discharge in air. Low current break arcs have to
pass one or more of these transient modes whether or not they may
eventually attain the steady gas arc mode
The arc duration, material transfer, contact morphology, surface
composition, and contact resistance of dissimilar contact combinations
of palladium to silver were experimentally examined with a breaking
inductive load from 0.5 to 4.0 A for 6×104 operations
in air at atmosphere pressure. The test results have led to the
conclusion that polarity significantly affects are erosion and
electrical contact resistance behavior. By utilizing results from a
previous study, it is known that cathode materials have significant
influence on are erosion, i.e., through material transfer, arc duration,
and erosion patterns. Contact resistance, however, is dominated by the
anode material due to surface contamination, specifically anode
oxidation in the late gaseous phase. Also, evidence is found in this
work that material transfer between two electrodes greatly influences
are erosion and contact resistance behavior
The paper discusses some of the difficulties encountered when extending the load capacity of vibratory convertors, and, in particular, those which are imposed by contact deterioration due to arcing. Two methods are described by which these difficulties may be overcome, both making use of special circuits designed to reduce the current carried by the contacts at the moment they break the circuit. Reference is also made to methods of dealing with certain vibrator defects, and with transformer surges. Finally, examples are given of practical applications of the circuits described.
The Closure and Partial Separation of a Metallic Contact 92, Part I, p. 301The Bridge Erosion of Electrical Contacts, Part IFine Migration at Moving Contacts
- A H Fairweather
- J J Lander
- L H Germer
FAIRWEATHER, A.: "The Closure and Partial Separation of a Metallic Contact," Journal I.E.E., 1945, 92, Part I, p. 301. H) LANDER, J. J., and GERMER, L. H.: "The Bridge Erosion of Electrical Contacts, Part I," Journal of Applied Physics, 1948, 19, p. 910. (4) HEUMANN: "Fine Migration at Moving Contacts," Ministry of Supply Translation TR. 556.
Physical Problems in Electrical Contacts. Critical Resume
- K R Schade
SCHADE, K. R.: "Physical Problems in Electrical Contacts. Critical Resume," E.R.A. Technical Report, Ref. U/T107, 1948.