Milenko Braunovic

Fretting damage in tin-plated aluminum and copper connectors

A number of bolted-type tin-plated aluminum and copper connectors commonly used for distribution transformers were examined using scanning electron microscopy, energy-dispersive X-ray analysis, Auger electron spectroscopy, X-ray analysis, and optical microscopy. In addition, the contact resistance of the connector contact zones was measured using a point probe. The connectors studied had been removed from service because of unsatisfactory performance under normal operating conditions as manifested either by overheating or instability. The results of the detailed examination show the presence of extensive fretting damage in the contacting surfaces. The fretting debris was composed mainly of tin oxide and oxidized base metal particles. Localized melting and wear of the tin plating down to the underlying substrate were also observed in some of the connectors examined.<<ETX>>

Effect of connection design on the contact resistance of high power overlapping bolted joints

The effect of design changes on the contact resistance of overlapping bolted/pad joints was investigated. It was found that slanting the edges of the bus-bars/pads under 45/spl deg/ and making slots in the overlapping areas significantly reduce the contact resistance of a joint and improve its mechanical integrity. These improvements are results of enlarged contact area and creation of a uniform current distribution at the contact interface.

Evalutation of Different Types of Contact Aid Components for Aluminum-to-Aluminum Connectors and Conductors

A number of contact aid compounds commonly used for aluminum-to-aluminum connectors and conductors were evaluated on the basis of their effect on the contact resistance-force/torque relationships, stability to thermal degradation, consistency, and ability to protect the contact against fretting. The results showed that the use of the contact aid compounds markedly improves the contact properties of a joint and significantly reduces its susceptibility to fretting.

Effect of Fretting on the Contact Resistance of Aluminium with Different Contact Materials

The effect of fretting on the contact resistance of aluminium wire (EC grade) and different plating materials (Ag, Cd, Ni, Sn, Zn) has been studied. The fretting tests were conducted with simulated connector configurations using a differential thermal expansion fixture to produce fretting damage. In addition, the waveforms of the contact voltage, scanning electron microscopy (SEM), and X-ray fluorescence analysis were used to study thc processes involved. The results show that the contact resistance of all combinations of aluminium-contact materials increases very rapidly after a given number of fretting cycles. The effects of fretting are reduced to some extent by applying higher electrical and contact loads. The waveforms of the contact voltage across the couples are severely distorted by the fretting action. The SEM and X-ray fluorescence analyses showed that the contact zones are severely damaged and that substantial transfer of material occurred as a result of the fretting action.

Evaluation of different contact-aid compounds for aluminum-to-copper connections

Many types of contact aid compounds, such as lubricants and inhibitors, are being used to ensure reliable connections between current carrying contacting members of power connectors. Most are commercially available, but little information has been published on their performance under accepted service conditions. Furthermore, no comparative study has even been made of the efficiency of these contact aids in maintaining uninterrupted operation of aluminum-to-copper connections under service conditions. The objective was to study the effect of such compounds on the stability of electrical connections under different operating conditions. For this purpose, a number of contact aid compounds was selected and subjected to parametric and stability tests. They were then evaluated and ranked on the basis of their spreading tendency, effect on the performance of a joint under current cycling conditions, and ability to protect the contact against fretting. The force torque and the contact resistance force relationship were also determined for different types of contact aid. >

Evaluation of different platings for aluminum-to-copper connections

A number of coating materials commonly used for aluminum-to-copper connections were evaluated on the basis of their effect on the contact resistance-force/torque relationships, the performance of coated joints under current-cycling and fretting conditions, and ability to protect the contact against galvanic corrosion in saline and industrially polluted environments. The results showed that the use of nickel-plating of both aluminum and copper and copper-plating of aluminum ensures the greatest stability while bare aluminum in contact with tin- and silver-plated copper shows the poorest performance under different operating conditions.<<ETX>>

Effect of fretting in aluminium-to-tin connections

The degradation of aluminium-to-tin plated electrical connections under fretting conditions is studied. In addition to contact resistance measurements, scanning electron microscopy (SEM) and X-ray fluorescence (EDX) analyses were used to study the processes involved. The results showed that fretting adversely affects the contact resistance of aluminum-to-tin plated connections, which show a rapid increase and substantial fluctuations after prolonged exposure to fretting. Two sustained plateaus in the contact resistance characteristics were observed: one coinciding with the melting voltage of tin and aluminum, and the other in the range corresponding to the voltage range of the melting, sublimation, and decomposition of the oxides, and vaporization of contact materials. The effects of fretting were reduced significantly by applying higher contact loads. SEM and EDX analysis revealed that considerable damage of the contact zones resulted from the fretting action and substantial exchange of material occurred. >

Effect of Contact Aid Compounds on the Performance of Bolted Aluminum-to-Aluminum Joints Under Current Cycling Conditions

A number of contact aid compounds most commonly used for aluminum-to-aluminum connectors and conductors were evaluated on the basis of their effect on the performance and stability of a bolted joint under current cycling conditions. The results show that a significant difference exists in the effect that some of the contact aids exert on the performance of the bolted joints under current cycling conditions. This difference manifests itself in higher contact resistance and rapid overheating of a joint during cycling. The observed difference was associated with the shear strength of the compound film and also with the properties of the additives such as metallic particles present in these compounds.

Effect of Current Cycling on Contact Resistance, Force, and Temperature of Bolted Aluminium-to-Aluminium Connectors of High Ampacity

The effect of current cycling on the contact 10ad, resistance, and temperature of bolted aluminum-to-aluminum joints of high ampacity has been studied. The results show that current cycling causes significant changes in the contact load, resulting in a drastic loss of contact force. Contact resistance is less affected by current cycling, and connector temperature shows a normal beating effect produced by the cycling current. A new accelerated current cycling test has been proposed. The test is based on the passage of relatively high current for short periods of time followed by longer cooling times. The results obtained from the new test indicate that this testing procedure is a valid alternative to those presently in use, and that it can be used as a method for detecting the deterioration of electrical connections. The results also show that the use of Belleville washers, plating, and lubrication i lnproves the performance of the bolted joints under current cycling conditions.

Fretting in copper-to-copper contacts under AC and DC current conditions

The effect of fretting on the contact resistance behavior of copper-to-copper wire-plate combinations under AC and DC current carrying conditions was investigated. The fretting conditions were as follows: frequency 1 Hz, slip amplitude 100 /spl mu/m and load 400 g (4N). The current level in both AC and DC conditions was 50 mA. In addition to contact resistance measurements, SEM and EDX were used to examine the surface damage in the fretted contact zones. The results indicate that the overall contact resistance behavior of copper-to-copper wire-plate couples subjected to the same fretting conditions but under AC and DC currents was practically the same. The characteristic feature of the samples under AC current conditions is a pronounced distortion of the contact voltage waveforms. The results of SEM surface analysis of the contact zones indicates that the surface damage resulting from fretting under AC current conditions was different from that under DC current conditions.