a new bronze age
the slip ring in a similar way to how grooves are cut during the electrical discharge machining process. This loss of contact and arcing can also destroy expensive “Wet A” cards which can cost as much as a slip ring assembly. The Wet A card is a circuit board in the control
system that can be overloaded by current and destroyed when ground currents are not properly shunted through the brush to the ground ring interface. Loss of contact on the ground ring and the associated arcing can expose the circuit board to excessive current that overloads the circuits and thus destroys the card.
BRONZE IS THE NEW GOLD Traditionally, slip rings have been made from steel as they are the most affordable. However, bronze is now being recognised as a much more efficient metal for some applications, reducing failure rates, the amount of time technicians spend in the turbine and making uptime a priority. Morgan’s Electrical Carbon is finding greatly improved brush life by using bronze slip rings in tandem with grade M50BR brush material, which can reduce the frequency of maintenance within the turbine and prevent failures. While initial outlay for bronze is higher, there is
❱❱ The USA expects a sharp rise in wind power and correspondingly more turbines on the landscape over the next decades, above; reducing failure rates has a significant advantage in maintaining the wind turbine generator assembly, left; bronze slip rings improve brush life and reduce failure rates, inset; and as slip rings are damaged, the chances of arcing and causing further damage accelerates
arcing and damage the rings. Even worse, friction chatter can cause a complete flashover which results in high-voltage electric short circuits that can destroy the slip rings, holders and other components in the generator brush box. This damage can lead to costly downtime, in addition to the cost of replacing expensive components. Similarly, the instability of brushes leads to
excessive brush and slip ring wear, which can produce conductive dust. This dust can cause current to track and flow in places where it is not desired, again increasing the risk of flashover. In addition to the positioning of brushes, brush
materials and brush holders are also a contributory factor to failure. With brush materials and the performance of each differing greatly, poor quality or mismatched brush materials can wear quickly and dust heavily.
INTO THE GROOVE When brush holders aren’t adjusted to the right height, or indeed compatible with the brushes in use, a phenomenon called grooving on the slip ring can occur. Grooving is where brushes lose contact with the slip ring, producing arcing which damages
no question that its ability to prolong the use of brushes makes it a worthwhile investment in the long term, and a greater pay-off in terms of whole life costs. In contrast to steel, bronze runs cooler and decreases the amount of heat-related damage to the carbon brushes. This is because the thermal characteristics of bronze are better for heat dissipation and with the proper brush material its ability to form a better patina reduces friction. This leads to longer brush life with less risk of potentially dangerous dusting. To prevent dusting, engineers should also look to
slip rings with specialised paint coatings. These prevent brush dust collecting and sticking to the slip ring, reducing the undesirable tracking of current.
REPLACING STEEL WITH BRONZE When looking to replace slip rings with bronze, it is important to consider insulation materials. Such materials have a higher dielectric strength, are more resistant to higher temperatures and have non-stick properties in order to keep conductive dust from collecting and current from flowing where it is likely to cause damage or destruction. The width of the slip ring is also important when
looking to decrease temperatures. To counter this, Morgan has developed a bronze slip ring with a 20 per cent increased width for additional thermal mass and added cooling holes for more efficient heat transfer. T&TH
June 2019 /// Testing & Test Houses /// 27
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