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  2. Heading • Theory and rationale for the carbon--graphite composite material (CGCM) • Structure, properties and arcing behaviour of CGCM • Actual environmental benefits and cost savings of CGCM • Conclusion Structure of this talk

  3. Heading INTRODUCTION Why bother with a new contact material? To address this question we will discuss the general problem of sliding contact materials in regard with minimising electrical and material losses, which eventually translate into two categories: - Environmental losses, and - $$$$$ losses

  4. Heading A few words about conductivity • Conductivity (σ) is the inverse of resistivity (or 1/ρ). It describes the ease of current flow in any element (unit 1/Ω·m). • For practical purposes it is easier to express σ as a percentage of the conductivity of commercial (annealed) copper, and this is unit called IACS (international annealed copper standard). • The following slide shows a few IACS values for some commonly used metals.

  5. Heading 5 Conductivity of some common conductors In other words, carbon, the main material used for current collection in the rail industry has basically the worst conductivity value among conductors. • To improve conductivity, carbon strips currently in use are impregnated with Cu to reach a IACS value of ~3%, but for the purpose of discussion we will assume a IACS value of 5%. • CGCM of recent production has a IACS value of above 80%.

  6. Heading 6 Back to basics: Power losses The power in any electric circuit is given by P = V × I and the loss is the heat generated, ΔΗ ΔΗ = α V× I = α I2 × R where α is Faraday’s constant. This means that the loss is proportional to the resistance. We will keep this in mind and discuss other aspects of CGCM meanwhile…

  7. Heading 7 Properties of CGCM • The copper-graphite composite material was invented in a joint project between the Public Transport Corporation of Victoria and RMIT in an attempt to reduce wire wear and minimize power losses. In contrast to the existing “carbons,” which consist mainly of hard carbon particles that abrade the (copper) overhead wires, CGCM contains graphite embedded in a copper matrix. These constituents provide a high-conductivity matrix with a solid lubricant.

  8. Heading 8 CGCM- continued • The idea of using a combination of copper and carbon is not new, and copper alloys (brass, bronze) with carbon have been in use for contact brushes. But Cu alloys are hard and abrade the wire, and Cu losses much of its conductivity with alloying. Pure copper has not been used in combination with carbon, because these elements do not dissolve in each other. Phase diagram of Cu-C (graphite) showing complete immiscibility.

  9. Heading 9 CGCM structure The novelty of CGCM is the fact that Cu and graphite could be mixed by powder metallurgy despite the lack of miscibility. The structure is made of Cu with islands of graphite that provide lubrication during sliding. Micrographs of CGCM showing islands of graphite embedded in a matrix of high-purity copper. The compositions are given in each photo. Taken at magnification × 50.

  10. Heading 10 CGCM properties • During sliding of the CGCM contact strip along the trolley wire, this microstructure forms a lubricating carbonaceous layer at the contact interface, with very low friction coefficient and wire wear. • CGCM samples were tested in sliding over contact wires in the lab and no wear could be measured after travelling 100,000 km at about 60 km/h. • A dedicated wear tester was designed and built for these experiments.

  11. Heading 11 Schematic of wear tests for CGCM

  12. Heading 12 Some friction test resultsCGCM vs. collector materials CGCM was tested alongside dry and lubricated collector materials and showed the lowest friction coefficient and lowest wire wear. (CCM is the carbon composite material in use today).

  13. Heading 13 Arcing • Arcing is one of the major factors causing wire failure, with temperatures during arcing reaching more than 600° C. • CGCM was tested for arcing at the Rail Technical Research Institute in Japan and was compared with materials used for the Japanese bullet train. • The next slide shows the apparatus and the results. There was very little difference in arcing behaviour between CGCM and pure copper. • CGCM arc duration was short. Arcing behaviour is also related to conductivity.

  14. Heading 14 Arcing test

  15. Heading 15 Costs and Emissions savings • Losses are inherent to electricity use. One of the major known losses in trains equipped with pantograph current collectors occurs at the contact point and is attributed to two main reasons: heat losses in the strip and sharp discontinuity in the current flow. • Overall, we assess that the total losses amount to ~6%, of which ~50% occur at the point of contact. (This is a modest assessment). • CGCM could reduce both types of losses: less heat is generated in the strip and also the discontinuity in material conductivity is less sharp.

  16. Heading 16 Assessing actual losses • The power (energy) ratio between carbon and CGCM strips is and assuming the same current in the train circuit we get PC / PCCGM = Rc / RCCGM but Rc / RCCGM = IACSCCGM / IACSC and therefore the power ratio is inversely proportional to the ratio of IACS values between the contact materials. PC / PCCGM = (Ι2 × Rc) / (Ι2 × RCCGM)

  17. Heading 17 Actual losses- continued • As mentioned, CGCM of current production has a IACS value of ~86%, but to simplify calculations we will assume only 80%, and for carbons we assume 5% although the actual value is below ~3%. • The ratio between IACS values is 80 : 5 = 16, i.e., heat losses at the contact point are reduced with CGCM to 6.25% of present losses, or in other words contact losses overall are reduced to 6.25 cents for every dollar lost with the present carbons. • the overall loss is reduced from ~6% of consumption to ~3.2%.

  18. Heading 18 Actual savings:1. Electricity • METRO, the present franchise train operator in Melbourne uses ~170 million kWh/yr • 6 % (present losses) of that is ~10.2 million kWh • 3.2% of that amount is ~5.44 million kWh • The difference between these two numbers is close to 5 million kWh and this represents only electricity savings with CGCM, without taking into account the savings from less copper wire wear.

  19. Heading 19 2. Cost savings • The exact dollar amount saved is difficult to calculate because Metro is charged according to different tariffs, but assuming $0.1—0.12/kWh on average, the total direct savings amounts to about 0.5--0.6 million dollars a year. • This is the direct expected reduction in electricity costs, but there are additional cost savings in wire wear and of course, in carbon emissions.

  20. Heading 20 Carbon emissions

  21. Heading 21 Carbon emissions • 1 kWh releases 890 g of CO2 • 1 black balloon represents 50 g CO2, 1 kWh= 17.8 black balloons With CGCM the system uses about 4.7 million kWh less per year, a saving of ~83.7 million black balloons. • Not considered here is the carbon powder generated on the roofs from the present carbons (carbon particle pollution).

  22. Heading 22 Conclusion (and additional notes) • An alternative contact material for current collectors was presented. • CGCM has significantly improved conductivity, reduces arcing, saves power and is more environmentally friendly. • In addition, it has better current-carrying capacity and the number of strips needed to operate the train can be reduced.(For example: The X-trapolis trains).

  23. Heading 23 Additional Notes • The patent is owned by M&H Materials P/L and Victrack and can be made available for trials simply by replacing the strips currently in use. • The author can be contacted at Mobile: 0413 898 393 Thank you for your attention.