Fasteners for high-voltage applications
The rise of the electric hybrid system has been rapid of late, though whether it is a lasting technology or simply a precursor to a widespread take-up of fully electric passenger cars is not yet clear. Electric motorsport has already established a firm footing though. For example, TT Zero gets faster each year, and although it remains a single lap it is close to half a conventional motorcycle Grand Prix distance. Endurance racing has had hybrids for 15 years, and 2013 is the fourth season for electric hybrids in Formula One. Until now they have almost been a sideshow in Formula One, but from next season they are a basic necessity, providing a huge chunk of a car’s performance.
All of this electric power requires high voltages – if we wanted to use 60,000 W of electric power at a conventional 12 V, for example, we would need 5000 A of current. Instead though, currents are kept to much more sensible levels, and for voltages the published literature from one component supplier for Formula One KERS states 500V, although the actual figure may be higher.
The advantage of a high voltage is that lower currents are required, and this means that the cross-sectional areas of conductors are reduced. The disadvantage is that high voltages like to jump across air gaps or creep along surfaces to conductors at lower potential. This means we need to leave a certain minimum air gap between any component at high voltage and one at lower voltage or connected to earth. Where two such components both touch the surface of an insulator connecting them, the problem is worse. If the air gap is respected, there is a larger distance across which charge will creep if there is a solid insulator.
Together, the effects of charge jumping a gap or travelling along the face of a conductor are known as creepage and clearance. Clearance is a fixed distance per voltage difference, given a known medium (for example air) over which the potential difference exists. For air this is 3000 V/mm, although it depends on temperature, air density and humidity. In any case, it is relatively easy to calculate and to leave a sensible safety factor.
Creepage is a different matter though, as it depends on the insulating material connecting the two conductors, how contaminated the surface is and what the contaminants are. The main area where creepage and clearance are likely to be a concern is the power electronics unit, because of the large number of high-voltage components in close proximity, but the same concerns exist in the motor, battery and voltage converter, if one is used.
Fasteners can often be the problem. We need them to hold lots of things together, but they can form an inconvenient conductive link between high- and low-voltage components. Where a non-conducting fastener can be found of acceptable strength, this is often a good solution. We should not confuse such fasteners though with nylon ‘registration plate bolts’ that we might find on passenger cars; there are non-conducting fasteners made from a number of high-strength polymers such as PEI and PEEK. These have a useful degree of strength but poor creep characteristics, so we need to make sure that the combination of service temperature and strength doesn’t cause them to lose load in service.
Of course, we have the option of using metallic fasteners and maintaining acceptable gaps where required, but this necessarily results in larger circuit boards that then need larger enclosures. Suddenly we have a physically larger and heavier hybrid component that tends to be more difficult to package and invites a lot of disparaging comments from the chassis men. In some cases it is possible to shroud conductive fasteners so that creepage and clearance effects are avoided, but often the best solution is a non-conducting bolt or stud.
Written by Wayne Ward