Current Ratings for In-Vacuum Cables

08/05/2015 - 05:48

What the maximum current rating for in-vacuum cables is, is not an easy question to answer.

For air side cables, the maximum current is typically defined by the current, which increases the temperature from 30°C ambient temperature to 70°C. If other insulation materials are used, the upper temperature can range from 90°C up to 180°C.

This current is dependant on: - Type of cable (round/flat) - Insulation material (giving the maximum temperature) - Number of conductors (normally 3 off) - Number of used conductors (normally only 2 off) - Wire diameter - Conducting Material (Copper/Aluminium...) - Mounting (free in air/in a tube/in an insulated tube...)

These rules also apply to vacuum. The main difference in vacuum is that the heat dissipation can no longer happen by convection, but only by radiation or by direct contact to other materials. As the intensity goes with T4 (Stefan -Boltzmann-Law), at low temperature the radiation is very low.

To achieve the higher temperatures, use Kapton insulated wires, which depending on the cables used can reach a limit between 250°C and 300°C. At elevated temperatures a reasonably high energy is dissipated however if the cable isn't baked out, trapped water will be evaporated and will affect the vacuum. To avoid these problems, motors and coils should be heated by applying current during the bake out to a higher value than during a normal run.

The heat created by the cable is calculated by the ohmic law, P =U*I =R * I2, so as an example, the thin 0.1mm wire 311-KAP-010 will create at 1A 2.27W/m (resistivity 2270 Ohm/km), whereas a 1mm ø multi strand wire 311-KAPM-100 with 30 Ohm/km will create only 0.03W/m. (Any RF effects are not taken into account in this calculation.)

The physical limit of use is the maximum temperature of the cable, either defined by the insulation material or the application. The current ratings given by Allectra are approximate values for free in-vacuum cables at continuous use. For short periods, significantly higher current can be applied, the limit will be the temperature of the cable.

If coils are made, the current is significantly lower, typically a maximum of 50% of the values can be used.

For critical applications, we recommend that the temperature of the cable is measured in testing.

**PDF - a table showing values of Kapton insulated wires for UHV use