Assessment of EMC and EMF issues
A detailed review of existing and planned automotive EMC standards has been carried out and documented. It was concluded that further development of standards is required in order to fully reflect the features of electric powertrains. Hence, testing procedures and suitable test equipment for filling the gaps detected during the first period have been proposed and are available on the project’s website.
Moreover, an approach for modeling the impact of low frequency magnetic field (due to traction currents, for example) on the human body has been proposed. These emissions may cause electro-stimulation of tissues such as muscles and nerves and should therefore comply with recommended exposure limits. Exposure risks are assessed by comparing the electromagnetic environment with field reference levels, which should ensure compliance with restrictions on in-body quantities such as induced current density and electric field in human tissues. However, the existing field reference levels have been developed for standing human under uniform field exposure, whereas for the in-vehicle environment the occupants are seated and exposed to non-uniform field distributions. Consequently, numerical simulations based on a seated human model under magnetic field exposures that are representative of electrical powertrain traction currents (see Figure 1) have been carried out to assess the validity of the field reference levels.
The conclusion of the research undertaken in the HEMIS project is that the magnetic flux density reference levels defined in the ICNIRP 1998 exposure guidelines (which are reflected in the EU recommendation 1999/519/EC for general public exposure) provide a more reliable safety margin (see Figure 2) in terms of the exposure limits of both ICNIRP 1998 and the more recent ICNIRP 2010 guidelines (for low frequency effects). However, the magnetic flux density reference levels specified in ICNIRP 2010 and the EU occupational exposure directive 2013/35/EU may not provide adequate protection with respect to the exposure limits.
Figure 1: Distribution of in-body electric field in vertical plane through right hip of seated human model exposed to PMOB project vehicle traction current paths at 200 Hz
Figure 2: Comparison of magnetic flux density reference levels for general public exposure with computed magnetic flux density threshold at which basic restrictions are reached for seated human model exposed to magnetic fields due to PMOB project vehicle traction current paths
Given the size and space constrains of road vehicles, the electric powertrain components are likely to be in relative close proximity to passenger compartment. Exposure measurements carried out on a 70 kW automotive electric powertrain under steady-state conditions for a range of speed and load combinations indicated exposures of the order of 150 times the magnetic field reference levels adjacent to the motor power cable entries (see Figure 3), falling to 100% of the reference level at distances of 20 cm (see Figure 4). Taking into account the results obtained in the HEMIS project, a number of possible mitigation techniques have been proposed.
Figure 3: Maximum magnetic field exposure related to reference levels measured at 5 locations under different operation conditions. Figure 4: Distance from motor cable entries at which measured exposure values fell to 100% of the magnetic field reference levels