Magnetic Field Exposure Assessment in Electric ...

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Magnetic Field Exposure Assessment in Electric Vehicles Andrea Vassilev, Alain Ferber, Christof Wehrmann, Olivier Pinaud, Meinhard Schilling, and Alastair R. Ruddle, Member, IEEE

Abstract—This article describes a study of magnetic field exposure in electric vehicles (EVs). The magnetic field inside eight different EVs (including battery, hybrid, plug-in hybrid, and fuel cell types) with different motor technologies (brushed direct current, permanent magnet synchronous, and induction) were measured at frequencies up to 10 MHz. Three vehicles with conventional powertrains were also investigated for comparison. The measurement protocol and the results of the measurement campaign are described, and various magnetic field sources are identified. As the measurements show a complex broadband frequency spectrum, an exposure calculation was performed using the ICNIRP “weighted peak” approach. Results for the measured EVs showed that the exposure reached 20% of the ICNIRP 2010 reference levels for general public exposure near to the battery and in the vicinity of the feet during vehicle start-up, but was less than 2% at head height for the front passenger position. Maximum exposures of the order of 10% of the ICNIRP 2010 reference levels were obtained for the cars with conventional powertrains. Index Terms—Electric vehicle, human exposure, hybrid vehicle, magnetic field.

I. INTRODUCTION UBLIC expectations to move toward the electrification of road transport are driven by a multitude of factors and concerns, which include climate change, primary energy dependence, and public health. On the other hand, there is widespread public concern regarding the possible adverse effects of electromagnetic fields (EMF), particularly low-frequency magnetic fields. The occupants of vehicles with electric powertrains will be exposed to low-frequency magnetic fields arising from currents flowing in the high-voltage power network, traction batteries, and associated devices such as inverters and electrical machines. Thus, there is a need to properly assess the level of magnetic field exposure that may result in electric vehicles

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Manuscript received September 2, 2013; revised April 14, 2014; accepted August 29, 2014. This work was supported in part by the European Community’s Framework Programme (FP7/2007-2013) under grant agreement number 265772. A. Vassilev is with the CEA LETI, Grenoble 38054, France (e-mail: andrea. [email protected]). A. Ferber is with the SINTEF, Oslo 0314, Norway (e-mail: alain.ferber@ sintef.no). C. Wehrmann and M. Schilling are with the Technische Universit¨at Braunschweig, Braunschweig 38106, Germany (e-mail: [email protected]; [email protected]). O. Pinaud is with the G2ELAB, Grenoble 38402, France (e-mail: olivier. [email protected]). A. R. Ruddle is with the MIRA Limited, Nuneaton CV10 0TU, U.K. (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TEMC.2014.2359687

(EVs), which include battery powered, hybrid, and fuel cell variants. The International Commission on Non-ionizing Radiation Protection (ICNIRP) has recommended limits for exposure to static [1] and time-varying [2] EMFs. These limits aim to provide protection from well-established acute physiological effects of EMF exposure, which include electro-stimulation of nerves (relevant for frequencies from 1 Hz to 10 MHz) and heating of body tissues (for frequencies from 100 kHz to 300 GHz). The recommendations relating to electro-stimulation effects, which are the primary interest in this study, were revised by ICNIRP in 2010 [3]. The exposure limits are actually specified in terms of in-body quantities that are not easy to determine. Consequently, ICNIRP has derived more readily measureable field reference levels from the exposure limits. It is considered that if the exposure environment complies with the field reference levels then it can be assumed that the exposure limits will not be breached. Exceeding the field reference levels does not necessarily mean that the exposure limits are also breached, but it is deemed that more detailed investigation is required in order to establish compliance with the exposure limits. In situations of simultaneous exposure to fields of different frequencies, these exposures are considered to be additive in their effects [2], [3]. Thus, the more frequencies that are present, the lower the levels that can be tolerated for any of them relative to the field reference levels. Furthermore, the influence of other fields that may be present in the environment also impact on what can be tolerated from equipment generating fields that people may be exposed to. This is very different to the evaluation of electromagnetic emissions against equipment EMC requirements, where compliance with the limits at each frequency is considered independently of all other frequencies, and independently of the other equipment that may be present in the intended operating environment. The field reference levels vary with respect to frequency and are valid only for pure sinusoidal signals. In the case of nonsinusoidal exposures, ICNIRP has proposed two exposure criteria that should remain below 100% to avoid undesirable electrostimulation effects. The first one (which also applies in the case of separate sinusoidal sources at multiple frequencies) consists in adding the ratios of the different spectral component (SC) magnitudes of the field in the environment to the field reference levels. A similar approach is also described in the IEEE standards relating to human exposure [4], [5], although in these documents the frequency range for the evaluation of electrostimulation threats is up to 5 MHz.

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The main drawback of this criterion for nonsinusoidal signals is that it assumes that all frequency components add in phase. This assumption is acceptable for signals with a limited number of SC, but for broadband signals this procedure could lead to an unnecessarily conservative exposure assessment. In [6] it is shown that for a measured field spectrum with SC that are all less than 1% of the ICNIRP 1998 [2] reference levels for the general public, the first criterion would suggest that the exposure is 634%. Relative to the ICNIRP 2010 reference levels the SC are all less than 0.1% of the general public recommendations, but the first criterion would suggest that the exposure reaches 99% of the general public recommendations. For these reasons a second criterion is defined by ICNIRP (the “weighted peak” approach [3]) in which a time-varying exposure measure is derived from an inverse Fourier transform that take into account the phase of each SC. This approach is more general and can be applied for every kind of magnetic field waveform. Using this approach, [6] indicates that the maximum value of the time-varying exposure measure is less than 20% of the ICNIRP 1998 reference levels for the general public, and less than 5% of the ICNIRP 2010 field reference levels. Published data concerning field exposure in EVs is somewhat scarce, and not always appropriate for assessment against the ICNIRP field reference levels. For example, the research conducted in [7] characterized the magnetic fields associated with various form of transportation. The results showed that EVs have a magnetic field spectrum roughly similar to conventional vehicles (CVs), but no exposure assessment was calculated. This study also found no evidence for measureable electric fields associated with vehicles in the band 5 Hz to 3 kHz. In a more recent paper [8], it is reported that magnetic fields measured in a hybrid car are much lower than the ICNIRP 1998 field reference levels, but no results taking into account additive effects were presented. Maximum fields of 15 μT (in the band 0.01–5 kHz) were recorded in a hybrid bus at the seat located closest to power cables during acceleration, deceleration, and driving at 25 km/h [9], but no information regarding the time variation of the magnetic field is provided. Two hybrid cars were measured in [10], which indicates that the exposure could reach 80% of the ICNIRP 1998 reference levels for the general public in the vicinity of the passenger’s feet during braking and acceleration. In [11] the second criterion was employed to evaluate measurements at 12 points that represent various body parts from head to foot for front and rear occupant location in five cars with motor powers ranging from 15 to 147 kW. Time-domain measurements were carried out over 1 s periods under constant drive conditions for frequencies from 1 Hz to 100 kHz. Exposure relative to the ICNIRP 1998 general public reference levels was reported to reach a maximum value of around 15% in the vicinity of the front occupants’ feet. These authors also report maximum exposure measures approaching 20% of ICNIRP 1998 general public levels at head height in a hybrid bus [12]. This paper describes the measurement of eight different EVs and three CVs, with characteristics as outlined in Table I, as well as the calculation of realistic exposure measures from these measurements using the second criterion (i.e., the ICNIRP

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TABLE I SUMMARY OF MEASURED VEHICLES Car code

Powertrain type

Electric motor technology

Power (kW)

Energy (kWh)

EV#1 EV#2 EV#3 EV#4 EV#5 EV#6 EV#7 EV#8 CV#1 CV#2 CV#3

Battery Plug-in hybrid Small hybrid Battery Battery Battery Fuel cell Battery Gasoline Gasoline Diesel

Brushed dc Permanent magnet Induction Permanent magnet 3-phase asynchronous Permanent magnet Permanent magnet Permanent magnet – – –

11 30 10 10 34 40 100 35 75 66 125

10 5 0.7 14 24.5 24 1.4 16 – – –

weighted peak approach). Section II outlines the measurement protocol and Section III summarizes the main sources of magnetic field in vehicles. The exposure calculations are presented in Section IV, and Section V summarizes the conclusions.

II. MEASUREMENT PROTOCOL A. Measurement Setup 1) Sensors: Four kinds of sensors were used: 1) Low-frequency (LF) 3-axis magnetic field sensors (Fluxgate magnetometers): a) Bartington MAG-03 (bandwidth 0–3 kHz; range: ±100, ±250 or ± 500 μT; noise < 10 pT/Hz); b) Sensys FGM 3-D/100 (0–1.2 kHz). 2) High-frequency (HF) 3-axis magnetic field sensors: a) Narda EHP-50D (5 Hz to 100 kHz); b) Spectran NF-5035 (1 Hz to 10 MHz). 3) Current sensor Fluke i310s (range 0–300 A, 0–20 kHz) used to establish the correlation between the magnetic field and the current flowing in the high-voltage power network. 4) Acceleration sensor (STMicroelectronics, model LIS3LV02DL, range ±6 g), used to establish the relationship between acceleration and power flow. 2) Acquisition system: The LF sensors and the current sensor were connected to an OROS analyzer (model name OR36) whose main functions were to: 1) low-pass filter the signals (cut-off frequency 2 kHz); 2) sample the signals (sampling frequency 5.12 kHz); 3) provide concurrent recording of the signals. The HF sensors had their own sampling and recording system. 3) Mannequin and Sensor Positions: In order to mount the different magnetic sensors and to ensure reproducible positioning, a nonmagnetic mannequin was developed (see Fig. 1), inspired by [7] and [13]. As tests were made while driving the cars, the mannequin was installed on the front passenger seat. The mannequin was equipped with three LF and three HF sensors located near the head, seat, and foot regions. A fourth LF magnetic field sensor was placed above the main battery in the trunk. The current sensor was placed on one of the battery cables.

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Fig. 1.

In-vehicle measurement setup.

Fig. 3.

Fig. 2.

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Magnetic flux density spectrum due to steering pump of EV#1.

B. Laboratory Tests For some of the EVs, laboratory tests were carried out in order to characterize the magnetic field emissions of all of the on-board electrical equipment. The car was raised on a lift in order to be able to place sensors below the car; everything was switched off except for the item under test. The magnetic field was then measured and this process repeated for every other item of equipment. This procedure is quite time consuming but very informative. For example, it revealed that a very specific harmonic spectrum is due to the steering pump of EV#1. This is illustrated in Fig. 2, which shows the root sum square (RSS) of the Fourier transforms obtained from the waveforms recorded for each of the three orthogonal components of the magnetic field. C. Outside Tests Outside tests are the most important because they are the most representative of real-world driving conditions. The main prob-

Spectrogram showing evidence of two 50 Hz high-voltage power lines.

lem with these tests is that there are several external magnetic perturbations that could influence the results. The approaches used for identifying these perturbations, and then performing the tests, are described later. 1) Identifying external magnetic perturbations: There are several sources of external magnetic perturbation in the environment, such as stationary or moving ferromagnetic masses (e.g., manhole covers, railway lines, other cars), as well as 50 Hz power distribution equipment (high-voltage transmission lines, power transformers etc.). These initial tests were carried out within a specific area with restricted access (for example the CEA complex in Grenoble) because the number of other cars was then limited. A specific driving route was defined in this controlled area and the car repeated the journey several times under different driving conditions (low and high speed, acceleration and deceleration). If the results contained a magnetic field feature that occurred every time at the same place, it was concluded that this field was due to an external perturbation. For example, a spectrogram obtained from a sensor located on the floor of EV#1 is illustrated in Fig. 3, which shows a clear permanent 50 Hz signal with two strong features located at around 30 and 100 s (on the vertical time axis). These features are probably due to two high-voltage 50 Hz power lines passing under the driving route. Once the external magnetic perturbations were identified, further tests were performed on a normal road. 2) On Road Tests: On road tests were performed which involved driving with maximum acceleration and deceleration in order to ensure maximum positive (traction) and negative (regenerative brake) currents. A straight road is better because in this case the magnetic fields due to the earth and due to the induced magnetization of the car are constant during the test. For the first few cars, measurements were also carried out on a steep slope and at high speed on a highway. However, it appeared that these two driving conditions did not generate higher fields



Fig. 4. Correlation between current and vertical magnetic flux density measured above rear pack battery on EV#1.

than for acceleration and deceleration on the flat road. For the remaining cars, therefore, the on road tests were performed only on a flat road and at moderate speeds (0–60 km/h).

Fig. 5.

Spectrogram (0.4–20 Hz) from a sensor close to a wheel of EV#1.

ICNIRP general public limits for frequencies below 1 Hz [2]. Nonetheless, traction current transients with SC above 1 Hz may contribute significantly to the in-vehicle magnetic field exposure [6].

III. IDENTIFICATION OF MAIN SOURCES OF MAGNETIC FIELD In this section, the main sources of magnetic field are listed, grouped by frequency content and presented in order of increasing frequency. A. Traction Currents It is well known that a current flowing through a wire or a loop generates magnetic field. Our measurements show that, for most of the cars, the high-voltage power network acts as a current loop. Moreover, the magnetic field close to the battery could be important: the ratio between the induced magnetic field and the traction current was found to be in the range 0.2– 1 μT/A, depending on the car. Therefore, if the traction current has variations up to ±300 A, the magnetic field could also have variations of up to ±300 μT. An example is shown in Fig. 4, where the vertical component of the magnetic field at each point in time is plotted versus the corresponding current. In this case, the traction current varies from −100 A to + 200 A whereas the magnetic flux density is in the range −40 μT to +100 μT; hence the field–current ratio is of the order of 0.5 μT/A. The magnetic field due to traction currents also presents a high spatial variability. In EV#1, for example, the magnetic field in the vicinity of the passenger’s foot ranges from 30 to 130 μT within distances of a few tens of cm. This is due to the fact that when two cables carrying opposite currents are close together (as is the case in the central tunnel of EV#1), the resulting magnetic field is minimized. But when the cables diverge (as is the case in the engine bay of EV#1), the field can increase significantly. Although this field is important, it is far below the ICNIRP general public limit of 40 mT for static fields [1] and below the

B. Wheels The permanent magnetization of steel belted tires is a wellknown source of in-vehicle magnetic fields (see [14], [15]). Our measurements show that this phenomenon is responsible for a magnetic field inside the car of up to 2 μT at the wheel frequency fw (which ranges from 0 to 20 Hz for speeds ranging from 0 to 130 km/h). There are also several harmonics present at lower intensities. The spectrogram shown in Fig. 5 (in dBμT) was obtained from a sensor placed near the passenger’s foot location in EV#1, close to the front right-hand wheel. The electric motor was switched off and the car was manually pushed at low speed (4–5 km/h) corresponding to fw 1 Hz. This clearly shows a fundamental signal 1μT at fw as well as the higher harmonics f2 , f3 , f4 , f8 , and f16 with decreasing magnitudes. As the ground was not flat, it was difficult to maintain a constant speed, which is why the fundamental frequency fluctuates over time. C. Internal Combustion Engine Measurements on the hybrid car EV#2 show that there is a correlation between the rotational frequency fm (varying up to 100 Hz) of the internal combustion and the magnetic field signals. This could be due to the motion of the pistons. The magnitude of the associated magnetic signal was 150 nT at frequency 2fm in this case. D. Specific Equipment Specific equipment of the car may also generate magnetic fields. The magnetic field emissions from the power steering pump (500 W, 12 V) of EV#1 have already been reported

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Fig. 7.

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Magnetic flux density spectrum (0–100 kHz) measured from EV#3.

TABLE II SUMMARY OF IDENTIFIED SOURCES Fig. 6.

Spectrogram (0.5–1 kHz) due to the steering pump of EV#1.

(see Fig. 2). This equipment can generate fields up to 1 μT inside the car at frequencies in the range 0.5–1 kHz (see Fig. 6). For EV#2, just after an accelerator pedal release, a wide-band magnetic signal of 1μT at frequencies up to 400 Hz was observed on the sensor above the battery. It is believed that this signal may be due to the regenerative brake. On EV#3, an unexplained coupling between an equipment working at constant frequencies (50 and 100 Hz) and the motor (whose fundamental frequency f1 is proportional to the wheel frequency, such that f1 = 4fw ) was found to lead to fields of the order of 0.5 μT at frequencies up to 600 Hz.

Sources Traction currents Wheels Internal combustion engine Steering pump Regenerative brake Electrical coupling Inverter Inverter

Maximum field

Frequency range

Cars involved

100–300 μT 0.1–2 μT 50–150 nT 1 μT 1 μT 0.5 μT 20 nT 60 nT

0–10 kHz 0–20 Hz 0–200 Hz 0.5–1 kHz 0–400 Hz 0–600 Hz 7–9 kHz 0.2–10 MHz

EV#1 to EV#8 All EVs and CVs EV#2, CV#1 to CV#3 EV#1 EV#2 EV#3 EV#1 to EV#4 EV#1 to EV#4

E. Inverter A power inverter is an electrical device that converts direct current (dc) to alternating current (ac). In EV, it is typically characterized by a switching frequency of around 10 kHz. To deal with these frequencies, it is necessary to use the HF sensors. These measurements were carried out for only four of the cars (EV#1–EV#4). Below 200 kHz, the field level was less than 20 nT; the maximum values observed in this frequency range (see Fig. 7) were at 1) 7–9 kHz, probably due to the inverter; 2) 16 kHz, due to the LF sensor electronics. Between 200 kHz and 10 MHz, the field level is a little higher but still low (less than 60 nT). A harmonic spectrum can be observed, but it is difficult to establish its origin. F. Summary The contributions from the different sources above 1 Hz that were identified are summarized in Table II and in Fig. 8. In this figure, each source is represented by a horizontal solid (blue or black) colored line. The vertical position indicates the maximum field measured, while the length indicates the frequency range. The dotted red line connects the different sources and forms a spectral envelope of the measurements. It can be seen that

Fig. 8. Summary of magnetic sources and ICNIRP 2010 [3] reference levels for magnetic flux density.

this envelope is decreasing with increasing frequency. Most of the magnetic field sources (wheels, brake, and steering pump) produce frequencies ranging between a few Hz and 1 kHz, with magnitudes of 0.1–2 μT. At frequencies above a few kHz, the magnetic field is less than 100 nT and only the inverter was identified as a source from the vehicles. Compared to the ICNIRP 2010 reference levels defined for the general public and for pure sinusoidal signals [3], which



related to the frequency dependence of BR (f ): W F (f, fr ) = BR (fr )/BR (f ).

(2)

The weighted magnetic field waveform can then be calculated as Bw e,f r (t) =

n

√ W Fj 2Bj . cos (2πtfj + θj + ϕj )

(3)

j =p

Fig. 9.

Amplitude and phase of weighting function, WF.

is plotted in Fig. 8 using dashed red line, the envelope of the magnetic field emissions is roughly two decades lower than the general public levels. However, this does not mean that the order of magnitude of the exposure is 1% of the reference levels. As already noted in Section I), it is necessary to take into account the additive effects of all frequencies from all sources in order to assess the exposure (see Section IV). IV. EXPOSURE CALCULATIONS A. Weighted Peak Approach for Nonsinusoidal Exposures The weighted peak exposure criterion defined by ICNIRP in the case of nonsinusoidal exposures (see [3] and [16]) is applicable for arbitrary magnetic field waveforms B(t). The general idea is: 1) defining a reference frequency fr ; 2) computing from B(t) a weighted signal Bw e,f r (t) that takes into account the frequency dependence of the magnetic field reference level BR (f ); 3) comparing this weighting signal to the reference level BR (fr ) at reference frequency. If it is assumed that B(t) is periodic on the measured interval [t1 ; t2 ], then B(t) may be approximated by the n spectral components (SC) derived from B(t) by means of a discrete Fourier transform. Thus, B(t) =

n √ 2Bj . cos (2πtfj + θj )

(1)

j =1

where j is the summation index of the SC (including the dc component), Bj represent the RMS amplitudes of the SC at frequency fj , and θj correspond to the phases of the SC. The weighting function WF is defined (see Fig. 9) such that the gain is inversely proportional to the chosen magnetic field reference levels BR (f ) and the corresponding phase values are

where W Fj represent the peak amplitude of weighting function at frequency fj and the terms ϕj represent the corresponding phase values. The parameters ϕj are specified as zero when BR (fj ) is constant, π/2 when BR (fj ) ∝ 1/f , π when BR (fj ) ∝ 1/f 2 , and −π/2 when BR (fj ) ∝ f (see Appendix A of [3]). ICNIRP recommends that the summation goes from 1 Hz (corresponding to j = p) to 10 MHz (corresponding to j = n). In this paper it was found that the magnetic field levels decreased rapidly for frequencies above 1 kHz. Therefore, the exposure sum ranges from 1 Hz to 2 kHz, and only the signals from the LF sensors are used. The weighted signal is then simply compared to the reference level at frequency fr . √ Bw e,f r (t) < 2BR (fr ), for t ∈ [t1 ; t2 ] . (4) Dividing by the right term of the inequality, we obtain a time-varying exposure function c(t), independent of fr that is required to be