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TEMPUS: HIGH FREQUENT CONDUCTED AND RADIATED EMISSION
1: Introduction
EMC stands for “Electro Magnetic Compatibility”. Two devices are compatible with each other when the
first one does not disturb the second one and vice versa. In case one device influences/disturbs the other
one, EMI (Electro Magnetic Interference) occurs which must be avoided. The main principle of EMC says
“You will not disturb the others and you will not be disturbed by the others”.
Due to technical evolutions, it becomes more and more difficult to comply with this EMC principle.
Electronic devices are more and more sensitive to disturbances i.e. their immunity level decreases. Indeed,
for instance the evolution from vacuum tubes to semiconductor transistors or the evolution from 5V
technology to 3V technology implies electronic devices are more susceptible to disturbances. At the other
hand side, a lot of electronic devices emit more and more disturbances i.e. the emission level increases. A
classical incandescent lamp caused (almost) no disturbances but today almost every electronic device
contains a SMPS (Switching Mode Power Supply), clocked processors and clocked busses which
increases the emission level.
Figure 1: Evolution of the emission level and the immunity level
Figure 1 visualizes the emission level tends to increase and that the immunity level tends to decrease the
last decades. When the emission level crosses the immunity level, the emission level is higher than the
immunity level and the devices disturb each other. Using EMC directives, the crossing of the emission
level and the immunity level must be avoided. The EMC directives aim to limit the emission level and to
avoid a further decrease of the immunity level as visualized in Figure 2. Based on this EMC philosophy,
the emission level stays lower than the immunity level.
Figure 2: EMC philosophy
To obtain that devices respect the EMC philosophy (or are expected to respect the EMC philosophy), there
exist normalizations which specify emission limits and immunity limits. Such limits are available when
considering (high frequent) conducted emission and when considering radiated emission.
2: High frequent conducted emission
2.1: The drive system
When considering the drive system visualized in Figure 3, the drive system does not only consume low
frequent currents (a 50 Hz component and harmonics) but also consumes high frequent currents flowing in
the power supply cable. Due to these high frequent currents, a voltage drop appears across the grid
impedance implying high frequent voltages.
The frequency converter in Figure 3 generates a PWM voltage (as visualized in Figure 4) to supply an
induction motor. When the motor cable is long (in some cases even a motor cable of 5 m behaves as a
long motor cable), the steep slopes of the PWM voltage pulses cause transmission line effects. These
transmission line effects imply overvoltages appear at motor side. Due to these overvoltages, the life
expectancy of the motor reduces. Using an output filter, these overvoltages will be limited or eliminated.
When the motor cable is sufficiently short, no output filter is needed.
Figure 3: Drive system with frequency converter
Figure 4: PWM voltage
2.2: Measuring high frequent conducted emission
The frequency converter and the entire drive system are sources of high frequent currents in the power
supply cable i.e. high frequent conducted emission. Figure 5 visualizes how this conduction emission level
can be measured. In order to make the measurements independent of the local grid impedance (implying
measurements performed by different persons at different locations can be compared with each other), the
drive system is fed by an artificial grid or LISN (Line Impedance Stabilization Network). By connecting
an EMI receiver with the LISN, it is possible to measure the high frequent voltages in a range from 150
kHz to 30 MHz. Due to the steep slopes of the PWM voltage pulses (the presence of steep slopes indicates
the frequency spectrum contains high frequent components), the high frequent conducted emission level is
considerable.
Figure 5: Measuring conducted emission between 150 kHz and 30 MHz
Figure 6 visualizes a LISN (on the right) and an EMI receiver (on the left). Due to the drive system of
Figure 3 (without EMC filter at grid side), the emission levels of Figure 7 are obtained. Notice the EMI
receiver measures voltage levels. Due to the high frequent currents in the standard grid impedance of the
LISN high frequent voltages appear. Since these voltage levels vary over a broad amplitude range, a
logarithmic scale is used. The measured voltage level 𝑈 is compared with a reference voltage level 𝑈𝑟𝑒𝑓 =
1𝜇𝑉. The equation
20 10log (
𝑈
)
𝑈𝑟𝑒𝑓
uses dBμV to express the emission level. Notice in Figure 7 that the emission level is considerable higher
than the allowed emission level (red).
Figure 6: EMI receiver and LISN
Figure 7: Conducted emission due to the drive system with frequency converter
2.3: The use of an EMC filter
As already mentioned, the emission level in Figure 7 is significantly higher than the allowed emission
level (red). In order to comply with the normalizations, it is useful to mount an EMC filter at grid side as
visualized in Figure 3. The EMC filter (it is important to have a good high frequent ground) reduces the
emission level of the drive system implying the emission level complies with the allowed emission level
(red) as visualized in Figure 8. Figure 9 visualizes such a high frequent EMC filter (the internal circuit is
visualized on a sticker).
In order to understand the internal circuit of such an EMC filter, it is important to make a distinction
between ‘differential mode’ currents and ‘common mode’ currents as visualized in Figure 10. The
differential mode current 𝐼𝐷𝑀 flows in the first conductor and returns using the second conductor. In case
of a single phase system, the normal 50 Hz load current is a differential mode current. Drive systems as
visualized in Figure 3 are also sources of high frequent differential mode currents.
The common mode current 𝐼𝐶𝑀 is twice flowing in the same direction as visualized in Figure 10. Due to
parasitic (small and undesired) capacitors, the high frequent common mode currents are flowing back to
the grounded voltage source. Common mode currents are typically high frequent currents. In general, high
frequent common mode currents are a larger problem then high frequent differential mode currents. When
the differential mode currents are flowing through two conductors close to each other, they generate two
(almost) equal magnetic fields having an opposite polarity. This implies there is (almost) no resulting
magnetic field i.e. the radial emission level is limited.
Figure 8: Reduced emission level due to an EMC filter
Figure 9: EMC filter
Since common mode currents flow back to the source using parasitic capacitances, the current flows using
all the metal parts and connections between the voltage source and the load (the DUT). This return path
can be the shielding of a shielded cable but the return path can be a water pipe, reinforcements in
concrete... The currents in the return path are not flowing at the same location as the 𝐼𝐶𝑀 currents in the
grid conductors. This implies the generated magnetic fields do not compensate each other and a higher
radiated emission level will be obtained.
Figure 10: Differential mode currents and common mode currents
Figure 11 visualizes a typical EMC filter containing
-
capacitors 𝐶𝑋 between the active conductors (e.g. between a phase conductor and the neutral
conductor),
capacitors 𝐶𝑌 between the active conductors and ground,
common mode coils (1 𝑚𝐻 in Figure 11),
differential mode coil (100 𝜇𝐻 in Figure 11),
resistors 𝑅,
voltage dependent resistors (VDR) to protect against overvoltages coming from the grid.
Figure 11: Internal structure of an EMC filter
A differential mode coil is a classical coil having a magnetic conducting core material in order to increase
the inductance. Common mode coils have a construction as visualized in Figure 12. Notice the presence of
a core and two coils. The first coil is situated in the first phase and the second coil is situated in the second
phase.
The differential mode current is flowing through the first coil and returning through the second coil. This
causes two fluxes in the core material. These fluxes have the same magnitude but have an opposite
direction implying these fluxes compensate each other which avoids saturation of the core material (and
avoids a decrease of the inductance). Since differential mode currents are (or can be) large (the 50 Hz
current is a differential mode current), the compensating fluxes avoid the need of a core material with a
large cross section. Due to the compensating fluxes giving a resulting flux which is zero, the common
mode coil do not reduce the differential mode current.
Due to the common mode current, there are two fluxes having the same direction in the core material as
visualized in Figure 12. In reality, the common mode currents are much smaller (high frequent and in the
𝑚𝐴 or 𝜇𝐴 range) than the differential mode currents. Due to the use of two coils, the flux due to the
common mode current doubles. Due to this resulting magnetic flux, a large inductor value is obtained for
the common mode currents which causes the desired reduction of these common mode currents. The
common mode currents are sufficiently small in order to avoid saturation of the core material due to the
magnetic flux.
Figure 12: Common mode coil
2.4: The use of EMC filters in photovoltaic installations
Figure 13: Single phase inverter between phase conductor and neutral conductor
Figure 13 visualizes a string of photovoltaic panels generating a DC voltage and an inverter converting
this DC voltage to an AC voltage of 230 𝑉. Between the photovoltaic panels and the inverter, an
EMC/EMI filter has been mounted. This filter reduces the high frequent currents in the photovoltaic cells
due to the inverter. This avoids an accelerated aging of the photovoltaic cells. There is also an EMC/EMI
filter mounted between the inverter and the grid. This filter limits the emission of high frequent
disturbances into the grid in order to comply with national and/or international EMC normalizations (e.g.
see http://www.schaffner.com).
3: High frequent radiated emission
Power electronic devices are not only a source of conducted emission, they are also a source of radiated
emission i.e. these devices emit electromagnetic waves. Also when considering radiated emission, there
exist emission levels which must be respected in order to avoid disturbances to e.g. the regular radio
communication devices.
3.1: Measuring high frequent radiated emission
Measuring this radiated emission is performed in a cage of Faraday as visualized in Figure 14. By placing
the DUT (Device Under Test) (also called EUT = Equipment Under Test) in the cage of Faraday together
with a receiving antenna, only the emission due to the DUT will be measured.
Figure 14: Measuring radiated emission in a cage of Faraday
The receiving antenna is connected with the EMI receiver visualized on the left in Figure 6. Using a PC,
the measurement results can be visualized in a user-friendly way. Figure 14 and Figure 15 visualize a
biconical antenna. By using such a biconical antenna, it is possible to measure the emission level in a
frequency range from 30 MHz to 200 MHz. Notice the presence of cones at the walls of the cage of
Faraday. Due to these cones, the emitted electromagnetic waves will not (or almost not) be reflected in
order to simulate an open field.
The biconical antenna measures the electrical field strength of the electromagnetic waves. This electrical
field strength is expressed in V/m. Since these field strengths vary over a broad range, a logarithmic scale
is used. The measured field strength 𝐸 is compared with a reference 𝐸𝑟𝑒𝑓 = 1𝜇𝑉/𝑚. The equation
20 10log (
uses dBμV/m to express the emission level.
𝐸
)
𝐸𝑟𝑒𝑓
Figure 15: Biconical antenna
Figure 16: Cage of Faraday
3.2: Measurement results
In case the biconical antenna is situated outside the cage of Faraday and measurements are performed in a
frequency range from 30 MHz to 200 MHz, a spectrum as visualized in Figure 17 is obtained. Notice the
presence of FM radio stations between 87.5 MHz and 108 MHz.
Figure 17: Measurement using the biconical antenna outside the cage of Faraday
As already mentioned, the radiated emission of a device (DUT or EUT) will be measured inside the cage
of Faraday in order to measure only the emission of the DUT (and excluding for instance the FM radio
stations in the measurement). As visualized in Figure 14, there is a distance of 3 m between the frequency
converter and the biconical antenna.
Figure 18 visualizes the emission levels (bleu: empty chamber) when there is no DUT in the cage of
Faraday which causing radiated emission. This gives a reference level for the measurements. When
placing a frequency converter (without motor cable and without induction motor) in the cage of Faraday
and when this frequency converter is fed by the grid voltage, the emission level increases (green: VSD in
standby) (VSD = Variable Speed Drive). When performing this measurement, only the low power
electronics of the frequency converter (VSD in standby) is fed, the inverter does not generate a PWM
voltage.
When switching on the inverter, a PWM voltage is generated which is an important source of conducted
and radiated emission. The radiated emission level increases considerably (red: VSD active). The emission
originates from the frequency converter itself and the three phase power supply cable.
Figure 18: Radiated emission of the frequency converter without motor cable and without motor
Figure 19 visualizes the radiated emission of the same frequency converter (bleu: only the VSD without
motor cable). By connecting the motor cable (still without induction motor), the emission level increases
slightly (green: VSD + cable). When this motor cable supplies an induction motor, the total emission of
the power supply cable, the frequency converter, the motor cable and the rotating induction motor is
visualized in Figure 19 (red: VDS + cable and motor).
Figure 19: Radiated emission of a frequency converter with motor cable and induction motor
References
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Kluwer, Antwerpen, 1990.
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PWM Inverter Fed AC Motor Drive System in a Frequency Range from 30 MHz to 200 MHz,
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the XXth International Scientific and Applied Science Conference: Electronics - ET 2011, Sozopol,
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an
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