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CHAPTER 4
ANALYSIS OF CONDUCTED EMI NOISE GENERATION
AND SUPPRESSION TECHNIQUES
4.1
INTRODUCTION
Rapid changes in voltage and current of a switching power
converter produce EMI noise transmitted in two forms: Viz. radiated and
conducted. The radiated EMI noise is emitted through free space to other
equipment, while conducted EMI noise is transmitted via the circuit
connection. The radiated EMI can usually be shielded by metal cabinets used
for housing the power converter and conducted EMI is reduced by using filter
to block its transmitting paths. However, because both the radiated and
conducted EMIs come from the same energy source and energy is dissipated
somewhere, blocking the conducted EMI can often lead to increase in
radiation. Therefore, an effective EMI suppression strategy has to rely on a
thorough understanding of the EMI mechanism, so that EMI will be reduced
at the source instead of only being blocked after it is generated. Since the
radiated EMI paths are difficult to define, a complete analysis of the EMI
mechanism based on radiation seems impractical Bertrand rovel et al (2011).
On the other hand, the conducted EMI should be helpful in locating the EMI
source and understanding the mechanism of EMI generation. Solution aimed
at reducing EMI generation at its source, will then become possible for
reducing simultaneously and effectively both the radiated and conducted
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EMIs. It is therefore the focus of this section is to analyze the mechanism of
EMI generation in AC motor drives based on conducted paths. The conducted
EMI can be generated by either coupling currents or inductive load current
switching.
With proper understanding of principles of EMI in motor drives, it
is not difficult to design a proper circuit for EMI suppression. It is important
to bear in mind that different sources of EMI require different circuit / filter
design and installations. A general rule of thumb for EMI suppression is first,
to provide a local path for the possible EMI current to circulate inside the
drive system and second, to increase the high frequency impedance between
the drive input and the AC power or battery source to block EMI currents.
This section serves to provide some general guidelines for EMI suppression.
A practical implementation requires the selection of a proper circuit and
parameters based on the encountered EMI characteristics.
4.2
EMI CAUSED BY CAPACITIVE COUPLING CURRENT
Recent studies have demonstrated the existence of the so-called
coupling current in PWM motor drive, in addition to the fundamental and
harmonic current components. A coupling current is the sum of currents in
parasitic capacitors when they are excited by the high dv/dt and high
amplitude square wave voltages. Although the coupling current does not
affect the basic functionality, it does produce certain unwanted second order
effects, such as the bearing currents and shaft voltages as well as the EMIs.
A coupling current is composed of two components: Viz. a
differential mode current and a common mode current. Both have to be
supplied by certain low impedance voltage source, either inside or outside the
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drive system. When it is drawn from the AC utility or the DC battery from
which the motor drive is powered, the coupling current shares the circuitry
with other equipment and becomes immediately a source of the conducted
EMI. Since, the differential mode coupling current travels in a different path
than that of the common mode, the levels of the EMI emission from both
modes of currents differ accordingly (Qiang Gao and Dianguo Xu 2007).
4.2.1
Differential Mode EMI is caused by Differential Mode
Coupling Current
The differential mode coupling current comes from the following
parasitic capacitors between inverter phases, such as C d which represents the
phase to phase parasitic capacitance of the motor windings. A high frequency
current is produced when the square wave line to line voltage energizes those
parasitic capacitors (Nobuyoshi Mutoh and Ogata 2004). Just like the inverter
load current, the current flows to source from the DC link. Assuming a stiff
DC link, most of the differential mode coupling current will circulate locally
in the loop formed by the DC link capacitor, inverter poles and motor as
indicated by the bold line figure 4.1.
However, as the impedance of the DC link is never equal to zero,
the DC link may not be able to respond to the full coupling current demand;
and a portion of the currents has to be drawn directly from the AC power
source through the rectifier side. Under such a condition, a portion of the
differential mode coupling currents must flow into the AC source as indicated
by the pink line in figure 4.1. This portion of differential mode coupling
currents flows outside the drive system and constitutes the conducted EMI
emission in differential mode.
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Figure 4.1
4.2.2
Differential Mode Coupling Current Contribution to
Conducted EMI
Common Mode EMI Coupling Caused By Common Mode
Coupling Current
The Common mode coupling current flows in parasitic capacitors
between drive components and earth ground, such as Cc which
represents
the capacitance between the motor windings and grounded enclosure. Unlike
the differential mode current, the common mode currents will not return via
the local path from the negative rail of the inverter poles to the negative DC
bus. Instead all of them flow into the ground and have to return via the ground
to the source. Assuming relatively high impedance between negative DC bus
and the ground, the main path for common mode coupling currents an inverter
drive with a rectifier input can be depicted as shown in figure 4.2.
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In addition, due to parasitic capacitance across the bottom rectifier
diodes, there may be some common mode currents which pass through the
DC link capacitor as indicated by the violet line. As this type of current
travels through the ground and the input power cable, any electronic devices
connected to the ground or the cable will be prone to interference from them.
In other words, the common mode coupling currents are definitely a source of
common mode EMI. Since the common mode current share most of their
paths with other equipment, the level of EMI emission from them is usually
higher than that from the differential mode coupling currents. It generally
constitutes the worst scenario for EMI suppression. Current measured
between lines to ground parasitic capacitance of the motor winding are shown
in figure 4.2.
Figure 4.2
Common Mode
Conducted EMI
Coupling
Current
Contributed
to
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In summary, coupling currents are the source of EMI. This type of
EMI contains the effects of both common mode and differential mode
currents. Since a coupling current governed by the capacitive coupling
impedance and its excitation voltages, the EMI generated is a function of the
parasitic impedance, common mode and differential mode voltages. In other
words, it is related to the dv/dt, DC link voltage amplitude and parasitic
impedance which is almost independent of the load conditions. Therefore, this
type of EMI can be considered as independent of motor load and operating
conditions.
In fact, the resonant frequencies of the parasitic coupling
impedance determine the EMI spectrum related to coupling currents. In a
typical induction motor drive, the capacitive coupling impedance of the
common mode path is very different from that of the differential mode path.
For example, an induction motor drive was seen to contain a common mode
current component of more than 10 MHz, while the differential mode
component has a frequency of several megahertz. This fact explains why there
is a large amount of EMI frequencies in the megahertz range, a phenomenon
not accountable by the PWM switching harmonics. In addition, the
differences in resonant frequencies make it possible to distinguish the
differential mode EMI from the common mode.
4.3
EMI NOISE CAUSED BY INDUCTIVE LOAD CURRENT
SWITCHING
The load current and PWM switching frequencies are known to be
major contributors to the EMI spectrum. In fact the load current switching
mechanism can lead to EMI produced in both PWM switching frequency and
load phase commutation frequency. Particularly, in each PWM switching
cycle, load current is transferred from one switch to diode in inverter. The
current drawn from the DC link must change abruptly. This process produces
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a pulsating DC link current in the PWM frequency with its amplitude
proportional to the load current. This pulsating current will find its way in to
AC/battery source and become a source of EMI.
4.4
COMMON MODE AND DIFFERENTIAL MODE NOISE
SEPARATION
Federal Communications Commission (FCC) electromagnetic
compatibility testing procedures do not require the separation of CM noise
and DM noise. However, it is necessary for us to better understand the
mechanism of noise generation and find an effective method to suppress the
noise with different characteristics. The concept of CM noise and DM noise
are shown in figure 4.3. The currents flowing equally in the two conductors,
but in opposite directions are known as the differential mode currents. The
currents flowing in the same direction along both conductors are defined as
the common mode currents. Common mode currents must flow through the
common ground of the conductors (Kempski 2005).
Figure 4.3 Common Mode and Differential Mode Noise Current
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Actually, there is only one current in one conductor. However, this
current can be viewed as the summation of DM current and CM current. In
figure 4.3 the I1 and I2 are the real currents flowing in the two conductors. V 1
and V2 are the real noise voltages across the 50W terminators provided by the
LISNs.
I1=ICM1-IDM
(4.1)
I2=ICM2 + IDM
(4.2)
V1=VCM1 - VDM
(4.3)
V2=VCM2 + VDM
(4.4)
Therefore, the CM current and voltage are defined as:
ICM = ½ (I1+ I2)
(4.5)
VCM = ½ (V1+ V2)
(4.6)
and the DM current and voltage are defined as:
IDM = ½ (I2- I1)
(4.7)
VDM = ½ (V2- V1)
(4.8)
Once the CM noise and DM noise are separated, different solutions
can be adopted to deal with different noises accordingly.
4.5
SUPPRESSION OF DIFFERENTIAL MODE EMI CAUSED
BY COUPLING CURRENTS
The differential mode EMI occurs only when differential mode
coupling currents flow into the AC utility or battery source. To limit its
generation, a low pass filter as shown in figure 4.4 is suggested to be installed
in the DC link between the rectifier battery and the link capacitor. Although
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this circuit looks like a filter, its function is to provide a local circulating
current to support the demand for differential mode current. In the circuit, C f2
is required to provide a local high frequency current source for the differential
mode coupling currents so that most of the currents will be forced to circulate
locally inside the drive. In the meanwhile, L and Cfl may be added to increase
the path impedance and block the EMI current drawn from the input side to
further reduce the level of EMI (Shaotang Chen 1999). Although similar
circuits can be installed in the AC input side, the one suggested, results in the
simplest structure. In case that the differential mode inductor L is needed, it is
important to choose the right magnetic cores to be able to attenuate the
megahertz components of coupling currents while maintaining a DC current
offset as rated.
Figure 4.4 Differential Mode EMI Filter in DC Link
4.6
SUPPRESSION OF COMMON MODE EMI CAUSED BY
COUPLING CURRENT
The common mode coupling current is a major EMI current
component (Zhang and Von 2002). Several approaches are proposed which
are summarized as follows:
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4.6.1
Local Circulation of Coupling Current
This approach provides a path for common mode current to
circulate locally between the inverter and the motor only and thus preventing
their flow into the battery and to the ground as shown in figure 4.5.
Figure 4.5
Circulation of Common Mode Coupling Current for
Reducing Common Mode EMI
A small capacitor Cn connected from the negative DC bus to the
ground will provide such a circulating path for the common mode current
assuming that the DC link has a high frequency capacitor C f to provide the
common mode currents (Ogasawara and Akagi 2001). A common mode
choke Lc inserted in the DC bus may be added to further enhance the
effectiveness of this method. In order to maximize local circulation and
minimize emission to the ground plane, it is suggested that the capacitor must
be close to where the motor is grounded.
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An AC version of the above suppression circuit is shown in
figure 4.6. Although not a preferred choice, the circuit will provide similar
capability in reducing common mode EMI when it is installed between the
utility and the inverter input. Although this method reduces EMI emission to
the utility side, it will increase the common mode coupling currents inside the
drive. It should be used only if a system is not sensitive to the relevant side
effects related to common mode coupling currents, such as bearing current
and shaft voltage problems. In practice, its use may also be restricted by some
related industrial regulations regarding leakage currents from an inverter to
the earth.
Figure 4.6 Common Mode Filters / Chokes
4.6.2
Shift Common Mode Current Spectrum
By using a common mode circuit as shown in figure 4.7, it is
possible to make the common mode EMI disappear from the measurement
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spectrum. This common mode circuit can be viewed as a lumped parameter
circuit equivalent to a transmission line (Yaxiu Sun et al 2006). With proper
circuit parameters, it is possible to change the characteristic impedance of the
transmission line seen by the inverter output when the circuit is connected the
output of the inverter and the input of the motor. This will result in different
ringing frequencies for the line and the coupling current frequencies will be
shifted to new band.
It is noted that the circuit will also affect the differential mode
coupling impedance and thus the current. Depending on the parameters, the
amplitude of the total coupling current could become considerably large
especially if the designed ringing frequency is low. The strategy is by no
means to eliminate or reduce the coupling current from the utility side but to
shift the frequencies to other un-harmful bands.
Figure 4.7 Circuit for Shifting Coupling Current Frequency
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4.6.3
Common Mode Voltage Cancellation
Another innovative solution to the common mode EMI is to use the
so-called common mode voltage cancellation technique. This method is to
cancel the common mode voltage at the source and thus eliminate all common
mode currents (Poon et al 2000). The concept of the common mode
cancellation first appeared and applied with certain success to motor bearing
current reduction. As bearing currents are essentially a type of common mode
current, the same techniques should apply to the common mode EMI.
4.7
SUMMARY
To understand the problems associated with the generation of
conducted EMI, its major causes like common mode capacitive coupling
current, differential mode capacitive coupling current and Inductive load
current switching were emphasized. The noise voltage separation using LISN
has been discussed.
The suppression techniques have been emphasized mainly, which
necessitates the need for local path for circulation of EMI current and also it
increases the high frequency impedance. The shift common current spectrum
technique gives rise to different ringing frequencies for the line and shift
coupling current frequencies to the new band. As the name implies the
common mode voltage cancellation, this method finds solution for the
cancellation of common mode voltage at the source side, which in turn
eliminates common mode currents.