EMI Generation ,Regulation and Control
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Transcript of EMI Generation ,Regulation and Control
Generation, Control and Regulation of EMI from AC Drives
G. Skibinski, J. Pankau, R. Sladky, J. Campbell
Rockwell Automation - Allen-Bradley Company
6400 W. Enterprise Drive
Mequon, WI 53092
(414) 242-7151 (414) 242-8300 Fax
Abstract: Adjustable Speed AC Drive (ASD)
manufacturer’s recently migrated from Bipolar Junction
Transistor (BJT) semiconductors to Insulated Gate
Bipolar Transistors (IGBTs) as the preferred output
switching device. The advantage of IGBTs is that device
rise / fall time switching capability is 5- 10 times faster,
resulting in lower device switching losses, a more efficient
and smaller drive package. However, faster output dv/dt
transitions and higher drive carrier frequencies increase
the magnitude of Common Mode (CM) electrical noise
and Electromagnetic Interference (EMI) problems.
Experience suggests all PWM drives with steep fronted
output voltage waveforms have these problems. This
paper provides a basic understanding of EMI generated
by these drives solutions to control EMI, as well as
regulation standards on allowable conducted and radiated
emissions to insure a successful drive system installation.
I. INTRODUCTION TO EMI NOISE
Electromagnetic Interference (EMI) noise is defined as
an unwanted electrical signal that produces undesirable
effects in a control system, such as communication errors,
degraded equipment performance and malfimction or non-
operation. References on the general principles of EMI are
available [1-3], as well as methodologies on calculating
radiated emissions [4]. IEEE Std. 518 applied these principles
to slow switching SCR DC drives in 1982 [5]. All ac PWM
drives have the potential to cause EMI with adjacent sensitive
equipment, when large quantities of drives are assembled in a
concentrated area [6- 10]. However, faster switching speeds
of new converterlinverter topologies require an updated study
of new system EMI problems created.
A. What is Common Mode Noise?
Common Mode (CM) noise is a type of electrical noise
induced on signals with respect to a reference ground. CMnoise problems imply a source of noise, a means of coupling
noise by conduction or radiation and circuits / equipment
susceptible to the magnitude, frequency and repetition rate of
the noise impressed. Each aspect of the noise problem is
covered in detail, starting with the effect of CM noise on
susceptible circuits.
B. Susceptible Equipment, Circuits& Systems
Fig. 1 shows potential CM noise problems increase with
susceptible equipment present, system input voltage, system
drive quantity, and, length of motor leads. Other factors are
type of ground system and cabinet layout practice.
Susceptible equipment may be computer systems,
communication links, ultrasonic sensors, weighing and
temperature sensors, bar code/vision systems, and capacitive
proximity or photoelectric sensors. Control interfaces include
encoder feedback, O-10 Vdc, and 4-20 mA signals.
Higher system ac line voltages have higher dc bus
voltages ( V~US).The higher output switching dv/dt increases
peak CM ground current (i = C~traY dv/dzj. Increasing drive
quantity increases the sum total of transient CM noise current
to ground. Higher drive carrier Iiequency WC), increases the
number of switch transitions and sum total of CM noise
current.
Motor cable lengths <20 ft exhibit low cable line to
ground capacitance and low CM noise risk ffom capacitive~edium R&
Figure 1. Applications with potential problems
07803-4070-1/97/$10.00 (c) 1997 IEEE
dv/dt ground currents. As cable lengths increase, cable
capacitance increases and CM charging current to ground
increases. At long cable lengths, the high frequency
oscillations of reflected wave voltage transients (-2 V& ) also
appear on motor terminals, to create CM ground noise
current through the stator winding and cable capacitance [7].
EMI mitigation must involve a discussion of safety
equipment ground, signal grounding and the effect of
grounding system type on CM noise.
C. EMI & System Safety PE Ground
Drive Power Equipment (PE) terminal in Fig. 2 serves as
equipment safety ground. Drive metal is bonded to PE, since
ungrounded metal accumulates electrical charge thru leakagecurrent that may exceed 50 Vdc (a safe touch potential).
Cable conduits, armor or cable trays should be bonded to the
cabinet, since it is shown later that these carry high frequency
noise currents. Drive PE, mounting panels and cabinet are
then bonded to system PE copper bus and connected (ground
conductor sized per NEC code) to True Earth (TE) zero
voltage ground such as building structure steel to insure safe
touch potential exist under ground fault conditions.
Drive logic common may go to PE or a separate isolated
TE bus in the cabinet that is single point connected to TE
ground along with the PE wire. This TE installation reduces
effects of PE noise between multiple drives and maintains
drive logic and susceptible interface equipment commons
close to TE potential.
D. EMI & System Signal TE Ground
Inside buildings, zero voltage or TE potential may be
obtained at structure steel, since steel girder grid connections
provide multiple paths to ground. Ground resistance is
affected by soil resistivity and dependent on moisture
content. TE may be low impedance until summer when
ground water tables dry up. Multiple ground rods in low
resistivity soil may be an adequate low impedance for 60 Hz
safety and signal ground for high frequency EMI noise
current. However. instances of ground rods driven in plant
IllIOIIIm,m Mode Vdtw !’,. —
/////////////////////////////
CO,”mm ,1=.!. C.rrc.l l.O
G.m,d P*C”,!4 #l (hnmd Ptien,i81 #2
Figure 2. CM current and CM voltage in safetyPE & signal TE grounds
floors have exhibited 1,000-5,000 Q between rod and
building steel, due to dry rocky soil under the building.
As shown in Fig. 2, often there are hidden CM ground
currents (lao) passing through a ground system from
Potential #1 ( Vl) to TE structure steel Potential #2 (VJ. VI
will have a high noise voltage relative to TE, if the ground
system impedance is high. A CM noise voltage VIZ is set up
between the two grounds. Drive logic common (tied to V])
and susceptible interface equipment common (tied to V2),
will have a CM noise voltage ( V12) to degrade the signal
interface. In real sites, finding the best ground point, with
drives located across a plant, is difficult. A good ground
system is essential for safety and noise free signal grounds.
E. EMI & Ground Philosophy: Ungrounded High, Solid
System grounding philosophy for multi-driveapplications is specified by users and based on concerns other
than EMI. An advantage of grounded wye systems in Fig. 3
is typical 20 dB attenuation of primary line to ground voltage
transients. However, it is shown that a wye secondary with a
solid ground neutral detrimentally completes a transient CM
noise current return path from the drive output to the ground
grid and back to drive by the ac input leads. CM current is
highest with grounded systems, but the noise loop is
contained at the transformer neutral (Xo) and noise does not
progress into the primary PE grid.
In Fig. 3, the high resistance ground system adds 150-
300 Q in the secondary Xo to ground circuit. Attenuation of
primary line to ground voltage transients is acceptable. This
resistor is now in series with the CM noise current return path
and significantly reduces peak CM current, so CM potential
differences across the ground grid become smaller.
A disadvantage of ungrounded systems is that primary
line to ground voltage transients are passed directly to the
secondary without attenuation. Safety concerns must also be
addressed with this system. However, the return path of CM
noise current path back to the drive input is broken, so CM
=z$)---%Ungrounded System
zPE
1 PE
Figure 3. Grounding philosophy affects system EMI
07803-4070-1/97/$10.00 (c) 1997 IEEE
I+:, ‘p-- ‘1? k’- *
I tao
v 1-2
‘+—---’Figure 4. Noise source: Drive induced CM current & voltage
noise current does not exist in the ground grid.
II. AC DRIVE AS AN EMI NOISE GENERATOR
A. Drive CM Voltage Inducing Common Mode Current
A PWM output voltage has abrupt transitions to and
from the de bus, essentially controlled by semiconductor
switching time and which are inherent sources of radiated
and conducted noise. Voltage transition time determines an
equivalent noise coupling fkequency fn = 0.318 / tri~e.IGBT
risetimes (tri~e) are 0.05 -0.2 ps, while BJTs are 1 -2 ps,
corresponding to fn of 6.4- 1.6 MHz and 320-160 kHz,
respectively. Output dv/dt is now 20 to 40 times higher.
Most drive related EMI is due to conducted noisecurrents in Fig. 4. Line to ground capacitance Cl.g of cables
and motors interact during positive or negative dv/dt
transitions to generate high frequency transient phase to
ground noise currents (Iao, Ibo, ICO) referred to as common
mode (CM), or zero sequence currents. Peak lao magnitude is
approx. (Cl.g ) times ( V1.g/ tJ and may reach 20 Apk.
CM noise current magnitude increases with faster tri~e
and higher bus voltage. Increasing drive carrier frequency
tic) increases EMI, since the CM current repetition rate is
faster. Higher localized drive quantity increases CM ground
current at an application site.
B. Conducted CM Current Inducing CM Voltage in Ground
A transient high frequency CM current path exists in Fig.
5 from each drive output phase during switching, thru stray
cable and motor CZ.g capacitance, into ground Potential #l
(Vl) and thru the ground grid to ground Potential #2 (Vz).
Ground grids are high impedance to CM high frequency
Unshielded Phase Conductor of Drive
-m IIao I
t-_T %g Critical Distance
I send ~f-2 ~J
Receive
I. ..--+ ----- ,----
1
●++*+* ***m ●*+ m+m●*T* ●*I ●**
I
is
i5. . . 4. . . . . . . . . . ...4... = . . . . . . . .-.--~ ----a k--
I ~4-2 I
ICommon Mode Vottage V
l=m
I . . L----.-k-=+ , ‘E
k% 2-------------- *
Common Mode Currmt Iao
Ground Potential #1 Ground Potential #2
Figure 5. CM current inducing CM voltage
noise current, so an instantaneous voltage difference ( P’Z2-
known as common mode noise voltage) exists across the
ground grid.
A (Send/ Receive) susceptible interface circuit, with
source and return signal current is, is referenced to TE zero
voltage ground (via building structure steel) at V., while the
Send end is referenced to noisy VI ground. Thus, CM noise
voltage impressed on both HI and LO signal lines, allows a
CM noise current ii-2 to appear in the same direction on both
lines and circulate back through ground. The signal may
develop a noise voltage due to ii-z. The interface equipment’s
ability to tlmction in the presence of high ffequency noise
depends on it’s Common Mode noise Rejection Ratio
(CMRR) threshold tested at noise fiequencyfm
III. SYSTEM NOISE COUPLING PATHS
A. Critical Operational Distance vs. Ckl Current Risetime
If both VI and V2 of Fig. 5 were maintained at TE
potential, then V1 =V2 = O and V12 = O, eliminating the signal
noise. Susceptible circuits may fimction with lao ground
noise present, if both V] and V2 have the same magnitude and
phase waveshape. In this case both VI and V2 are not= O, but
V12 -O, so the minimal noise present is rejected by the circuit
CMRR. Thus, high peak l.O ground currents with slow
risetime noise may still have V12 -0, depending on distance
separation. Low peak ZaOwith fast 50 ns risetimes may have
large instantaneous voltage differences at either end, even for
short ground distance separation.
The term (U8) defines a maximum critical distance lC
where magnitude and phase relationships are equal, such that
VZ2 -0 between two separated single ended interface circuit
grounds. Wavelength (1) in meters is calculated ash= c /fn,
where c = 3.108 m/s and fn is in Hz. Fig. 6 shows the lC chart
07803-4070-1/97/$10.00 (c) 1997 IEEE
700
““”l
Region Suseqttible600 toCMNoise500 / E’!
“0.01 0.1 1 10“
Drive Output Voltage Risetiu CM Noise (uS)
Figure 6. Interface distance vs. CM voltage risetime
for various PWM voltage risetimes.
Consider an IGBT drive with tri~e= 100 ns, logic
common to noisy PE, connected to a O-10 Vdc single ended
WO wire interface circuit of 200 ft length, and with receive
end referenced to a different TE ground. Fig. 6 shows there is
a possibility for CM noise voltage interference with these
conditions after 40 ft. In contrast, a BJT drive with tri~eof 2
p has VIZ -0 and minimal CM noise up to 900 tl of
interface length.
This chart applies to single ended systems and does not
imply equipment will not operate properly above lC if
systems containing CM filters, galvonic or optically isolation
or differential circuits are used.
B. CM Current Capacitively Coupled to Signal Voltage
High dv/dts from drive unshielded output leads in Fig. 7
will capacitively couple lao thru stray capacitance Cl~ onto
both signal lines in close proximity and produce an error
voltage depending on load impedance balance. Worst case
~0 - (Cls ) (Vi-g I A %.,), where CI, is proportional to thez
length of parallel power and
distance.
Standard noise reduction
Twist signal leads together to
signal leads and separation
solutions available are: (1)
provide balanced capacitive
Unshielded Phase Conductor of Drive
9 -99-- --Iao [~ l~c
I %s[ 1-s
L 4. ~. .R-m. . . . . . . . . . . . . . .* . . ..- ■ . . . . .
i ; 4.... .<.. .. .J%-.=.=J. &-_ . . . . .
Lo b-y D- “- -
ao
I II
A’ n!’~ro””d,Own;i,Ground Potential #l
TE
Figure 7. CM current capacitively coupled to signals
Unshielded Phase Conductor of IMve
Critical Distance
%.
I H!--- q -.-,
120 VAC U ‘-Interface u.Power Lads
:*”=ommon Mode Voltage V
:.,. (. -....4
,-’
Ground Potential #1 ‘ Ground Potential #2
Figure 8. CM current capacitively coupled to interface power
coupling C1~ to each signal lead, (2) Shield signals so
electrostatic coupled noise currents flow on shield to ground
instead of signal leads, (3) Separate control ffom power wires
in open air, conduit or cable trays, (4) Use shielded power
cables.
C. CA4 Current Capacitively Coupled to Interface Power
In Fig. 8, unshielded 120 Vac power leads in a conduit
or cable tray with unshielded drive power leads cause EMI
problems when dv/dts of 10,000 V/Vs or greater are present.
High dv/dt from drive leads capacitively couple to 120 Vac
power leads and through susceptible load power supply
capacitance, to impress noise voltage on W and LO signal
lines at TE.
D. Noisy Shield Ground
Signal shields reduce external electrostatic coupling but
may introduce EMI, if the shield is connected to a noisy
ground potential. As discussed, drive dv/dt at “noisy” VI
creates a transient CM ZaOpath to “quiet” V2 and induces a
—Ton
r1I1IIIII
Unshielded Phase Conductor of Drive
‘Cpg Critical Distancesend ~ Receive
..-[ ~,~ J-------- ---------/m//////////////////////////////N/
k 7!7 /79. - 9 ----------- b
Common Mode Currwst ~Ground Potentiat #l r!hmd P#entkd #2
Figure 9. Noise coupling: Noisy shield ground
07803-4070-1/97/$10.00 (c) 1997 IEEE
VIZ CM noise voltage.
Shield connections to noisy VI potential in Fig. 9 cause a
CM current i12 path thru shield capacitance C~.H1 & C~-Lo
creating susceptible load noise. Current i12 continues thru
zero voltage ground V2 and back to VI. Load noise due to
shield induced noise is verified by removing the shield
ground.
Solutions include: (1) Galvonic or optical signal isolation
modules, (2) Inductance on power leads to reduce IaO
risetime to ground, so noisy VI is closer to quiet V2 potential
and V12 -0, (3) CM choke on both signals and shield at
SEND end. CM choke inductance in the i12 ground path
reduces the effect of V12 dv/dt reducing i12 coupling through
C~-H1 & C.-LO , reducing susceptible load noise. CM cores
do not affect line to line signal quality.
E. Noisy Source Ground
Signal shields reduce external electrostatic coupling but
still may introduce EM1, if the shield is connected to a noisy
ground potential to TE ground potential, while interface
equipment source is referenced to Fig. 10 noisy ground. The
fast di/dt edges of CM l.O current set up a high dv/dt V12
voltage as demonstrated before. The ilz paths due to non-zero
V12 are shown in Fig. 10. Noisy VI end in Section III-D had
a metallic shield path to couple noise in the entire length of
signal cable, while now noisy VI end must first get through
the Send end power supply ground impedance, so that noise
levels will be lower with this configuration.
Previous solutions also apply in this case. Signal quality
may be improved by grounding the shield at both ends in
cases of CM noise with fast rising edges or high frequency
ringing. Shield low impedance co-axial braid, parallels the
high ground impedance between V12, but forces VI - V., so
CM noise voltage VZ2 -0. However, interface grounds ride
~iII11I1I
Unshielded Phase Conductor of Drive
c l-g Critical Distance
_S&oL!!~ ~~
bm9---m---------A7 Common Mode Crrrnmt 110 A“
Ground Potential #1 Ground Potential #Z
Figure 10. Noise coupling: Noisy source ground
Unshielded Phase Conductor of IWW
‘ao h hGround Potential #1 Ground Potential K!
Figure 11. Conducted CM current creating radiated emissions
up and down with identical noise voltage, so coupled noise
into differential signal leads is minimal.
Disadvantages of multipoint ground schemes are VI to
V2 ground loops may produce high shield current limited by
shield resistance and “quiet” zero voltage ground V2
becomes polluted with “noisy” VI ground voltage and affects
other sensitive equipment tied to V2.
F. Conducted CM Current and Radiated Emissions
Unshielded drive wires act as antennas for the electric
fields set by the steep dvldt of the PWM output voltage.
Radiated emissions occur at llntri~e and its higher harmonics.
Unshielded drive input / output cables carrying CM ZaOmay
act as loop antennas for radiated emissions, due to the current
path in these wires returning via the ground grid in Fig. 11.
Drive CM output cores and conduit, armor or shielded cable
solutions substantially reduce radiated noise, but full
compliance to FCC / European CE regulations may require
EMI filters.
G. Noise Coupling Paths in a Drive System
Fig. 12 shows system CM noise current paths taken
when poor wiring practice using three unshielded phase
output wires, randomly laid in cable tray, and a local motor
Frame
Build
Figure 12. Noise paths due to poor wiring practice
07803-4070-1/97/$10.00 (c) 1997 IEEE
ground wire to the ground grid is used. Transient CM current
IaO is sourced tiom the drive during an output voltage
transition, e.g., phase “A” IGBT turns on to (+) dc bus. ZaO
current couples through cable capacitance to the grounded
cable tray at Potential #2 and capacitively couples through
the motor stator winding capacitance into Potential #3 PE
ground grid via the motor ground wire. Conducted CM
current continues through the ground grid bypassing drive PE
until returning at the feed transformer secondary grounded
neutral XO, where a low impedance path back to the drive
source can occur on phase A, B or C. Inside the drive, the CM
current selects the bridge rectifier diode that is conducting
back to the (+) dc bus source. Building structure steel
provides a True Earth (TE) ground for the solidly grounded
transformer neutral.
The ground grid is a high impedance to high frequency
ground noise current IaO , so that an instantaneous voltage
difference, known as CM noise voltage, is created across the
ground grid Potential #1 through Potential #4. CM voltage is
impressed on susceptible interface equipment between drive
logic ground Potential #I (which is noisy compared to
structure steel) and interface ground Potential #4 (referenced
at zero voltage TE potential). Common mode voltage is also
impressed between the encoder case at Potential #3 and drive
PE logic ground Potential #1. Successful encoder operation
depends on how much CM voltage is capacitively coupled
from the noisy encoder case into encoder circuitry. The chart
of Fig. 6 may help determine probability of CM problems.
Additional equipment users referencing to ground grid
potentials VI, V2 and V3 may also experience CM voltage
problems. Ability of interface equipment to fi.mction in the
presence of noise is ultimately determined by it’s CMRR
threshold tested at noise ~n. Poor wiring practice (shown in
Fig. 12) also exemplifies the radiated emissions problem. A
system loop antenna is formed between both drive output /
input wires and return ground grid. Thus, a better wiring
practice is desired prior to drive installation.
IV. NOISE COUPLING DEMONSTRATION
This section shows the advantageous effect of insuring
solid PE panel grounds, using proper shield grounding
techniques on signal interfaces, and using drive CM cores.
2k
(m)TE Potential 2
Figure 13. Single ended interface circuit tested
A. Noisy Source Ground
In Fig. 13, the ASD Analog Out lCommon is connected
with a 200 ft, twisted, shielded pair to a 2 kQ single ended
load. Load Common is bonded to remote building structure
“quiet” TE potential. A Noisy Source Ground potential for
drive logic common was created with a 600 tl drive PE
Ground wire. This creates a high inductance ground to high
frequency CM transient current. Signal cable length exceeds
Fig. 6 (Critical Interface Distance for IGBT risetimes) so CM
voltage V12 is impressed on single ended signal V~ = 10 Vdc.
Source ground Potential #l is noisy, while receive ground is
TE zero voltage Potential #2. Table’ I shows pk-pk noise
voltage on signal VS for various shield terminations and
configurations.
Table I. Noise Voltage on Signal Voltage
Shield Noisy Source Noisy Shield DriveConnection Ground Ground CM Core
I WPP) (Vpp) WPP)Drive 30 I 26 8Open 16 14 6
Both 5 4 0.2Load 8 4 0
Shield connection options as demonstrated in TABLE I,
are not effective if interface distance is long and drive logic
PE source ground is noisy due to high inductance or high
impedance PE ground. Bonding shield ends to both Send /
Receive commons through the low impedance shield brings
these potentials closer in instantaneous magnitude and phase.
CM voltage on V. is reduced ( VZTO), even though both
grounds are not at absolute zero potential. However, shield
currents may flow and TE ground is now polluted for other
users.
B. Noisy Shield Ground
Section IV-A conditions were repeated with a 50 fl PE
ground to plant grid as in Fig. 14. Shield connection to noisy
No Shield
oShield ondrive sideonly
oWieldconneciedto both
o sides
Shieldo connected
to load side.Only
10 V/Div. 500 P @iv.
Figure 14. Noise demonstration: Noisy shield ground
07803-4070-1/97/$10.00 (c) 1997 IEEE
Ov Shield Open
Shield
Ovconnectedto drive
Ov Shield connectedto both sides.
Shield connectedOv to load side.
10V/Div. 500ps/Div.
Figure 15. Noise demonstration: CM core solution
drive PE ground impresses CM voltage on V. as before.
Shield connection to “quiet” load side TE ground vastly
reduces CM noise.
C. Equalizing Grounds with CM Core Solution
Section IV-B conditions were repeated with a CM core
added on the drive output leads in Fig. 15. This reduces CM
ZaOrisetime to 2 ps. Using Fig. 6,2 MSrisetimes indicate CM
noise is not an issue up to 600 ft of interface cable. CM Noise
is now significantly reduced for open shields or drive end
shield connections. CM cores allow instantaneous PE & TE
potentials to track each other ( VIZ-O). CM noise is eliminated
with load side shield connections, without disadvantages of
multipoint shield bonding.
V. SOLUTIONS TO CONTROL EMI
There are four basic steps to the philosophy of noise
mitigation and abatement that are discussed.
(1) Proper grounding
(2) Attenuate the noise source
(3) Shield noise aw~fiom sensitive equipment
(4) Capture and return noise to the source ( drive)
A. Proper Grounding
Figure 16, Drive cabinet grounding
to the cabinet frame, Programmable Logic Controllers (PLC)
or other susceptible equipment. All metal is bonded to PE
ground bus for fault safety. Two choices exists for
instrumentation and drives with TE commons. TE & PE
buses may be tied together at one point in the control cabinet
or brought back separately to the PE ground point. Motor
cable fourth green wire meets NEC requirements for
grounding motors. Some high hp motors with very long leads
sometimes are additionally bonded to nearest low inductance
ground, since ground wire “inductance” and high motor Csg
winding capacitance may allow voltage buildup under PWM
operation.
(3) Drive Panel Layout & Susceptible Equ@ment: A PLC
chassis fi-ame is also it’s logic common. PE panel layouts that
route high fi-equency CM noise current, returning on both
conduit/armor and motor ground wire, are important factors
for reducing PLC backplane noise and preventing CM noise
interface problems with external equipment at other ground
potentials. Grouping input and output conduitiarmor to one
side of the cabinet and separating PLC and susceptible
equipment to the opposite side will eliminate CM noise going
through the PLC fiarne as in Fig. 17. CM noise returning on
output conduit or armor will flow into the cabinet bond and
exit through the adjacent input conduitJarmor bond near the
cabinet to find the transformer Xo neutral. Thus, proper panel
Common ModeCurrent on Armor
The importance of ground system selection, single point
grounding, and drive / equipment panel layout grounds as
related to CM noise are discussed.
(1) Ground System: Fig. 12 shows system CM noise fi-om
the drive output returning through the solid ground neutral of
the drive feed transformer. Thus, use of a floating secondary
will reduce the metallic conduction path and CM noise
magnitude. High resistance grounding leaves a conducting
noise path but greatly attenuates CM noise.
(2) Single Point Grounding /Panel Layout: Fig. 16 shows a
system single point ground scheme with drives in a cabinet,
recommended input / output conduit or armor cable bonded
‘yop;:dlu:
Steel if Required
Figure 17. Cabinet layout with drives & controls
07803-4070-1/97/$10.00 (c) 1997 IEEE
Dnd
1 Y-1///////////
A’ mGroundPotentiaI#l Ground Potentiat W
Figure 18. CMcoresolution forpower /signal leads
layout insures noise isawayfiom sensitive equipment. CM
current on the return ground wire tlom the motor will flow to
the copper PE bus and backup the input PE ground wire, also
away from sensitive equipment. If a cabinet PE ground wire
to the closest building structure steel is necessary, then a right
ide wire under the conduits and drives will shunt CM noise
away from the upper left PLC backplane of Fig. 17.
B. Attenuate the Noise Source
The best way to eliminate system noise is to attenuate it
at the drive source before it enters a system grid and takes
multiple high frequency “sneak” paths, which are difficult to
find in installations. CM chokes on drive output and CM
cores on interface equipment in Fig. 18 are highly effective in
reducing CM noise and ensuring filly operational tripless
systems in the medium to high risk installations of Fig. 1.
(1) CM Chokes on the Drive Output: Common Mode Chokes
(CMC) are inductors with phase A, B and C conductors
wound in the same direction with one or more turns through a
ferrite or common magnetic core. Typically, one or more
toroid shape cores in a stack. Drive PWM output voltage
transitions of 50-100 ns do not change when a CMC is added
to the output. However, the CMC provides a high inductance
(high impedance) to the line to ground noise current
generated during PWM high dv/dt voltage transitions.
Magnitude and risetime of CM noise current is substantially
reduced below equipment noise thresholds. Voltage
waveform quality of line to line output is unaffected, while
ground based noise is “choked” off. CMCS are physicallysmaller than three phase line reactors. Line reactors reduce
both line to ground and line to line capacitive coupled noise,
but phase inductance reduces fimdamental motor voltage and
Inverteroutput
voltage
Mode
Current
-20 Apk
+6 MHz VW
Current ~ - 1.st. 5us + 1PEAK
With i ICommon /
Mode I~7
W SPECTSUM
Chokas ‘n
~
1/3 I ~mK
Figure 19. Effect of CM core of system Iao noise
available motor output torque.
Typical CM high fkequency line to ground current
magnitude in Fig. 19 is substantially reduced from 20 Apk to
<5 Apk, as well as the rate of rise (di/dt) which is limited by
CMC inductance. Fig. 19 shows CMC peak ground current
now occurs at 5 ps at a di/dt rate of 1 Alps versus 100 ns at a
di/dt rate of 200 Alps without a CMC. The ground grid is a
high impedance to the 100 ns high peak current creating large
instantaneous CM voltage differences. However, with a CMC
reduced ground current magnitude and low di/dt rate
maintain ground potential difference fluctuations close to
zero voltage or TE ground. As a result, common mode
voltages are reduced and error free operation of an ASD,
interface, and sensitive equipment is possible. A CMC
inserted in Fig. 12, would reduce voltage differences between
drive Potential #1 and interface Potential #4 several hundred
feet away and thus reduce CM noise.
(2) CMC on 120 Vac and Drive Signal Interjace: A CMC
around drive HI-LO signal interface lead and shield in Fig. 18
has been shown to be beneficial in reducing CM noise
voltage on signal level components. CMCS around the 120
Vac power feeding susceptible interface equipment may also
reduce EMI interference, if lead separation from unshielded
drive output leads is not possible.
C. Shield Noise Away from Sensitive Equipment
After high frequency CM noise is attenuated with CMCS,
the third mitigation step is to control the noise path taken,
done by diwting the noise away from sensitive equipment
referenced to ground. Spacing control and signal wires apart
from high dv/dt power wires is a good practice and will
07803-4070-1/97/$10.00 (c) 1997 IEEE
reduce the capacitive coupling problem. Predictable noise
control from power wires is best done using four conductors
in a conduit, or better yet a four conductor shielded / armor
cable with an insulated PVC jacket.
(1) Better Wiring Practice: Three Conductors plus ground in
Conduit: Fig. 19 shows this condition with transient CM
current ZaOsourced from the drive as before. The conduit is
bonded to the drive cabinet and motor junction box and the
green ground PE wire is connected to the drive cabinet PE
bus and the motor ground stud. Part of l.O flows through
cable capacitance to the grounded conduit wall and part
through motor stator winding capacitance to frame ground.
The green wire and conduit absorb most of this capacitive
current and return it back to the drive out of the ground grid,
thereby reducing “ ground noise” for the drive to motor run
shown. However, conduits may accidentally contact the
ground grid structure due to straps, support, etc. AC
resistance characteristics of earth are generally variable.
Thus, it is unpredictable how noise current divides between
the wire, conduit wall or ground grid. Thus, inadvertent
conduit grounding at Potential #2 will induce CM voltages
for users referencing this node in Fig. 19. Also, if drive PE
cabinet wire is grounded to building structure steel, then CM
currents returning back from the motor conduitignd will go
into the ground grid at Potential #l, through feed transformer
Xo and back to the drive through input phase conductors.
CM voltage problems may still exist for susceptible
interface equipment referenced between Potential #l or
Potential #2 (which are noisy compared to structure steel)
and interface TE zero voltage ground Potential #4, dependent
on Fig. 6, the drive risetime vs. critical interface distance
chart. Thus, 3 wire plus gnd wire in a conduit from the feed
transformer source is recommended with conduit and green
wire bonded to secondary Xo neutral and another wire from
Xo to the ground grid structure. This presents the CM noise
current a low impedance predictable metallic return path out
of the ground grid. Locating the drive isolation transformer
closer to drive cabinet will shorten ground noise current paths
and help contain noise. Using CMC in high risk applications
eliminates concern over noise leakage to ground through
AR~M,R
DRIVE FRAME SHIELD MOTOR
Jr 4 J # PE GRID:Jgm@l PE TIE IN PE TIE IN
USER #2 USER #n
Figure 20. Solution: Shield controls EMI noise path
accidental conduit contact.
(2) Shielded Cable Controls Conducted Noise Current Path:
Shielded / armor drive output power leads in Fig. 20 reduce
the amount of capacitive coupled CM IaO ground current
flowing in a ground grid system, where conducted EMI noise
problems can occur. Shielded or armor cables with insulated
outer jackets, on both output and input sides, provide an
isolated predictable metallic CM noise current path to and
from the drive, so noise is not re-introduced back into the
ground grid by accidental contact.
High ffequency CM line to ground currents (1=0, Zbo, l..)
sourced from the drive during PWM voltage transition have
three return path options back to the drive, the 60 Hz green
safety wire, the cable shield/armor or customer ground grid.
Predominant return path is the shield/armor, since it is the
lowest impedance to the high frequency noise. The
shiekl/armor is isolated horn accidental contact with grounds
by an insulating PVC outer coating so that the majority of
noise current flows in the controlled path of the cable and
very little noise goes into the customer PE ground grid. Thus,
ground potential differences are minimized between true
building structure earth ground and customers grounding at
Users #2 and User #N points.
Noise current returning on the shield or safety ground
wire is routed to drive PE terminal, to cabinet PE ground bus,
out the cabinet PE ground wire, to customer ground grid at
User #1 and then to source transformer Xo grounded neutral.
Noise return path back to the drive dc bus source is via input
phase A, B or C , depending on which bridge diode is
conducting. If drive feed transformer is far away, then
ground grid pollution at User #1 exists and use of drive input
shielded power cables back to the main supply is desirable.
At short output cable lengths, 50% of return noise
current flows through the safety ground wire path and 50°/0
thru the shieklhrmor. At long cable distances, the safety
ground wire inductance looks like an open circuit to high
frequency noise and 95% of total noise current flows in the
shield and 5°/0 in the customer grid in Fig. 21. Zero sequence
Iao, Ibo, Ico source currents return in the opposite direction on
SHIELD ~ X=lo REIURNw D .10 aRCE
COAXIAL LCWINOUCTANCE STRUCTUREFOR ZERO SEQUENCE CLWRENT
r--l” SHIELD PREDOMINATES 95% 10
IDMVE lao-- 1 AC MOTOR
lxl---,Y
Figure 21. Shield controls conducted & radiated noise
07803-4070-1/97/$10.00 (c) 1997 IEEE
the shield braid/armor to form a coaxial low inductance
structure. Continuous welded aluminum armor was found to
have lower zero sequence inductance than interlocked armor
cable. Thus, the shield is the predominant conducted high
fi-equency noise return path as compared to the customer
ground grid. Thus, the use of CMC to attenuate the noise
combined with drive input and output shielded/armor cables
to control the noise path are effective noise reduction
mitigation methods.
(3) Shielded Cable /Conduit Control Radiated Emissions:
(a) Magnetic Field: Drives generate perfectly balanced phase
voltages so that fundamental frequency phase currents are
also a balanced set, e.g. la + zb + ZC= O. External magnetic
field emissions radiated from a shielded cable are minimal
since fundamental frequency currents sum to zero and 95°/0
of the high frequency zero sequence currents sourced by the
drive return in opposite direction on the shield. Thus loop
antenna area between magnetic galvanized steel or aluminum
armor selection is not critical, since cable currents are almost
balanced. Magnetic field emission efficiency is also reduced
with shieldlconduit systems, since drive output CM current
returns in a small loop area, either to the green wire or
armor/conduit wall.
(b) Radiated Field: Electric field emissions radiate
perpendicular from phase conductors and are completely
attenuated with continuous welded galvanized steel or
aluminum armor type MC cable for frequencies ffom the
drive carrier frequency up to the 6 MHz noise current
frequency Jn. Thus, the capacitive coupling noise to signal
and control interface is reduced. Braided shields and conduit
wall systems are also effective in attenuating emitted electric
field noise.
D. Capture and Return Noise to the Source (drive)
The fourth mitigation step is to capture and return the
noise back to the drive source. Shielded cables or conduit
returns noise out of the ground grid and back to drive PE as
shown in Figs. 19 and 20. CM capacitors connected from
drive PE to drive input lines or from PE to (+) and (-) dc bus
terminals act as high frequency noise bypass capacitors. They
short circuit the noise path from drive PE through the ground
grid and to transformer Xo connection. They are used in
extreme cases of CM noise problems.
VI. REGULATIONS FOR EMI COMPLIANCE
A. How Do EM1 Filters Work ?
Proper grounding and cabinet layout, proper shield
,.G . . . .
Y,%*’”’PE I m
Figure 22. Filter controls EMI path& magnitude
termination of control wire, use of shielded input/output
power cables and CM cores on drive power and drive
interface leads fix the majority of drive EMI problems.
However, an additional EMI input filter may be required to
reduce EMI conducted and radiated emissions low enough
for European CE Class A and Class B conformity standards
or for drives installed in residential areas where potential AM
radio and TV interference problems exist.
Previously, CM line to ground current Zao was shown to
be transiently sourced from the drive output during inverter
semiconductor rise and fall times, with ZaOreturning via the
ground grid to supply transformer X. connection and back to
the drive, via one or all of the three phase input lines. CM
cores on the drive output reduced lao peak and slowed the
effective di/dt risetime to ground. Shielded drive input cables
to transformer supply X. and shielded output motor leads
collected most of Iao and kept it out of the ground grid where
CM voltages maybe developed.
An EMI filter plus output shielded cable of Fig. 22 work
on the same series path described. However, instead of a high
impedance CM core to limit ground current at the drive
output leads, the EMI filter contains a large CM core
inductance and individual phase inductors that are high
impedance “blockers” to limit the high frequency series
ground return current to extremely low values in the ac mains
supply. EMI filters also contain CM line to ground capacitors
which fimction as low impedance bypass capacitors to re-
route most of the high ti-equency ground noise current Iao ,
returning on the output shielded cable, back to drive ac input
R,S,T terminals and out of ground grids.
Line Impedance Stabilization Network (LISN)
equipment at the EMI filter input detects noise voltage ( Vn )
developed in the plant ac mains supply. LISNS measure CM
noise voltage, since CM is greater than normal mode noise
and is the predominant field problem.
B. Conducted& Radiated Emission Levels
Maximum allowable drive P’n conducted into power
lines, without interference to external line equipment, is
defined in dBV or dBp z due to large noise attenuation ratio’s
of Table H. A 100 pV noise level above 1 pV is expressed as
40 dBflVusing (1) with Vin = 1 ~V, Vout = 100 VV .
V n (dB) z 20 Log10 (Vout / Vin) (1)
07803-4070-1/97/$10.00 (c) 1997 IEEE
dB(uV) 100KHZ lMHZ IOMHZ 30MHZ Table III. Allowable CE Emission Levels120 I I I
110
30 I I Ill 1111
20 !Wrtlllllll lllllllr’ll\l J’nl+++twt+~10 2 46810 2 46810 2 46810 2 3
Figure 23. Conducted emissions vs. frequency(A) No filter (B) Std. Filter (C) Std. Filter/ shielded cable
(D) Special filter / shielded cable
Conducted Emission Limits [ dBpV ] over 150 kHz -30 MHz
Class 150 kHz – I 0.5 – I 5–
500 kHz 5 MHz 30 MHz
A AV (66), AV (60), AV (60),QP (79) QP (73) QP(73)
B AV (56-46), AV (46), AV(50),QP (66-56) QP (56) QP(60)
Radiated Emission Limits [ dBpV /m] over 30MHz -1 GHz
Class 30MHz -230 MHz 230 MHz -1 GHz
A @30 Meters 30 37
B 6? 10 Meters 30 37
CLASS A = EN 50081-2, CISPR 11, GROUP 1CLASS B = EN 50081-1 , CISPR 22, GROUP 2
C. Frequency Characteristics of Noise Source
Table II. EMI Performance vs. Noise Level PWM output voltage, internal Switch Mode Power
Supplies (SMPSS), and drive semiconductor transients are theAttenuation Attenuation EMI main EMI noise sources in the 150 kHz to 30 MHz range.
(dBV) (Voltage Ratio) Protection Output switching voltage of Fig. 4 and Fig. 19 induceOto 10 1:1-3:1 Poor CM currents to ground through stray capacitances that driveloto30 3:1-30:1 Minimum30 to 60 30:1-1000:1 Average
input LISNS detect. Spectrum analysis of Fig. 4 indicates a
> 6(I 1000:1 Goodrisetime ffequency component at fr = 0.321tri~e, decaying at -
40 dBldecade above fr. Thus, EMI components in 3.2 -6.4
LISNS measure V. and spectrum analyzers convert it to
dBp V units over the sanctioned conducted emission
frequency band of 150 kHz to 30 MHz. Limits fi-om 10 kHz
to 150 kHz are proposed but not required at this time. Quasi-
peak (QP) detectors streamline EMI measurement time but
have higher QP dBp V limits than Average dBuV of Table III.
Fig. 23 shows allowable conducted emission limits in QP
dBuVvs. frequency.
Radiated electric field emissions are expressed in dB
pV/m, rather than V/m, for EMI standard comparison. Thus, 1
m V/m using Vout = 1000 pV and Vin = 1 pVin (1) results in
60 dB p V/m. Radiated emission interference problems are
noticeable on AM radio, TV and radio-controlled devices
more so than for industrial instrumentation. Radiated troubles
begin at 0.1 to 3 V/m [ 5 ].
MHz range for IGBT trj~e= 50-100 ns are seen. Pulse width
(~) changes over a cycle, from 400 ns to 200 us,
corresponding to f. components = 800 kHz to 1.6 kHz and
which decay -20 dB/decade above fr Pulse width variance
cause spectrum “smearing’ over a wide frequency range. EMI
components centered at drive fc (1 to 12 kHz) and harmonics
of fc are also seen.
Other noise sources are semiconductor recovery voltage
spikes, creating noise in the 20 - 30 MHz range that exits
both input and output power leads to ground. Logic board
SMPSS powered tiom drive dc bus power, also have PWM
vokage waveforms similar to Fig. 4. Thus, fr, f, , and fc ( 10
kHz to 100 kHz) noise ffequency components may also exit
drive input and output power leads to ground.
European Union basic EMC standards applied to drives
are listed in EN550 11, while specifications that declareD. Line Impedance Stabilization Network
emission limits are found in gene~ic EMC standards applied
to drives listed in EN50081 -1 and EN5008 1-2 [11]. Class BLISNS in Fig. 22 stabilize line impedance at 50 Cl for Vn
limits for residential, commercial and light commercial sites
follow EN50081- 1 while Class A limits for heavy industry
sites follow EN50081 -2. Class B limits are mostly needed to
eliminate AM radio and TV interference problems.
‘m.
Figure 24. Single phase schematic of LISN
07803-4070-1/97/$10.00 (c) 1997 IEEE
Figure 25. Standard 3 phase EMI filter schematic
measured > 1 MHz. Variations in measured Vn due to
different user line impedances or EMI filter interactions are
thus eliminated. Fig. 24 shows a CISPR 16 single phase
schematic of a three phase LISN with Drive Under Test
(DUT) and ac mains phase to ground connections.
Components change with current rating and frequency range.
LI simulates typical line inductance of 50 PH. L2, C3, R.j, C2,
R3 form an ac mains filter preventing external noise ftom
affecting 10 kHz to 150 kHz DUT Vn measurements. In the
150 kHz to 30 MHz range, L2, C3, Rj are not used and R3 =
O. LISNS measure conducted drive noise via high frequency
bypass capacitor Cl, which routes CM high frequency V. to
RI + R2 = R = 50 Cl measuring device. In the 2nd range,
LISN impedance is a parallel L1 inductor and resistor R = 50
Cl at frequencies >1 MHz.
E. Typical EMI Filter Schematic
EMI filters are comprised of single stage L-C filters,
each with 40 dB/decade attenuation from resonant frequency
A= 1 / (27c(LC)05 ). Thus, if 40 dB attenuation at undesirable
noise ffequency fn is desired, then filter L and C are selected
for f, = J. / 10. EMI filter designs must minimize capacitor
Ileahge to ground for safety reasons and insure filter
resonates with drive noise sources do not occur under any
operating condition to prevent underdamped oscillations.
In the multistage EMI filter of Fig. 25, load side yll~ad
capacitors are high frequency bypass capacitors to CM IaO
noise generated during drive output switching. Line toground impedance (Zc = 1 /(2 n y. CY )) is lower here, than
a CM current path from transformer XO , to three phase ac
main lines and through the high impedance blockers of the
filter inductors (ZL = 2 n fn L ). yjload capacitor in series
with XzlOad line to line capacitor also is a CM line to ground
bypass filter for IaO. Thus, L1’ line to ground noise voltage is
very low and equal to Iao times ZC . Differential and CM
inductors along with Ylline, Xjlfne, and Y21ineform a CM line
to ground filter that attenuates VLI ‘-ground noise volt% to
required dBp V levels.
Line to line high frequency noise sourced from the drive
Frquency [Hz]
Figure 26. Typical radiated emissions with filters
is reduced in amplitude by the Xjload bypass capacitors. CM
inductors insert minimal inductance line to line, so that phase
inductors and X2~ine capacitors attenuate line to line noise to
required dB,u V levels.
F. Measurement of Conducted Emissions
Fig. 23 shows measurements of conducted emissions for
various cases with /without filters and shielded cables:
(1) No Filter: Curve A shows the ASD exceeds Class A & B
margins. The wide band of noise frequency is due to PWM
pulse width changing over a given cycle. Noise frequency
spectrum related to drive output 50-100 ns Iao risetime, peaks
at 3-6 MHz and decays at an expected -40 dB/decade.
(2) Standard Filter: Curve B shows the ASD still exceeds
Class A & B margins even with a standard EMI filter. The 5
MHz noise frequency correlated to lao risetime is now more
prevalent. A 12 MHz peak is due to semiconductor risetime
of the switchmode power supply.
(3) Standard Filter & Shielded Cables: Curve C shows 20
to 30 dB,u V improvements by using shielded cable on ASD
input and output power leads. The IGBT risetime peak at 5
MHz is reduced 30 dBpV, as well as 20 dBp V attenuation of
the switchmode risetime peak at 12 MHz. This indicates that
the low impedance of the co-axial shielded armor takes
almost all CM ]aO current directly to the EMI filter CM caps
and back to the drive input as expected, leaving little high
ffequency noise current coupled into the ground grid and ac
mains supply before the LISN. Continuous welded aluminum
armor Type Metal Clad cable has reduced “EMI emissions
over both conducted and radiated ffequency range. The co-
axial nature reduces conducted emissions while the seamless
characteristic attenuates radiated electric fields due to noise
by eddy current shielding.
07803-4070-1/97/$10.00 (c) 1997 IEEE
(4) Special Filter & Shielded Cables: Class B requirements
are met using a special designed EMI filter matched to the
ASD, shielded armor cable on drive input and output power
leads, solid wire bonding practices to metal of both drive and
EMI filter, and using a metal cover for the drive.
G. Measurement of Radiated Emissions
EMI filters meeting conducted emission limits are
essential to passing the specified 30 MHz to 1 GHz frequency
band radiated emission test requirements in Fig. 26.
However, logic board clock transitions, shielded logic cables
and PC board layout are a dominant influence at these ultra
high frequencies.
VII. CONCLUSION
Generation of Common Mode (CM) EMI noise that is
sourced from the ac PWM drive’s high dv/dt output voltage
waveform was discussed in this paper. CM current that is
capacitively conducted into the system ground grid is a major
noise component. CM current induces CM voltages
throughout the plant ground grid, making instrumentation
reference to a “quiet” ground a difficult task. Conducted and
radiated characteristics of the noise source were analyzed.
Noise coupling paths for the generated noise were
discussed and demonstrated in detail for various industrial
control systems.
Solutions to control the EMI involved discussions on: (1)
Proper grounding of drives along with proper panel layout of
drives and controls, (2) Attenuating the noise source with CM
cores on drive output leads and interface leads, (3) Shielding
the noise away from sensitive equipment by physically
separating drive power and signal control wires, using three
wires plus ground in output/input conduit. Output power
leads using three wires plus ground in a shielded/armor cable
with an insulating outer jacket to isolate ground noise current
provides the most predictable control over the noise path
taken. These solutions are found to fix the majority of drive
related EMI problems.
FCC and CE regulations constraining allowable
conducted and radiated emission levels were defined and
typical EMI filter and shielded cable approaches to meet
these more stringent EMI levels were demonstrated.
Acknowledgments
Acknowledgment is given to Prof. Geza Joos of Concordia
University, who encouraged me to write this summary article.
Thanks also goes to the Allen Bradley internal EMI/CE team
consisting of R. LaPerriere, J. Meier, B. Weber, J. Erdman,
Dr. R. Kerkman, J. Johnson, D. Jaszkowski, R. Nelson, D.
Anderson, D. Leggate, D. Schlegel, K. Pierce, D. Dahl, and
H. Jelinek who worked through the CE and common mode
noise issues.
References
[1] W. Ott, Noise Reduction Techniques in Electronic
Systems, Wiley, 1976, ISBNO-O-471-65726-3
[2] H. Schlicke, Electromagnetic Compatibility (Applied
Principles of Cost Effective Control of Electromagnetic
Interference and Hazards) , Marcel Dekker, 1982
[3] B. Kaiser, Princ@les of Electromagnetic Compatibility,
Artech, Massachusetts, 1983,79-12032
[4] M. Mardiguian, Controlling Radiated Emission by
Design, Van Nostrand Reinhold, 1992
[5] IEEE guide for the installation of electrical equipment to
minimize electrical noise inputs to controllers from
external sources , ANSI / IEEE Std 518-1982, IEEE
Press, John Wiley
[6] D. Anderson, R. Kerkman, L. Saunders, D. Schlegel, and
G. Skibinski, “Modem Drives Application Issues and
Solutions Tutorial”, IEEE-IAS-Petroleum and Chemical
Industry Conference (PCIC), Philadelphia, PA, Sept. 26,
1996.
[7] Gary Skibinski, “Installation Issues for IGBT AC
Drives”, Allen-Bradley, Rockwell Automation, Duke
Power Seminar, May 8, 1996
[8] G. Skibinski, “Installation Considerations for IGBT AC
Drives - A Summary Paper”, Association of Energy
Engineers conference, Plant & Facility Expo, Atlanta,
GA, Nov. 7, 1996
[9] G. Skibinski, J. Pankau, and W. Maslowski, “Installation
Considerations For IGBT AC Drives”, IEEE Annual
Textile, Fiber, and Film Industry Technical Conference,
May 5, 1997
[1 O] Russel J. Kerkman, “Twenty Years of PWM AC Drives:
When Secondary Issues become Primary Concerns”,
IEEE Industrial Electronics Conference (IECON),
Taipei, Taiwan, August 5-9, 1996, pp. Ivii- lxiii.
[11] EN55011: Limits and methods of measurements of
electromagnetic disturbance characteristics of industrial,
scientific and medical radio fi-equency equipment,(Modified version of CISPR 11, equivalent to VDE 0875
Tll)
07803-4070-1/97/$10.00 (c) 1997 IEEE