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