common-mode current inductively

8/11/2019 Common-Mode Current Inductively

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Common-Mode Current Inductively

Coupled Emission of AC PWM DrivesMeng Jin, Zhang Lei, Ma Weiming, Zhao Zhihua, and Pan Qijun

Research Institute of Power Electronic TechnologyNaval University of Engineering

Wuhan 430033, China

[email protected]

AbstractThe most common solution for modern adjustablespeed drives is the use of induction motors fed by voltage-source

inverters. The inverters using fast switching insulated gatebipolar transistors (IGBTs) generate pulse-width modulated

(PWM) voltage pulses. The high carrier frequency along withfast rise and fall time of the PWM switching results in nontrivial

common-mode or ground currents noise. The current component

can cause electromagnetic interference (EMI) noise to neighbourequipments and installations by either radiation from the motor

cable or by cross-talk (inductive and capacitive coupling) with

other conductors. This paper discusses the inductively coupled

EMI noise from an ac drive system where there susceptibledevices and a common ground plant are presented. The modeldescribing the sophisticated EMI phenomenon is proposed and

measurement results are presented.

I. INTRODUCTION

The standard solution used in industry for ac adjustable

speed drives (ASD) remains the pulse-width modulated

(PWM) voltage source inverter because of many advantages:

high efficiency (up to 98%), open-circuit protection, small

relative size, excellent regulation capabilities, wide speed

range, constant high input power factor, common bus

regeneration, etc [1]. To achieve high efficiency levels, fast

switching insulated gate bipolar transistors (IGBTs) are used.

As a consequence, at the output of the inverter, PWM pulses

with high dv/dt (5kV/s to 15kV/s) [2] will have thepotential to cause electromagnetic interference (EMI) issues

with adjacent sensitive equipments, when large quantities of

drives are assembled in a concentrated area.

Conducted EMI emissions of the ASD output power leads

contain several frequency components [2]. There is a circular

low-frequency magnetic field component around the wires

due to the inverter fundamental output frequency, a mid-

frequency magnetic field component due to the carrier

frequency current and a high-frequency electric and magnetic

field due to the transient di/dtcommon-mode (CM) current

conducted on the power leads. The CM noise is a type of EMI

noise induced on signals with respect to a reference ground, is

sourced from CM voltage. In three phase inverters, PWM

generates CM voltage because of asymmetrical switching

states. Past experience has shown unknown conducted noise

in the plant ground as the predominant EMI issue for

susceptible equipment, which may be computer systems,

communication links, ultrasonic sensors, weighing and

temperature sensors, bar code/vision systems.

Fig. 1 Common-mode current paths in a grounded PWM drive.

Fig. 2 30 hp ASD output waveforms (200s/div).

II. CM CURRENTPATHS IN ADRIVE SYSTEM

Fig. 1 shows the high frequency CM current paths in agrounded PWM ac drive system. The high rates of rise and

fall of line-line voltage pulses in the range of a few hundreds

of nanoseconds give rise to ground currents due to 1) cablecapacitance to ground Ccgand 2) motor winding capacitance

to ground Cmg. If not properly mitigated, high frequency

ground currents can also interfere with the power systemground and affect other equipment on the power system

through inductive coupling and capacitive coupling paths.

Fig. 2 shows the CM voltage and CM current waveforms

from a 400V/50Hz 30hp ASD system. A convenient way ofinvestigating ASD CM voltage is by establishing a virtual

neutral reference node from awye connected node of three 1M resistors to ground. A current probe (CT) encircling all

ASD output cables normally measure CM current, since the

2008 Asia-Pacific Sympsoium on Electromagnetic Compatibility &

650 19th International Zurich Symposium on Electromagnetic Compatibility, 1922 May 2008, Singapore


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2008 Asia-Pacific Sympsoium on Electromagnetic Compatibility, 1922 May 2008, Singapore

instantaneous sum of the fundamental frequency components

of each phase sum to zero. The transient CM current in Fig. 2

may reach 1 Apk at every CM voltage switching transient

become possible EMI problems.

The CM voltage with high and high amplitude in the cable

between the inverter and the motor is a potential source of

EMI, therefore usually shielded motor cables are used. Suchcables are expensive and increase the overall installation cost.

Usually, unshielded cables are between 30% and 70% cheaper

than shielded cables. Unfortunately, their use is restricted by

EMC-related concerns [3].

High dv/dt from a drive unshielded output leads would

capacitively couple through stray capacitance onto both signal

lines in close proximity and produce an error voltage. The

capacitively coupled emission has been detailed discussed [4],

[5], [6]. A complete investigation from an EMC point of view,

about the inductively coupled EMI from the motor drives, has

not been presented in the literatures until now.

III. INDUCTIVELYCOUPLED EMI

Long unshielded drive output cables carrying CM current

may act as loop antennas for radiated emissions, due to thecurrent path in these cables returning via the ground plane in

Fig. 3. Drive CM output cables and others circuits conductors

also inductively couple and introduce EMI issue.

PWM DriveMotor

Cmg

Common Mode Current

Steel Ground Plane

Load

U

V

W

Drive Cable

Signal Cable

Signal Current

Common Mode Voltage

Mutual

Inductance SENDRECEIV

Fig. 3 Configuration of CM EMI loop coupling for ASD system.

The ground plane is a high impedance to high frequency

ground noise current [7], so that an instantaneous CM noise

voltage is created across the ground points of the drive and

motor. CM voltage is impressed on susceptible interface

equipment between drive logic ground and interface ground.

As shown Fig. 3, a susceptible interface circuit, SEND and

RECEIVE, with source and return signal current. Thus, CM

noise voltage impressed on both signal lines, allows a CM

noise current to appear in the same direction on both lines and

circulate back through ground. The signal may develop a

noise voltage. There also has mutual inductance coupling

between the output cables and signal cables, which can induce

another noise voltage into this interface circuit, has not been

presented in the previous literature until now.

Fig. 4 show a interface signal with 50% duty cycle,

frequency = 15 kHz and amplitude = 10 mV. Fig. 4(a) referred

to the drive OFF state, and Fig. 4(b) referred to PWM drive

RUNNING. A series of pulses with a maximum magnitude of

80 mVpk can be seen in Fig. 4(b).

(a) Drive off. Vertical scale: 10mV/div; Time scale: 10s/div

(b) Drive running. Vertical scale: 20mV/div; Time scale: 10s/div

Fig. 4 Measured waveforms of receiver load signal.

104

105

106

107

0

10

20

30

40

50

60

70

8090

100

Frequency [Hz]

Magnitude[dBV]

Drive OFF

Drive RUNNING

Fig. 5 Measured spectrum of the signal voltage in Fig. 4.

Frequency characteristics of the signal voltage spectrum for

drive RUNNING state and OFF state are compared in Fig. 5.

The frequency components of coupled noise voltage exceed

the original signal more than 30 dB. There must be a serious

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19th International Zurich Symposium on Electromagnetic Compatibility, 1922 May 2008, Singapore

risk of the interface equipments ability to function in the

presence of the high-level noise voltage.

IV.MODELLING ANALYSIS

The investigated topology is shown in Fig. 6. The purpose

is to model the RECEIVE load voltage when the unshielded

cable carrying a noisy CM current couples to the susceptiblecircuit.

Steel Plane

Unshielded Phase Conductor

SENDRECEIV

t

Cip

SR

VSVE

SIGI

LOADR

Cmp

HS

H0

l

ICM

Fig. 6 Configuration of CM EMI inductively coupling.

The EMI coupling mechanism can be theoretically modeled

by Fig. 7.ZMG is the impedance of ground plane. Mt is the

total mutual inductance between the loops formed by the drive

cables and the signal cables [8], [9], detailed model is shown

in Fig. 8.

SR

MGZ

t CMj M I

VSVE

Steel plane

Cable

RECEIV SEND

SIGI

CM SIGI I

LOADR

Fig. 7 Equivalent circuit model for the susceptible circuit.

1L

CMI

SIGI

2L

3L

15M

5L 6L

7L

4L

26M

24M

37M

17M

35M

Fig. 8 Partial inductance model for the coupling loops.

From Fig. 7, the noise voltage at the RECEIVE load can

be obtained as

S LOAD MG SIG MG t CM S 0R R Z I Z j M I V (1)

where,

t 15 26 17 242 2M M M M M (2)

MG in ex in exZ Z Z Z j L . (3)

whereZinis internal impedance of the ground plane [10], can

be expressed by

in DC HFZ R jX (4)

6

DC

r

17.210

lR

wt

(5)

9

HF

36910

1

r r

t

flX

w e

(6)

where r is relative conductivity, l, w, and t represent the

length, width, and thick of the plane, respectively, all units are

mm, r is relative permeability, f is frequency, and is

skin depth,r r66 f , unit is mm.

The external impedance of a flat steel plane mainly formed

by the external inductanceLex[10], due to the fields external

to the cable, can be calculated [11], [12]

10

ex 0 w

0 w

2 2

0 w

( ) tan 2

ln(4( ) ) ln( )4 2

l w

L H rw H r

w wH r w w

(7)

whereH0is height of the cable andrwis cable radius.

Usually, the total impedance of a flat steel planeZMGis very

small compared to RS and RLOAD, as shown in Fig. 9, its

magnitude is less than 11 below 10 MHz. Assuming

S LOAD MG( )R R Z and (1) can be approximated

S LOAD SIG MG t CM S 0R R I Z j M I V (8)

SubstitutingE LOAD SIG

V R I into (8), yielding

SE S MG t CMS LOAD

RV V Z j M I

R R (9)

Therefore, the expression of the coupled voltage on thesusceptible load is obtained and can be calculated based on

practical parameters.

104

105

106

107

0.1

1

10

100

Frequency [Hz]

ImpedanceMagnitude[]

Fig. 9 Calculated impedance for steel ground plane.

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common-mode current inductively

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