EMC Filters Guide
REO INDUCTIVE COMPONENTS AG
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Tel: 0049-(0) 2 12-88 04-0
Fax: 0049-(0) 2 12-88 04-188
REO USA
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IN46226-6550, USA
Tel: 001 317 8991395 Fax: 001 317 8991396
REO (UK) LTD Units 8 -10 Long Lane Industrial Estate,
Craven Arms, Shropshire SY7 8DU UK
Tel: 01588 673411 Fax: 01588 672718
Email: [email protected] Website: www.reo.co.uk
REO UK LTD
Electromagnetic field theory
2
Electromagnetic field theory
5
What makes up the electromagnetic spectrum?
7
Electromagnetic compatibility (EMC) and the law
8
Where does electromagnetic interference come from?
9
EMC filters
12
Testing
13
Points to consider when selecting a filter
14
Standards
15
Installation
16
Standard filters
Hans Christian Oersted was a
professor of science at Copenhagen
University. In 1820 he arranged a
science demonstration to friends and
students in his home. He planned to
demonstrate the heating of a wire by
an electric current, and also to carry
out demonstrations of magnetism, for
which he provided a compass needle
mounted on a wooden stand. While
performing his electric demonstration,
Oersted noted to his surprise that
every time the electric current was
switched on, the compass needle
moved. He kept quiet and finished the
demonstrations, but in the months that
followed worked hard trying to make
sense out of the new phenomenon. But
he couldn’t! The needle was neither
attracted to the wire nor repelled from
it. Instead, it tended to stand at right
angles (see drawing below). In the end
he published his findings (in Latin!)
without any explanation.
+
N
REO (UK) LTD
Units 8-10 Long Lane Industrial Estate,
Craven Arms, Shropshire SY7 8DU UK
Tel: 01588 673411 Fax: 01588 672718
Email: [email protected] Website: www.reo.co.uk
S
-
It was the scientist Michael Faraday
who first studied in detail the
phenomena involving the interaction
between electricity and magnetism.
Amongst his many achievements, he is
credited with the construction of the
first electric motor and the discovery of
both the principle and the method
whereby a rotating magnet can be
used to create an electric current in a
coil of wire (still the basis of modern
electricity generating plants).
Finally, a Scottish mathematician,
James Clerk Maxwell, picked up on
Faraday’s ideas and was successful in
developing a theory of electromagnetism which enabled testable predictions to be made. Inspired by Faraday’s
lines of force, he developed a model
that unified magnetic and electrical
forces. Maxwell unified the study of
electricity and magnetism in four tidy
equations. In essence he discovered
that electric and magnetic fields were
intrinsically related to one another, with
or without the presence of a conductive
path for electrons to flow. Stated simply
Maxwell’s discovery was this:-
He also observed the way in which iron
filings arrange around a magnet. They
appear to follow lines of magnetism
leading out from one magnetic pole
and back to the other pole. He
suggested that the effect of a magnet
on a wire carrying a current is the
result of lines of force . Faraday spent
the latter part of his life working to
devise an experiment that would
confirm his theory of electromagnetic
fields but without success.
Electric
Field
Magnetic Field
Direction
of Wave
A changing electric field produces a
perpendicular magnetic field
and A changing magnetic field
produces a perpendicular electric field
Quite interestingly Maxwell predicted
that electromagnetism would be propagated through space at a finite rate and
was struck by the similarity between
the predicted speed of electromagnetism and the speed of light. From this
connection sprang the idea that light
was an electric phenomenon and the
subsequent discovery of radio waves.
Michael Faraday
What makes up the electromagnetic spectrum?
The electromagnetic spectrum is a
family of waves that travel through
space by way of the production of
electric and magnetic fields. Changing
electric fields are set up by the oscillation of charged particles and these
changing electric fields induce
changing magnetic fields in the
surrounding space. Changing magnetic
fields then set up more changing
electric fields and so on. The net result
is that the wave energy travels across
space.
All electromagnetic waves travel at the
same speed through the same medium
or substance but they have a variety of
frequencies which provide a corresponding variety of wavelengths. If the
original charged particle vibrates
rapidly, the frequency of the wave is
high. Because there are many oscillations per second, the corresponding
wavelength is short.
Conversely, if the original charged
particle vibrates slowly, the frequency
of the wave is low and the corresponding wavelength is long.
The whole range of frequencies and
wavelengths is called the electromagnetic spectrum and different parts of
the spectrum are given different
names. These parts of the spectrum
have different properties and, consequently, they have different uses.
Therefore, it can be seen that there is
the need for the coexistence of all
kinds of radio services, which use the
electromagnetic spectrum to convey
information, with technical processes
and products emitting electromagnetic
energy as an undesirable by-product.
Furthermore, the problems of EMC are
not limited to interference with radio
services because electronic equipment
of all kinds is becoming more susceptible to malfunctions caused by external
interference. This is particularly
relevant in the case of electronic equipment that is required to continue
running for economic or safety
reasons. Banking systems and aircraft
computers are two notable examples.
Frequency Hz
Wavelength M
long-wave radio
10
10
6
short-wave radio
10
10
9
0
microwaves
millimeter waves
10
3
12
10
-3
red
orange
yellow
green
blue
indigo
violet
infrared light
10
10
15
ultraviolet light
18
10
VISIBLE LIGHT
21
The Electromagnetic Spectrum
x rays
gamma rays
10
-9
10
-12
Electromagnetic Compatibility (EMC) and the law
There is now the European Directive
89/336/EEC, which specifically deals
with EMC. Like a number of other
documents produced by the European
Commission this is a new approach
directive, which sets out the essential
requirements that must be satisfied
before products can be marketed
anywhere within the EC. It also says
how evidence of conformity will be
provided.
In the case of EMC the essential
requirements are that electrical and
electronic equipment shall be
constructed so that:
The electromagnetic disturbance it
generates does not exceed a level
allowing radio and telecommunications equipment and other apparatus
to operate as intended;
The apparatus has an adequate
level of intrinsic immunity to electromagnetic disturbance to enable it to
operate as intended.
The manufacturers or their authorised
representative are required to attest
that the requirements of the Directive
have been met. This requires two
things:
A Declaration of Conformity must be
kept and made available to the
enforcement authority.
A CE mark must be affixed to the
apparatus, or its packaging, instructions or guarantee certificate.
Compliance
In essence, for goods to comply with
the EMC Directive they should be
tested but in reality this is difficult
because laboratory conditions are not
the same as the real working environment. Also it is quite possible that
numerous types of other equipment
could be connected to the goods and it
is impossible to cover all possible
configurations.
However, where possible the goods
should be tested either in house or by
a competent body against the relevant
standards. A Technical Construction
File must be produced and be ready for
inspection, if required.
Standards
The generic standards relating to EMC
are divided into two sections, one for
immunity and one for emissions, each
of which has separate parts for
different environment classes.
EN 50 081 Emissions
EN 50 082 Immunity
Part 1
Part 2
Part 3
Domestic
Commercial
Light Industrial
Industrial
Special
Where does Electromagnetic Interference come from?
Electromagnetic interference (EMI) can
manifest in a variety of ways and the
emission source is usually frequency
dependent. The interference can be
conducted, through mains cables and
earthing connections, or radiated. Most
electronic hardware contains elements
which act in a similar manner to an
antenna, such as cables, pcb tracks,
internal wiring and mechanical structures. These elements can unintentionally transfer energy via electric,
magnetic or electromagnetic fields,
which couple with other circuits.
EMC Filters
Instead of designing a filter stage for
every new piece of electronic equipment that is manufactured, there is a
convenient solution available in the
form of a ready made module that can
be connected between the mains
supply and the electronic unit. The
components are selected by the filter
manufacturer to give the best reduction
of mains conducted interference for
most situations, across the frequency
spectrum, using the optimum selection
of components. More importantly filters
comply with the numerous safety rules
and the approval costs have been
spread over a large number of units
because the filters can be universally
applied.
The capacitors oppose the AC flow of
electrons more at lower frequencies
and less at higher frequencies.
Inductors on the other hand react
against the rate of change of current;
they are more effective at opposing AC
flow of electrons at higher frequencies.
Therefore, a combination of series
connected inductors and shunt
connected capacitors is chosen to
provide suppression over a wide
frequency spectrum. The resistors
serve to discharge the capacitors when
the supply is disconnected and for
damping resonances. The enclosure is
normally produced from metal to
provide good earth bonding.
Filters work by providing an impedance
mismatch between the power line and
the equipment, which reflects the
generated noise back to its source. In
order to maximise the impedance
mismatch the choice of filter circuit
should take into account the impedances of the source and load.
The main components inside the filter
are chokes, capacitors and resistors.
A typical REO filter built into metal enclosure
(lid removed). The main components; capacitors and inductors can be clearly seen.
The filter circuit is designed to contend
with two types of noise. There is
COMMON MODE NOISE, which
manifests itself as a current in phase in
the live and neutral conductors and
returns via the safety earth. This
produces a noise voltage between
live/neutral and earth. It is often caused
by capacitive coupling to the case
earth. The other is DIFFERENTIAL
MODE NOISE produced by current
flowing along either the live or neutral
conductor and returning by the other.
This produces a noise voltage between
the live and neutral conductors.
The chokes fall into two groups; current
compensated or common mode and
series or differential mode types. The
current compensated choke has two or
three windings on a toroidal core. The
direction of each winding is chosen to
give an opposing current flow, hence
balancing the flux. The result is that a
much smaller choke can be used.
Furthermore, the common mode
currents, which are in phase in the two
or three conductors, have an additive
effect, thus presenting higher impedance against the common mode noise.
Source
Load
Common Mode
( asymmetrical interference )
Differential Mode
( Symmetrical Interference )
Cp = parasitic capacitance
Earth Current
Symmetrical
Interference
current
Cy
Cx 1
L
Cx 2
Cy
Asymmetrical Interference current
The differential mode chokes are larger
due to the higher current handling
requirement. Using a core made from a
highly permeable material will reduce
its size.
Testing
Capacitors also fall into two groups; X
Class and Y Class. The X Class capacitors are connected between live and
neutral, or between phases, to reduce
differential noise. They are tested to
withstand mains voltage. Y Class
capacitors on the other hand are more
critical because they are connected
between live/neutral and earth to
reduce common mode noise. Because
of this they have to be tested to ensure
that they cannot fail to short circuit.
Needless to say they are more expensive.
For higher levels of attenuation, several
stages of chokes and capacitors can
be added and this is known as a multistage filter.
Another important factor is the earth
leakage current. The larger the Y Class
capacitor the more the 50 Hz current
that will leak to earth, raising the potential of the filter enclosure. The
maximum permissible leakage current
depends on the application but to give
an indication the maximum earth
leakage for double insulated equipment, such as hedge trimmers and
drills, is 0.25 mA. Equipment that is
permanently connected to the mains
supply may have a leakage current of
up to 5mA. Industrial equipment
normally has higher leakage current
limits but in each case particular care
must be taken to ensure that earthing
regulations are observed.
The frequency range of interest for
emissions from most products is
150kHz to 30MHz. The emissions are
measured using a spectrum analyser
and then they are compared with the
Class B limits for domestic or light
industrial applications or Class A limits
for industrial applications (Standard EN
55 022). The spectrum analyser will
give a sweep of the entire spectrum
range and indicate the emission levels,
usually as a print-out. The limits are
also indicated and frequencies with
unacceptable emission levels are
highlighted.
The picture shows a screen capture
from a spectrum analyzer. Note that
the sweep range is 150 kHz to 30 MHz
and the red line (quasi-peak) and
purple line (average) indicating the
limits for light industrial and domestic
use (Class B). The two frequency
curves for the equipment under test are
quasi peak in blue and average in
green. The vertical scale is in dBV. In
order to conform the curves should not
exceed their corresponding limits.This
is the format for the results as
prescribed in EN 50 081-1 for
conducted emissions.
The electromagnetic field strengths
associated with the emissions
measurements range from a few
millionths of a volt per meter to a few
thousandths of a volt per meter. It is
difficult to represent this scale in a
linear manner and so a logarithmic
scale is adopted. The spectrum
analyser plot has decibels as the unit
of measurement along its vertical axis.
A decibel is a ratio of two power
values, which in this case is the
increase in power caused by the
emission compared with the power of
the reference voltage. For convenience, because power is proportional to
the square voltage, the ratio of the
voltages is squared instead of
measuring power.
Screen capture from a spectrum analyzer
Points to consider when selecting a filter
The performance and hence quality of
a filter may be assessed by looking at
its insertion loss characteristics. This
data is published by all reputable filter
manufacturers and the following is an
example.
Damping
[dB]
symmetrical
asymmetrical
100
80
The acceptable level of earth leakage
current for the installation must also be
known and the filter selected, accordingly.
60
40
20
0
.15
The current rating is also very important especially in instances where the
load produces high peak currents and
the RMS is not a true indication of the
real current. Higher than expected
currents can cause core saturation, in
the inductors, thus drastically reducing
the filter s effectiveness. Most filter
manufacturers will take this into
account when designing a filter but this
data is not normally published.
1
10
30
Frequency [MHz]
The curves can be used to assess
whether a particular filter will provide
sufficient suppression for a known
interference pattern. However, precise
performance can only be established
by testing under real conditions. The
higher the dB value the greater the
degree of suppression achieved.
The components in a filter are classified for a range of climatic and
mechanical requirements, including
lower temperature limit, over temperature limit and relative humidity. Unless
otherwise stated REO filters conform
with Environment Category IEC 68-1.
Over temperature
limit + 85C
Lower temperature
limit - 25C
25/85/21 The current has to be derated for
ambient temperatures of greater than
40C
Relative Humidity
95% 21 days/year
2 [ 2]
= √(85-)/40
The permissible current rating is
subject to ambient temperature. Below
40C a filter can be used at its rated
current. The current reduces for higher
temperatures. At 85C the current is
reduced to zero. (For 25/85/21 )
Ambient
40C
85C
Standards
All REO filters are built to the following
standards:VDE 0565 Part 1 - 3
DIN EN 133200
IEC 68-1
All materials are UL listed and in many
cases filters are UL approved, or
approval can be obtained in a short
time.
Installation
Standard Filters - Single Phase
A filter should be positioned as close
as possible to the connection to the
supply; ideally before the mains switch
in the front panel and fuses. Otherwise
the connecting cables could provide a
coupling path via stray induction to the
unfiltered cables.
CNW 101
CNW 102
Single phase, 250 V, cost
effective unit for general
purpose.
Single phase, 250 V, high
performance unit suitable
for most applications.
Wiring on each side of the filter should
be well separated and extend straight
out from the filter s terminals. If this is
not practical the output cables should
be run at 90 degrees to the input
cables to reduce the likelihood of
coupling.
The filter should be correctly rated for
the local supply voltage.
A good ground connection is required
between the filter casing and earth.
The ground connections must have
large contact surfaces and be made
onto bare metal, not painted.
Type
CNW
CNW
CNW
CNW
CNW
Current
101/3
101/6
101/10
101/16
101/20
3A
6A
10 A
16 A
20 A
Size (inc terminals)
W
H
L
80
45
30
80
45
30
92
50
30
92
50
30
92
50
30
Type
CNW
CNW
CNW
CNW
CNW
Current
102/3
102/6
102/10
102/16
102/20
3A
6A
10 A
16 A
20 A
CNW 201
CNW 116
230V / 440V 2 line
bookcase style filter.
Single phase, 250 V,
optimised unit for use with
motor drives.
Type
CNW
CNW
CNW
CNW
Current
16 A
30 A
50 A
63 A
201/16
201/30
201/50
201/63
Size (inc terminals)
W
H
L
126
255
50
126
255
50
126
255
50
126
255
50
L
L'
L
Cy
R
Cx1
Cx2
PE
N(L2)
Cy
N'(L2')
Type
CNW
CNW
CNW
CNW
Current
116/8
116/12
116/20
116/30
8A
12 A
20 A
30 A
Size (inc terminals)
W
H
L
93
50
40
93
50
40
93
50
40
118
50
40
118
50
40
Size (inc terminals)
W
H
L
57
120
105
57
120
105
57
140
105
57
140
105
Standard Filters - Three Phase
CNW 103
CNW 104
CNW 105
CNW 204
3 phase, 3 x 440 V,
3 line mains filter with
high attentuation.
3 phase, 3 x 440 V,
3 line mains filter with
very high attentuation.
3 phase, 3 x 440 V,
4 line mains filter with
high attentuation.
3 phase, 3 x 480 V
bookcase style filters, with
very high attentuation.
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
Current
3x3A
3x6A
3 x 10 A
3 x 16 A
3 x 25 A
3 x 36 A
3 x 50 A
3 x 80 A
3 x 120 A
3 x 150 A
103/3
103/6
103/10
103/16
103/25
103/36
103/50
103/80
103/120
103/150
Size (inc terminals)
W
H
L
118
53
40
118
53
40
118
53
40
170
98
70
170
98
70
290
148
70
290
148
70
320
168
110
320
168
110
320
168
110
L1
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
Size (inc terminals)
W
H
L
170
98
70
170
98
70
170
98
70
170
98
70
170
98
70
290
168
110
290
168
110
350
168
140
350
168
140
350
168
140
Current
104/3
104/6
104/10
104/16
104/25
104/36
104/50
104/80
104/120
104/150
3x3A
3x6A
3 x 10 A
3 x 16 A
3 x 25 A
3 x 36 A
3 x 50 A
3 x 80 A
3 x 120 A
3 x 150 A
Type
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
L1'
L1
L1'
L1
L2
L2'
L2
L2'
L2
L3
L3'
L3
L3'
PE
Cx1
LINE
Cy1
Cx2
Cx2
Cy3
Cx3
PE'
4x3A
4x6A
4 x 10 A
4 x 16 A
4 x 25 A
4 x 36 A
4 x 50 A
4 x 80 A
4 x 120 A
4 x 150 A
105/3
105/6
105/10
105/16
105/25
105/36
105/50
105/80
105/120
105/150
Cy2
Cx2
L3'
Cy
PE'
CNW 703
3 phase, 3 x 480 V
bookcase style filters,
with very high attenuation.
Over voltage protection device,
3 x 440V for easy installation
into new or existing equipment.
Efficient protection against high
voltages, lightning strikes and
transients. Attenuation of interference down to approximately
50dB at 1MHz.
L1
LINE
L1'
L1
L2'
L2
L2
L1
L2
Cx1
Cx2
Cx3
L3'
L3
R
N'
N
PE
CNW
CNW
CNW
CNW
Cy1
Cy2
PE'
Size (inc terminals)
W
H
L
50
80
230
50
80
230
60
150
280
56
150
330
Current
203/16/SE
203/25/SE
203/36/SE
203/50/SE
3
3
3
3
x
x
x
x
16 A
25 A
36 A
50 A
L
L2'
L3
PE
L3'
Cx2
Cx1
R
PE
Cy
For details of our full range of EMC
filters and custom design solutions
please;
Call 01588 673411
Email [email protected]
Visit our website www.reo.co.uk
Type
L1'
LOAD
4x6A
4 x 10 A
4 x 16 A
4 x 25 A
LINE
106/6
106/10
106/16
106/25
LOAD
CNW
CNW
CNW
CNW
Type
3x7A
3 x 16 A
3 x 30 A
3 x 42 A
3 x 55 A
3 x 75 A
3 x 100 A
3 x 130 A
3 x 180 A
N'
N
CNW 203
Size (inc terminals)
W
H
L
150
65
240
150
65
240
150
65
240
150
65
240
204/7
204/16
204/30
204/42
204/55
204/75
204/100
204/130
204/180
Size (inc terminals)
W
H
L
50
255
126
55
305
142
60
335
150
70
330
185
80
330
185
80
330
220
90
380
220
110
440
240
110
440
240
L2'
Cx1
3 phase, 3 x 440 V,
4 line mains filter with very
high attentuation.
Current
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
CNW
Current
R
CNW 106
Type
Type
L1'
PE
Cy2
Cy1
L3
Size (inc terminals)
W
H
L
118
53
40
118
53
40
118
53
40
170
98
70
170
98
70
290
148
70
290
148
70
320
168
110
320
168
110
320
168
110
Current
L
L2
Cx1
PE
PE'
L1
LINE
R
LOAD
L
LOAD
R
LINE
Type
LOAD
Type
CNW 703
Max. Surge
Current kA
80
Size (inc terminals)
W
H
L
205
90
75