Electromagnetic interference (EMI)
is the disruption of operation of an electronic device when it is in the
vicinity of an electromagnetic field (EM field) in the radio
frequency (RF) spectrum that is caused by another electronic device.
All of us encounter EMI in our everyday life.
Common examples are:
 |
| magnetic field effect |
• Disturbance in the audio/video signals on radio/TV due an
aircraft flying at a low altitude
• Noise on microphones from a cell phone
handshaking with communication tower to process a call
• A welding machine or a kitchen mixer/grinder
generating undesired noise on the radio
• In flights, particularly while taking off or
landing, we are required to switch off cell phones since the EMI from an active
cell phone interferes with the navigation signals.
Type of Electromagnetic
interference
a) In which there is physical contact between
the source and the affected circuits, and
b) Radiated – this is caused by induction.
The EMI source experiences rapidly changing
electrical currents, and may be natural such as lightning, solar flares, or
man-made such as switching off or on of heavy electrical loads like motors,
lifts, etc. EMI may interrupt, obstruct, or otherwise cause an appliance to
under-perform or even sustain damages.
Recent technologies like Wi-Fi are more robust,
using error correcting technologies to minimize the impact of EMI.
All integrated circuits are a potential source
of EMI, but assume significance only in conjunction with physically larger
components such as printed circuit boards, heat sinks, connecting cables, etc.
Mitigation techniques include the use of surge arresters or transzorbs
(transient absorbers), decoupling capacitors, etc.
RFI detection with software is a modern method
to handle in-band RFI. It can detect the interfering signals in time, frequency
or time-frequency domains, and ensures that these signals are eliminated from
further analysis of the observed data. This technique is useful for radio
astronomy studies, but not so effective for EMI from most man-made sources.
EMI is sometimes put to useful purposes as
well, such as for modern warfare, where EMI is deliberately generated to cause
jamming of enemy radio networks to disable them for strategic advantages.
Type of Electromagnetic
interference on the basis of frequency:
High
frequency -(Khz Mhz called RF)
Low
frequency –electrical system standard frequency
Electrical gadget produces
interference which may affect operation of electronic equipment.
High
frequency interference:
It affects wireless transmission
such as radio and TV. The interference is generally generated by device like
variable frequency drive. Frequency in the drive is controlled by means of a
technique called pulse width modulation (PWM).
 |
| typical pulse output of inverter |
Above wave form contains
harmonics.. the harmonic greater than 100KhZ can scape from the drive in two
manners-
Radiation- this is an electromagnetic
radiation. the drive acts as screen.
Conduction-the conducted interference will
travel to other electrical equipment connected to the network i.e. to the mains
through the incoming cable and to the motor through the cable.
Government wants to keep the
radio, TV transmission frequency band, and wireless communication frequency
band and telephone system free of interfrerence.IEC standards are available and
adopted by the government. Generally the following equipment s don’t requires
any certification –
-Switch mode power
supply (SMPS)
-Electronic
transformer or ballast
-Commutator or
slip ring motor
-Broadcast service
local
-Speed controller
-Microprocessor or
small device
Low
frequency interference
a) Mal operation of sensitive device
and medical equipment
b) Flicker on computer screen ,
exception LCD based computers
c) Interference with communication
system
d) Interference with low current and
voltage system such as instrumentation cable and PLC cabling.
e)
5-10 milli gauss can disturb the
electronic gadgets.
The interference is caused by
electromagnetic wave generated by electrical wiring. Cables and overhead lines
carry current and rated frequency (50-60 Hz) and may carry harmonic currents. The
current in cables and overhead line produces electromagnetic field and its
strength is measured in Tesla or Gausss, 1T=104 Gauss.
 |
| calculation of magnetic field |
The frequency of the magnetic field is the same as
frequency of the current and magnitude is the directly proportional to the
magnitude of current.
The typical example of
interference -
a) cable running in the ceiling space
b)
electronic
items near with switchboard ,transformer etc
c)
any
communication cable running parallel with electrical cable
d) power and communication cable
running on same pole
Calculation of magnetic field in
milli Gauss-
I=
ampere, d= distance between conductor
Field
at a distance R in meter
a) field due to a single conductor
B=2 X (I/R)
b)
field
due to two conductor in opposite
direction
B=2 X (I x d /R2)
c)
three
phase three wire
B=3.046 X (I x d /R2)
d)
three
phase 4 wire
B=2 X (I/R) (I=unbalance current
in neutral)
EXAMPLE
a)
field
due to a single conductor
say, I=250A, distance R=2M Then
B=2x250/2 =250 milli Gauss
b)
field
due to two conductor in opposite direction, 150mm apart
Say, I=250A, distance R=2M Then
B=2x (250x150)/1000x22 =18.75 milli Gauss
c)
three
phase three wire
Three wire circuit with 250A
current per phase, 150mm distance between 2 adjacent conductors, at 2 meter
B=3.046 X (250 x 0.15 /22)
= 28.56milli Gauss
Generally this situation finds at
high voltage terminal of transformer.
d)
three
phase 4 wire
Neutral current =50A, field at 2
meter distance
B=2 X (50/2) =50milli Gauss
The
magnetic induction may be reduced by –
a) Increasing the distance between
the conductor
b)
Reducing
the distance between 2 conductors
c) Balancing the load of three phase
supply
Typical magnetic field -
a) LV switchboard =0.5 to 1.5
milligauss
b)
11KV
switchboard =0.5 to 1.5 milligauss
c)
Distribution
Transformer without enclosure =10 to 80 milligauss
d) Distribution Transformer with
enclosure =0.5 to 1.5 milligauss
Protection against interference
for low voltage circuit
a) Maintain distance 300 mm -600 mm
between power and control cable
b)
Reduce
gap by 150 mm
c)
Prepare
separate route for power and control cable
d) Use screen cable for signal
/communication cable
Table 1. Reference levels for occupational exposure
|
Frequency
|
B
limit (H limit), milligauss
(Am-1)
|
E
limit, kVm-1
|
|
50 Hz
|
10.0
(800)
|
10
|
|
60
Hz
|
10.0
(800)
|
8.3
|
Table 2. Reference levels for general public exposure
|
Frequency
|
B
limit (H limit), milligauss
(Am-1)
|
E
limit, kVm-1
|
|
50 Hz
|
2.0
(160)
|
5
|
|
60
Hz
|
2.0
(160)
|
4.2
|
Ref:
ITU-T, Geneva
