Minimizing EMI System Design Guidelines

Minimizing EMI System Design Guidelines
Headquarters
Unitronics (1989) (R"G) Ltd.
Unitronics Building, Airport City
P.O.B. 300, Ben Gurion Airport, Israel 70100
Tel: + 972 (3) 977 8888 Fax: + 972 (3) 977 8877
Minimizing EMI: System Design Guidelines
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Unitronics products are designed to operate in a typical industrial environment.
This document gives guidelines on how to design your system for optimal
performance in noisy environments.
After installation, check your system periodically, in particular after installing
new machinery close to the system.
Devices Emitting high EMF ......................................................................... 1
Wiring Separation ..................................................................................... 2
Enclosing and Earthing .............................................................................. 2
I/O Wiring: General Guidelines.................................................................... 2
Signal (Communication, I/O) Line Filtering....................................................3
Wiring Power Supplies for I/O Expansion Modules ..........................................4
Power-line filtering .................................................................................... 5
Connecting the EX-A1 communication cable: .................................................7
Devices Emitting high EMF
High voltage, high current, and high frequency circuits such as high voltage /
high current power supplies, high power converters and amplifiers, contactors
and solenoids, motors and motor-drives may cause severe electro-magnetic
disturbances that may affect the operation of other nearby computerized devices
such as PLCs or I/O modules.
Those high voltage, high current, and high frequency circuits should not
share the same cabinet with PLCs or I/O modules.
If this is unavoidable, within the cabinet, either:
•
•
Physically separate these sources from PLCs and I/O modules with large
metal earthed plates. Such a plate should be large enough to partition
the cabinet into two cubicles.
Separate these sources and their cabling from PLCs and I/O modules as
described in the section Wiring Separation.
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Wiring and Grounding Guidelines
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Tech Support Document
Wiring Separation
Use separate wiring ducts for each of the following groups:
•
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Digital inputs, digital outputs, 24VDC (power supply for the PLC and I/O
Expansion Modules), communications, analog inputs, and analog
outputs.
Lines that are connected to the power grid, 230/115, 24VAC, all AC
lines such as motor driver outputs, noisy DC lines such as DC servo
drives and motors.
Separate these groups by at least 10cm (4"). If this is not possible,
cross the ducts at a 90˚angle.
Enclosing and Earthing
•
•
•
It is recommended to install the PLC and I/O modules in a closed
metallic cabinet. This will significantly improve interference immunity.
Make sure that the cabinet and cabinet door are properly earthed.
Please refer to the cabinet manufacturer instructions for proper
installation and earthing.
When you earth devices, minimize wire impedance by using a wire that
is as short and thick as possible, 3.3mm2 (12 AWG), up to 10cm long
recommended. Connect the line to nearest possible grounding point in
the cabinet, preferably a grounding plate or the cabinet body. Be sure
to remove any paint or other non-conductive coating between the wire
terminal and metal as this may cause poor conduction.
I/O Wiring: General Guidelines
•
•
•
•
•
2
Route each I/O signal / signal-group along with a dedicated common
wire (e.g. 0V). This serves as a signal return path and increases
interference immunity.
Connect common wires at the respective common pins of the module
where the specified I/O port is located. Please refer to the module
installation guide for details.
For all types of analog and high-speed I/Os:
Use shielded twisted pair cable
Do not use the shield as a signal or as a return conductor
Ground the shield at the closest grounding point to the I/O port,
preferably a grounding plate or the cabinet body.
This connection method ususally gives the best interference immunity.
However, in some cases, grounding the shield at both ends of the cable
is preferable. In this case, be sure that both points have the same
potential in order to eliminate ground currents through the shield.
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•
Wiring and Grounding Guidelines
I/O Expansion Module
Route those signals separately from high voltage / high current and AC
wiring, as explained in the section Wiring Separation.
Signal (Communication, I/O) Line Filtering
Some environments may induce greater EMI than the typical industrial
environment. Extra power and/or signal line filtering may improve the system’s
immunity to EMI.
If signal-line filtering is required, please use the following guidelines in addition
to the guidelines provided by the filter manufacturer:
•
•
•
Place the filter as close as possible to the target device(s); maximum
wire length is 10 cm.
Signal lines can be filtered using rounded ferrite cores. To maximize
their effect, wind the wire through the ferrite core multiple times to
attentuate high frequencies as shown below.
Always pass both the signal and signal return wires thru the ferrite
core.
If multiple I/O lines share the same common return wire, pass all of
these I/O lines and their return wire through the same ferrite core.
Multiple Windings to Maximize Filtration
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Wiring and Grounding Guidelines
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Tech Support Document
Wiring Power Supplies for I/O Expansion Modules
The examples below are based on the EX-A1.
•
If the power-supply is closer to the PLC than it is to the module’s power
supply (EX-A1):
- Create 0V and 24V junctions at the PLC respective terminals (see the
following figure) and directly connect the PLC main supply lines to the
EX-A1.
- Continue daisy-chaining the supply lines to the expansion I/O units.
Please ensure that the wire segment which carries the 0V
between the EX-A1 and the PLC does not branch.
PLC Connected First (when power-supply is near the PLC)
•
4
If the power-supply is closer to the EX-A1 than it is to the PLC:
- Create 0V and 24V junctions at the EX-A1 respective terminals (see
the following figure) and directly connect the EX-A1 main supply lines
to the PLC.
- Split the supply lines at the EX-A1 terminals and continue daisychaining them to the expansion I/O units.
Please ensure that the wire segment which carries the 0V
between the EX-A1 and the PLC does not branch.
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Wiring and Grounding Guidelines
I/O Expansion Module
EX-A1 Connected First (when power supply is near the EX-A1)
Power-line Filtering
Some environments may induce greater EMI than the typical industrial
environment. Extra power and/or signal line filtering may improve the
system’s immunity to EMI.
If power-line filtering is required, please use the following guidelines in
addition to the guidelines provided by the filter manufacturer:
•
•
•
•
•
Use of power-line filters that comprise an earthing terminal is
recommended. Ground the filter earth terminal as explained in the
section Enclosing and Earthing.
Place the filter as close as possible to the target device(s). The
maximum wire length is 10 cm.
When using the EX-A1 expansion adapter:
Place one filter for both the PLC and the EX-A1 as instructed below.
Please note that you must use a single line filter for both the PLC and
EX-A1.
If the power-supply is closer to the PLC than it is to the EX-A1:
- Install and connect the power-line filter as close as possible to the
PLC. The maximum wire length is 10 cm. Please refer to the filter
manufacturer documentation for installation information.
- Create 0V and 24V junctions at the PLC respective terminals (see the
following figure) and directly connect the PLC main supply lines to the
EX-A1.
- Continue daisy-chaining the supply lines to the expansion I/O units.
Please ensure that the wire segment which carries the 0V
between the EX-A1 and the PLC does not branch.
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Wiring and Grounding Guidelines
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Tech Support Document
PLC Connected First (when power-supply is near the PLC)
•
If the power-supply is closer to the EX-A1 than it is to the PLC:
- Install and connect the power-line filter as close as possible to the EXA1. The maximum wire length is 10 cm. Please refer to the filter
manufacturer documentation for installation information.
- Create 0V and 24V junctions at the EX-A1 respective terminals (see
the following figure) and directly connect the EX-A1 main supply lines
to the PLC.
- Split the supply lines at the EX-A1 terminals and continue daisychaining them to the expansion I/O units.
Please ensure that the wire segment which carries the 0V
between the EX-A1 and the PLC does not branch.
EX-A1 Connected First (when power-supply is near the EX-A1)
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Wiring and Grounding Guidelines
I/O Expansion Module
Connecting the EX-A1 Communication Cable:
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Note that the communication cable is ended by RJ45 plugs. The plug
comprising a yellow-green grounding wire must be connected to the
PLC as shown in the following figure.
This yellow-green wire must be grounded to the metal door panel or
cabinet body (which must be also well grounded) using a screw. There
cannot be any paint or other non-conductive coating between the wire
terminal and metal as this may cause poor conduction.
DO NOT extend the wire.
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