Electromagnetic Compatibility Guidelines for Machine Design
C-5
Installation Manual for Models ODM-005, ODM-005i, ODM-010, ODM-010i, ODM-020 and ODM-020i
Shielding and Segregation
The EMI radiating from the drive enclosure drops off very quickly over distance. Mounting the drive in
an enclosure, such as an industrial cabinet, further reduces the radiated emissions. The cabinet should
have a high frequency ground and the size of the openings should be minimized. In addition, the drive is
considered an “open” device which does not provide the proper IP rating for the environment in which
it is installed. For this reason the enclosure must provide the necessary degree of protection. An IP rating
or Nema rating (which is similar to IP) specifies the degree of protection that an enclosure provides.
The primary propagation route for EMI emissions from a drive is through cabling. The cables conduct
the EMI to other devices, and can also radiate the EMI. For this reason, cable segregation and shielding
are important factors in reducing emissions. Cable shielding can also increase the level of immunity for
a drive. For example:
•
Shield termination at both ends is extremely important. The common misconception that
shields should be terminated at only one end originates from audio applications with fre-
quencies <20 kHz. RF applications must be terminate the shield at both ends, and possibly at
intermediate points for exceptionally long cables.
•
When shielded cables are not terminated at the cable connection and pass through the wall of
a cabinet, the shield must be bonded to the cabinet wall to prevent noise acquired inside the
cabinet from radiating outside the cabinet, and vice versa.
•
When shielded cables are terminated to connectors, the shield must be provide complete
360° coverage and terminate through the connector backshell. The shield must
no
t be
grounded inside the connector through a drain wire. Grounding the shield inside the connec-
tor couples the noise on the shield to the signal conductors sharing the connector and virtu-
ally guarantees failure to meet European EMC requirements.
•
The shield must be continuous. Each intermediate connector must continue the shield con-
nection through the backshell.
•
All cables, both power and signal, should use twisted wire pairing.
The shield termination described above provides a coaxial type of configuration which provides magnetic
shielding, and the shield provides a return path for HF currents that are capacitively coupled from the
motor windings to the frame. If power frequency circulating currents are an issue, a 250 VAC capacitor
should be used at one of the connections to block 50/60 Hz current while passing HF currents. Use of a
properly shielded motor cable is essential to meet European EMC requirements.
The following suggestions are recommended for all installations.
1. Motor cables must have a continuous shield and be terminated at both ends. The shield must con-
nect to the ground bus bar or drive chassis at the drive end, and the motor frame at the motor end.
Use of a properly shielded motor cable is essential to meet European EMC requirements.
2. Signal cables (encoder, serial, analog) should be routed away from the motor cable and power
wiring. Separate steel conduit can be used to provide shielding between the signal and power
wiring. Do not route signal and power wiring through common junctions or raceways.
3. Signal cables from other circuits should not pass within 300 mm (1 ft.) of the drive.
4. The length or parallel runs between other circuit cables and the motor or power cable should be
minimized. A rule of thumb is 300 mm (1 ft.) of separation for each 10 m (30 ft.) of parallel run.
The 30 mm (1 ft.) separation can be reduced if the parallel run is less than 1 m (3 ft.).
5. Cable intersections should always occur at right angles to minimize magnetic coupling.
6. The encoder mounted on the brushless servo motor should be connected to the amplifier with a
cable using multiple twisted wire pairs and an overall cable shield. Encoder cables are offered in
various lengths that have correct terminations.
Persistent EMI problems may require additional countermeasures. The following suggestions for system
modification may be attempted.
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