On Tue,7/12/2016 8:55 AM, nm8rmedic via RFI wrote:
I would appreciate some input from the group on talking points. This could
also be the chance to influence wiring design for many of this manufacturer's
lift station packages.
Key issues are:
1) Include in your specification for equipment that all shield
connections are bonded to the shielding enclosure at the point of
entry/exit. This addresses what we in the pro audio world call "the Pin
One Problem." It is a MAJOR cause of RFI, both emissions and
susceptibility. Likewise, require that all "green wire" (equipment
ground) conductors be bonded to the chassis/frame/shielding enclosure at
the point of entry, NOT via internal wiring, NOT to a circuit board.
Also, specify that this connection be carefully inspected to verify that
is it is not insulated from the frame/chassis/enclosure by paint!
2) Require the use of twisted pair for ALL circuits. When the circuit is
"ground" referenced, require that the ground-referenced connection be to
the shielding enclosure at the point of entry/exit.
3) Pay VERY careful attention to every element of variable speed motor
systems, including their control wiring. Specify twisted pair, a pair
per phase of the wiring between the controller and the motor, specify
that controllers and the motors they control be as close together as
physically possible, and that the chassis of the controller and the
frame of the motor be bonded together, with the bonding conductors
following the path of the power conductors.
4) Specify that all wiring be installed in steel conduit that is
continuous over its entire length and bonded to the equipment on both ends.
The common element here is that most emissions at HF and below are
common mode and are radiated on wiring connected to the noisy equipment.
Even DC and low frequency AC circuits should be assumed to have strong
RF noise components as a byproduct of how the power is generated,
controlled, and used. Those variable speed motor controllers are an
extreme case -- the power they provide to the motors are rectangular
waveforms, with their width adjusted to control motor speed. Repetition
rates are typically in the range of 10-20 kHz.
Henry Ott's classic EMC text is a great reference, and is widely
considered the "bible" by most EMC experts.
http://www.hottconsultants.com/book.html It's published by Wiley, and is
available from all major book sellers. You want the latest (2009) edition.
73, Jim K9YC
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