[RFI] Wiring practices to minimize emissions
wd8dsb at gmail.com
Tue Jul 12 13:25:08 EDT 2016
I would like to add to Jim's (K9YC) comments that most variable speed
drives (VSD's or VFD's) were made EMI/EMC compliant by the drive
manufacturer via the use of an input line filter during the compliance
testing stage of their drive and while the variable speed drive
manufacturers will sometimes specify the appropriate input line filter to
make their product EMI/EMC compliant, most system integrators are clueless
about EMI/EMC compliance and therefore do not include the input line
filters in the final system design (due to ignorance or cost cutting).
The last two commercial RFI issues I tracked down were variable speed
drives that were not using input line filters, and the addition of input
line filters solved the RFI issues in both cases. The drive manufactures
were understanding of the RFI problem, but the system integrators (folks
that designed the complete system) were clueless.
I should also mention that there is special cable available (called VFD
cable) for use between the variable speed drive and the motor that has
numerous benefits (including shielding), and the use of this cable is
highly recommended when the motor is not located close to the variable
speed drive. Belden is one manufacturer that produces VFD cable.
Don Kirk (wd8dsb)
On Tue, Jul 12, 2016 at 12:53 PM, Jim Brown <jim at audiosystemsgroup.com>
> 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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> RFI at contesting.com
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