Hi Jim,
I've addressed RFI issues with this sort of motor controller in several
of the RFI tutorials/app notes on my website. As W0LEV has noted,
filters are required and they probably help. Maybe enough, maybe not.
Twisting power conductors inside conduit is quite acceptable under NEC
provided that the conductors used are rated for the voltage and the
conditions of installation. It's VERY rarely done, so electricians will
give you blank stares, and inspectors may tell you it's not permitted. :)
There ARE environmental requirements for the specific types of cable
that may be installed. They are based on flammability, toxic fumes, and
flame spread, and were instituted after the real world "Flaming Inferno."
Using twisted pair for the current-carrying conductors DOES greatly
reduce noise at both AF and RF. It's also quite important that neutrals
be bonded to ground at one, and ONLY one, point within a system. A
double-bonded system allows neutral current to divide between neutral
and ground, which creates a massive magnetic field. In a proper system,
the magnetic field is confined to a narrow region between the
conductors, because the currents are equal and opposite.
If, for example, it's a wye-connected 3-phase motor, each phase should
be twisted with a neutral. If delta, each side of the delta should be a
pair.
Another potential problem is the a bond can be formed at RF by
capacitance between neutral and ground at the end without the DC bond.
At RF, this produces the same mess as the DC bond.
In some jurisdictions, metallic conduit is either required or commonly
used.
Another issue with using twisted pair is that 1) it's harder to pull; 2)
larger overall diameter; 3) conduit may not be large enough; it can be
difficult to pull new conductors through the existing conduit system.
73, Jim K9YC
http://k9yc.com
On 12/18/2017 4:09 PM, jimk8mr--- via RFI wrote:
I've recently discovered a major RF noise source at our Florida condo - pumps
or other swimming pool related equipment about 200 feet from my attic dipole.
I'm not planning to do any followup with management until after the holidays,
but I've been doing some thinking about it.
There are some pumps outdoors, but any breaker panels or disconnect switches
are inside a locked room I've not yet been inside to inspect. I suspect it
would be easier to install toroid chokes inside - if so, are there any issues
with NEC, or any other issues, about using twisted pair AC wiring inside (PVC)
conduit between those panels and the outdoor motors?
73 - Jim K8MR
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