To add to what Jim had said about this "smells like a Variable Frequency Drive
[VFD]" (which I also think is probably what you're seeing here):
VFDs operate by varying the power to a three phase motor to control speed and
torque. This is done through PWM (Pulse Width Modulation) at a frequency in the
kHz+ range. They rely on the inductance of the motor windings to integrate the
switching waveform into essentially a sine wave of varying voltage and
frequency. Since the motor itself is part of the filter network, the wiring
between the VFD and the motor can sometimes radiate. VFDs will commonly specify
that the wiring between the drive and the motor be of no more than a specific
length (which is usually relatively short such as 20-50 feet or so), and that
it be in a *metal* conduit which helps with shielding. You should check that
the cabling between the VFD and the motor was installed properly.
The other possibility is that noise is coming from the supply lines (the normal
AC supply wiring) to the VFD. This is probably less likely than the
VFD-to-motor wiring, but it's something to check. You can get a "reactor" to
use for helping with AC line distortion that might also help with reducing
radiated noise if it's coming from the power wiring. The reactors are just
small inductors that help to "round off" the switching transients produced by
the rectifier in the VFD.
Good luck in your worthy quest to eradicate some RFI!
> -----Original Message-----
> From: Topband [mailto:email@example.com] On Behalf Of Don
> Sent: Saturday, December 13, 2014 8:35 PM
> To: topband
> Subject: Re: Topband: Boradband noise on 160 meters tracked down to
> apartment complex
> Update on the apartment complex noise that I tracked down to a new
> apartment complex in late October.
> Finally was able to meet with the maintenance manager for this
> facility this past Thursday, and we were able to track the source of
> the interference to an HVAC air handler unit located on the roof of
> the complex. Once again used my portable HF receiver (without my
> preamp) with 14" diameter tuned loop, and had to back the gain way
> down on the receiver to locate the source. We were able to turn on
> and off the unit and observe the RFI turn on and off as we turned the unit on
> and off.
> I then drove back to my house, and we ran the tests again and sure
> enough the interference at my house was on when we turned the air
> handler on, and RFI went away when the air handler was turned off.
> Now going back to the contractor that installed the equipment in an
> attempt to better understand the hardware in the unit (hope to get
> some prints,
> etc.) before having them actually attempt to tackle the problem.
> Fortunately I have a couple of years of HVAC experience from an
> electrical controls standpoint (had to revamp a faulty design for our
> manufacturing plant), and suspect this past experience will come in
> handy during our investigation.
> Just FYI,
> Don Kirk (wd8dsb)
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