[RFI] RFI from variable speed drive 1.1 miles away (very interesting case).

[Lee STRAHAN]

Hi Guys
   Well, very interesting to me as well. I have been trying to get a VFD drive situation "Fixed' for about 5 years now. It took me a year to figure out why I could not just hear a buzz on AM for this noise. It just covers up everything weak with a white noise sound. It all started after an air compressor burned its housing building structure and itself to the ground at the wood products company about 1500 feet away from my farm. They replaced their compressor unto my knowledge with a 100 Horsepower unit on a VFD drive. I suspected that the new building may be the problem when the noise started but had no proof.
  My findings after all this time and the involvement of the power company which I must say Pacific Power is VERY responsive is that it is definitely the VFD on this compressor that leaves me with 20 dB of extra wideband noise over background on 100 Hz CW bandwidth on 160 meters. I reached out to the ARRL but without a usual recording of humming noise or digital occurrences what I had was useless to the ARRL at the time, years ago. I moved my RX antennas 1000 feet away from the power lines in three different directions and the noise would still cover most any DX signal.
    What I eventually found was that it was not radiated noise that is the problem from the plant but it is conducted noise on the power lines that serve both my farm and the wood products business next door and continue to a substation about 2 miles away. I finally bought a used SDR-IQ and I was able to finally see the interference that would ordinarily just sound as wide band noise on any receiver. I mounted one of my Hi-Z amps with a 10 foot whip on my truck and installed the SDR-IQ and laptop for output in the truck cab. It was the immediately clear that the power line was definitely involved as I was able to drive about 2 miles from my farm toward the substation and still detect that line noise pattern on my SDR-IQ. It was also immediately apparent that as I drove into the facility that it was coming from there.
 The company has been somewhat responsive but it has been a year now since their last response. They have unpowered and powered the whole air compressor system with VFD device on and off and we have definitely verified it as the source. Their electricians tend to believe in their ways and not mine even with my license since 1962 and jobs in RF type signals in electronics for 50 years. There are companies that make filters for this at 480 volts 3 phase and the electricians as near as I can tell tend to not believe that these would help and need to try their own fixes. They have tried inductors, transformers, and wire dressing with no avail. 
    I have a rather lengthy and sort of crude video of that truck trip and I will try to trim it, then rescale its size some and  post it on one of my websites within a few days. I will share a link here when this happens.
  I have a promise from the guy in charge of the facility that he will get back to me after his vacation within a few days. I guess we will see about that. 
     Bottom line here is that it sometimes takes all the toys to find power line problems. I have low frequency receivers, HF receivers with a small  DF loop, VHF , UHF  and ultrasonic frequency receivers for AM. None could track this noise except the SDR as it comes from everywhere off the lengthy power line.  Even when the power company opened the 15 KV to 480 V at the plants metal security box transformer for me there was no indication of any arcing. The noise starts about 1.5 MHz in the BC band and continues up just outside the 80 meter band. 
   I will be back no doubt.
Lee   K7TJR  OR

-----Original Message-----
From: RFI <rfi-bounces+k7tjr=msn.com at contesting.com> On Behalf Of Jim Brown
Sent: Thursday, November 17, 2022 11:27 AM
To: rfi at contesting.com
Subject: Re: [RFI] RFI from variable speed drive 1.1 miles away (very interesting case).

On 11/17/2022 7:52 AM, Wes Stewart via RFI wrote:
>   This is very interesting to me as I've posed an earlier, as yet unanswered, question about potential RFI in a Bosch inverter-driven compressor heat pump,.  I am considering this as an alternative to a two-speed conventional compressor unit.  The Bosch is addition to the government mandated ECM blower also uses variable speed condenser fan.  The schematic shows a couple of toroid chokes on the line input, so they've made some attempt at suppression but how effective, who knows.

The analysis I've heard from two very competent engineers (the late Neil Muncy, and an industrial consultant who had encountered VFDs in the
wild) is that a major mechanism is the magnetic field produced by current loops in the path between the controller and the motor.  Both Neil and that EE noted that it is common for motor and controller in industrial installations to be widely separated. If there IS a twisted pair, the resulting transmission line will confine HF currents to it, but if it's NOT, a strong magnetic field will be produced in the random return path depending on "ground" between motor and controller, in addition to the EM field which radiates.

This is in addition to "Pin One-like" problems with termination of the mains "green wire" at the chassis at the point of entry, or not. In products with line filters, it's pretty common for noise current to be emitted on the green wire, which radiates.

73, Jim K9YC


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