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Author Topic: Commercial Building Electric Motor Noise  (Read 793 times)
AJ4TG
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Posts: 4




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« on: August 16, 2010, 06:40:47 AM »

We have a Yaesu FT-2000 that is part of a amateur radio station at the company I work for (the radio equipment is mainly intended to support local disaster communications...non-business).  We recently moved the equipment from one building to another and are now experiencing a high level of noise on 80m (S9+20) and 40m (S9+10).  The noise sounds like an electric motor running through the speaker.  The coax feedlines run in a bundle through the building to an electrical service room, and then up 2 floors to the roof.  I suspect the noise is coming from a piece of equipment in the electrical service room (when entering the room I can hear a motor running at what sounds like the exact same RPM). 

I have read about electric motor noise in mobile installations and trying to fix the problem by installing filters in-line with the offending motor(s).  However, this would not be an option for us because the equipment I suspect is a large UPS for the building network servers.  Also, the path for the feedlines is fixed...there are no other routing options that our facilities management folks would even consider (like boring new holes through the floors).

Are there any other options to explore?

If this topic has been discussed elsewhere, I was unsuccessful in finding it.  If so, could someone point me to the right place or suggest some alternate search keywords?

Thanks!
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W6RMK
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Posts: 651




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« Reply #1 on: August 16, 2010, 07:28:01 AM »

Most AC motors (bigger than 1/2 HP or so) are induction motors of one sort or another and don't have brushes, etc., and don't generate RF noise.

Are you hearing interference with a varying pitch?  (like alternator whine or ignition noise in a mobile?)
You might be getting interference from a Variable Frequency Drive: a motor speed controller that changes the frequency of the AC going to the motor.  Basically a 3 phase inverter. They're very common these days (fan speed, pump, elevators, positioning equipment, hoists, etc.) and the output waveform is often not super low in harmonics (after all, you're driving a big inductive load, which ignores all the high frequency).

A 3 phase interference source is going to sound higher frequency than a single phase source (because you get 3 times as many noise pulses/cycle).. 360 Hz is pretty common and is around F above middle C.


As to what you can do..
Odds are, you're getting the noise through your power supply or similar. But, you need to rule out getting the noise through the antenna.  Put a dummy load at the end of the coax run and see if the noise goes away.  If it does, then perhaps a different antenna or different antenna location will help.

Have you tried running the rig off a battery sitting on the table with the rig (no long wires, no charger, etc.)

What does the grounding look like? Are you inadvertently coupling noise in through a big grounding loop?  You imply that you're in a commercial building, so what did they do about the coax coming in from outside?  How is it grounded? Where is the antenna discharge unit?

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WA3SKN
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Posts: 5486




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« Reply #2 on: August 16, 2010, 09:04:10 AM »

First, disconnect or short the antenna jack at the radio... do you still have the noise?
This should eliminate the power supply and the ground.
Then, reconnect and go to the antenna itself and terminate the coax into a dummy load... do you still have the noise?  This eliminates local pickup through the coax.
This will isolate the problem to noise pickup via the antenna, the typical situation.
Now you will have to determine or locate the noise source.
Distance (can you move the antenna or noise source?), shielding (can you shield the noise source?), and filtering (can you filter the noise source?) are then the solutions to the problem.
Nowadays, 95-99 percent of the time the noise is incoming via the antenna.  Filtering the power only works if the noise is incoming via the power leads.
73s.

-Mike.
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