[RFI] Help on speed drive motor controller RFI
jim at audiosystemsgroup.com
Sun Jan 28 17:38:55 EST 2007
On Sun, 28 Jan 2007 00:00:24 +0100, Juan EA5RS wrote:
>A new water pumping station using a variable speed drive was recently
>installed on the roof of the apartment building where I live, just a few
>meters from my HF antennas. RFI from the speed drives is tremendous, putting
>more than -60 dBm on a 500 Hz BW on my RX on 3,5 MHz. The drive has no RFI
>Based on manufacturer description these drives are based on "space vector
>technology" "using carrier frequencies up to 15 KHz"
>What I will try is to get rid of the speed drives,
You won't get rid of them -- they are far too closely integrated into the
design of the unit. You MAY be able to reduce the RF trash a lot. I suggest
that you download and study the tutorial on RFI that I have recently posted to
This "15 kHz technology" is basically a square wave driving the motor, with
all the associated harmonics, with LOTS of current going through the wires
between the controller and the motor(s). Those wires are antennas, and are
radiating the trash. The trash may ALSO be radiated by wiring within the
controller (due to poor shielding), and it may be conducted back through the
power line and radiated.
What typically happens with these variable speed motors is that the controller
and the motor are separated by 10-30 meters, and are both bonded to building
steel. This combines with capacitance between line, neutral, and "ground" to
form a large RF current loop and antenna. So you have both magnetic coupling
(you're close enough to be in the near field) and antenna action.
Suppression may not be easy, thanks to the large currents involved. One good
start would be to slow down the rise time of the square wave. But the mfr
would need to be involved in that.
Once you've studied this material, you could try contacting the manufacturer
to find out what RF suppression is available for the unit. And you should also
learn what the laws in your country say about RFI.
Jim Brown K9YC
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