[RFI] Help on speed drive motor controller RFI

Jim Brown 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
>filters fitted
>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 
my website.  


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

More information about the RFI mailing list