[RFI] Help on speed drive motor controller RFI

Tom Rauch w8ji at contesting.com
Tue Jan 30 05:00:33 EST 2007

> The product brochure Juan linked to shows RFI filters as 
> accessories. It
> appears they need to be ordered separately and assembled 
> on-site. The
> installation manual may also specify that the motor leads 
> be shielded. That
> is a common requirement for this type of device--one often 
> ignored by the
> installers, to save time and money.
> So the first step is to be sure the unit is installed as 
> specified by the
> manufacturer, including all recommended filters and wired 
> per the manual.

I've been through that too with almost identical 
controllers, and the problem is getting electricians to 
understand shielding.  The motor and leads are the big 
issues. They carry the high current high voltage square 
waves. They have to be in metallic conduit that has solid 
connections all the way from the controller (where it MUST 
be attached to the power line filter ground and the 
controller ground with very short leads) to the motor 
housing, which must not have any large insulated gaps in the 
metal housing and a shaft with good electrical connection to 
the housing..

When I swept standard line filters from another manufacturer 
of similar devices (SEW-Eurodrive) the better filters had 
high attenuation up to the area of the AM BC band and then 
quickly fell apart.

Rather than shield the motors and leads properly I elected 
to build low pass filters that had very high voltage rating. 
The filters had very high impedance series inductances 
throughout HF and very low shunt capacitor values ( a few 
hundred pF). The filters were inductor input. This prevented 
loading the FET's with high values of capacitance  (which 
could damage them). Then the motor leads can be anything.

I used a very good standard shielded CE approved line filter 
for power lines.

The entire thing went in a large aluminum chassis with 
screen cover for viewing and ventilation and smaller sub 
chassis for output filters and control line filters. That 
went into a conventional enclosure.

73 Tom 

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