[RFI] Help on speed drive motor controller RFI

dgsvetan at rockwellcollins.com dgsvetan at rockwellcollins.com
Mon Jan 29 17:31:33 EST 2007


The square waves are the result of efficient electronic drive to the
motors.  Power electronics will, in general, "toast" if the drive waveform
has a slow risetime.  Semiconductors that handle many amps of current need
to switch quickly so as not to develop excess heat in their junctions (the
heat being the product of current and whatever the on-resistance is of the
particular devices).  Since the motors track the drive frequency, the
semiconductors only have to be either on or off.

I work as an EMC engineer.  Prior to my current employment, I worked for a
company that specialized in electromagnetic shielding systems and power
filter line filters.  I was asked to do an RFI assessment of very large
automotive dynamometers that were going to be used within the confines of
an anechoic test chamber.  The dynamometers were driven by large, variable
speed electric motors using variable frequency drives.  The radiated energy
was horrible anywhere around the dyno itself or the drive electronics.
Since the idea of the anechoic test chamber was to measure EMC performance
of vehicles under varying driving conditions, it was important that the
test instruments in the chamber not pick up the radiated emissions from the
dyno drive system.  It took large filters on the drive lines to the motors
and a lot of welded metal conduits in which to run those lines to get the
noise down.  Of course, the motor had to be well-grounded and some extra
shielding added around it.  The drive electronics were outside the chamber,
so as long as they were on a a separate set of power feeders (to avoid
putting noise on the power feeds to the RFI test equipment), all was well.

Good luck on this.  The information from Jim, Tom, and Ian is right on the
money.  If you really can get the motor changed out to a fixed speed type
with simple on-off control, most of your problem should go away.  If not,
you'll need to consider either workign to get RFI supression added to the
variable speed system or moving your antennas further away from the motors.

73, Dale

             "Juan EA5RS"                                                  
             <ea5rs at ono.com>                                               
             Sent by:                                                   To 
             rfi-bounces at conte         "'Tom Rauch'"                       
             sting.com                 <w8ji at contesting.com>, "'Ian White  
                                       GM3SEK'" <gm3sek at ifwtech.co.uk>,    
                                       <rfi at contesting.com>                
             01/29/2007 04:00                                           cc 
                                       Re: [RFI] Help on speed drive motor 
                                       controller RFI                      

Thank you very much for all responses, both direct and thru the reflector.
It was very enlightning to hear your opinions.

When I said the speed controller did not have any RFI filters fitted, I
meant it was not using the separate, optional RFI and "dV/dt" filters
offered as options by the manufacturer. I actually don't know if the
controller has any type of RFI filtering inside.

I have to agree that even performing to EMC standards, and I am sure this
one is not, the RFI from this controller is absolutely unacceptable for
signal reception.  Just from the beginning I anticipated this would be a
very tough RFI problem to deal with due to proximity and tremendous QRM

That is why, to be practical, it seems to me that the only feasible total
cure is to avoid the speed controllers all together, and this is going to
my goal. According to an Hydraulic Engineer I have been consulting with,
pressure controlled system and specifically the pump motor does not demand
variable speed drive at all, and a simple relay "on-off" controller could
the job and this would completely eliminate the problem. Now I need to be
persuasive with the contractor and my neighbours.

Still, the doubt remains why this controller generates so much QRM while
those others I am sure also exist near my antennas (i.e. elevator motor
controllers) are apparently QRM free. I also would like to know if the fact
the controllers supply currents near square wave is a real requirement for
motor control or it is just the way it is being done. I have a hard time
believing slightly rounding the waveform could impair motor performance.

Juan EA5RS

-----Mensaje original-----
De: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com] En
de Tom Rauch
Enviado el: lunes, 29 de enero de 2007 11:37
Para: Ian White GM3SEK; rfi at contesting.com
Asunto: Re: [RFI] Help on speed drive motor controller RFI

> Please read what was actually written, Tom.

The EA5's post must have choked in a spam filter, but  my
points are twofold.

1.) even when the supply meets or exceeds FCC and CE specs,
it can cause bad RFI problems.

2.) even if we don't see an obvious filter it may have one

I certainly agree a unit that fails CE and is labeled CE in
theory gives a complainer great power, but I can tell
everyone from first hand experience units that meet or
exceed CE can cause slight problems a mile way and can be
devastating within a few hundred meters.

There is a chicken farm about two miles from that has a
controlled made in the Netherlands that does pass CE and FCC
but still has birdies several dB out of my noise on 160

I worked on good three phase controllers at another location
that were several hundred feet from the receiving antenna
and caused bad problems in a moderate background noise

It is virtually impossible for most people to test for CE or
FCC compliance or to know if there are really filters in the
unit (if they are necessary to pass). The bottom line is if
it bothers the radio... and a device that passes certainly
can be a problem. In such cases, and they are common,
filters from the manufacturer often are useless.

73 Tom

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