Topband: lift station noise

[GEORGE WALLNER]

On Wed, 08 Apr 2009 17:59:04 -0700
  "Jim Brown" <jim at audiosystemsgroup.com> wrote:
> On Wed, 08 Apr 2009 19:12:16 -0400, w2pm at aol.com wrote:
> 
> I don't know, but my guess would be yes. If you are on 
>good terms with your 
> neighbor, you may get some relief with a good line 
>filter. 

If the motor is supplied via a variable frequency drive 
(VFD) it will require some special filtering! These drives 
input single phase (but sometimes three phase) 230 V 60 Hz 
AC and output a vartiable frequency (10 to 70 Hz) 
simulated three phase 230 V AC that is generated using 
streams of constantly modulated 2 to 10 kHz pulses. 
Filtering the input is relatively simple: as Jim said, a 
good Corcom filter will do it. You can not, however put 
one those filters on the outputs! The high value 
capacitors in the filters will load the outputs too much 
(10 kHz!), the filters get hot, the VFD gets hot and the 
motor will not run properly. (Indeed the drive's internal 
CPU may shut if off.) You need to install a two stage 
filtering system: first, connect to the output what is 
called a reactor. It is essentially a three wire very low 
frequency common mode choke (see Newark.com). Following 
the reactor, you need to install three 2200 pF, 1 kV mica 
or ceramic capacitors, one between each output line and 
ground. That should be followed by a HF common mode choke 
that consists of about 8 turns of three wires (suitably 
sized and insulated) wound as a common mode choke on a 2" 
ferrite ring (No. 31 or similar), followed by another 
three capacitors. Now to top if off, you need to run the 
power to the motor using either a shielded "frquency 
drive" cable (amazingly expensive!) or three separate 
RG-58 coax lines, where the inner conductors carry the AC 
power and the shields are tied to ground. Good grounding 
is essential! Shielding the whole thing will help too. I 
have done all that, and it sort of works! VFD's are high 
power (100W+) noise generators! There is a good tutorial 
on VFD's on the omega.com web site.

GL,

George
AA7JV