At 11:05 PM 6/24/2001 EDT, you wrote:
>
>I really having a hard time getting my 120VDC motor to meet the 48dbuV
>conducted emission requirements for FCC.
>I put in common and differenital mode chokes, that helps but does not
>completely fix the problem.  Also tried chassis caps and such.
>
>Anyone have any suggestions?
>Thanks,
>Paul
>

This worked for us in the past, but will depend on the type and size of
motor, your enclosure, etc. I assume this motor has brushes; brushless DC
motors are much quieter. I also assume this motor is driven by clean DC,
not just rectified AC...

1. Put 2 parallel caps across the motor leads where they exit the motor, as
close to the motor as possible. These should be ceramic caps of about 250
volt rating, one .001uF and one .01uF.

2. Also put a transient voltage suppressor across the caps. This should be
a zener type, not a MOV, since MOV's will deteriorate over time with
repetitive spikes. You could look at Digi-Key part # P6KE150CADICT-ND, this
has a 150 volt rating. (www.digikey.com).

However, if using rectified unfiltered AC, the rating of the transient
suppressor should be about 200 volts...

This will clamp spikes from the commutation and also protect the caps.
Depending on the size of the motor, you should check the temperature of
this while the motor is running to make sure it can handle it.

3. Put large ferrite beads on each lead of the motor, as close to the motor
as possible. We used 3/8" diameter X 3/8" long Fair-Rite 77 material toroid
cores with 2 or 3 turns through each. But any high-perm material should
work...

4. Put another .001uF cap across the motor leads right after the beads.

5. Run BOTH leads through another ferrite core, 2 or 3 turns. Wind both
leads in the same direction. Add .01uF caps from each lead to GND after the
core.

6. Ground the case of the motor, either directly or through a .01uF cap.
Hopefully you're using a metal or shielded plastic enclosure (?).

The above should help; it sounds like a lot of stuff but if the motor is
mounted on a PCB, most or all of this stuff can go on the PCB; just keep
the parts close to the motor. Or you can mount a small separate PCB on the
motor to hold the parts...

Also, is this motor controlled by PWM? If so, the controller itself may be
producing RFI and may need filtering at the output. You didn't give
details, but if you're using phase-controlled rectified AC, you would need
some serious filtering of this before running it to the motor....

Good luck...
Maris

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