EMI Filtering

[Mike Betts]

All,

FIRST has reversed its position on EMI filters on motors (http://forums.usfirst.org/showthread.php?t=4867).

I have lobbied for this for a few years now and applaud FIRST's decision to allow it.

I would invite technical mentors who have proven and demonstrated success at EMI filtering (example) on a FRC robot to post details on this thread.

What parts did you use?
How was it installed?
What was the result of your investigation (what problem did it solve)?

To keep confusion to a minimum, please limit posts to factual rather than theoretical. I know everyone has an opinion but time is limited before the next regional...

Thank you.

Mike

[Al Skierkiewicz]

Mike,
I normally would recommend a ceramic cap from each motor lead to the motor case as near to the motor as possible. The value can be .01-.1 mfd at anything over 200 volts, but the leads should be kept as short as possible. This will, in effect, create a low pass filter that will dump the arc energy back into the motor case. However, our robot frames are not at ground potential (but do present a capacitive effect to ground).
It may be just as effective to place one cap across the terminals. This would shunt the energy and would not force the noise into an ungrounded chassis. Insulation on this type of connection is of prime importance. On motors like the chalupas with motor leads already installed, I would recommend that teams twist the wires together as they leave the motor. Try this before using caps. It is possible to achieve 2 to 3 turns per inch which will give some capacitive coupling between the leads. For the FP motors, the caps can be applied at the motor tabs. Use teflon or other insulating tubing on the cap leads and the solder directly to the tabs. (Use caution, heat will transfer to the motor brush and breaker assys. A larger tip, well wetted with solder and a tinned lead on the cap will make this a safe and quick operation.)
You can add the cap lead to a crimp terminal when attaching the wire for a motor (crimp both the wire and cap lead in one terminal). Again, insulate the cap leads. I think the Keyang motors have some internal suppression but likely will not produce as much interference as the larger motors.

Be advised...This is a large departure for FIRST to allow the use of these devices. They have been illegal for as long as I can remember, even predating the 883 Victors.

Be advised II...You may think that bigger is better in this application but there is significant diminishing returns. Stick with the ceramic caps and never use electrolytics. I hate the smell of hot electrolyte in the morning, it stays with you all day and wrecks the look of aluminum.