Shielding radio waves

[simon]

Hi
I just bought a delightful pair of sound protective headphones with an internal radio to defeat the noise of the bot when it is working and entertain me at the same time.
However I have discovered an issue of interference when the control box is switched on, and within about 10 feet of it it is almost impossible to hear the radio at all.
Has anyone got any suggestions to shield the control box to silence the interference?
Simon

[richards]

If you're running a spindle, install the appropriate line and load reactors. Don't forget to install an EMI filter also.

[gus]

I would try some other radios before I get all excited. A poorly operating receiver can hear all kinds of things it shouldn't.

Real radios glow in the dark!

[simon]

This is a really good quality radio. I havent experienced any interference anywhere else.
The router makes no impact at all (Porter Cable 3.5hp). The control box seems to be the only thing that interferes. Switch it on and all hell breaks loose between the ears!
Mike what is a emi filter? What I thought was to put the control box into a metal box or a cage - or is this too simplistic?
Simon

[gus]

Simon,

Just so you understand where my comment comes from I have spent better than 20 years servicing RF communications systems. (Radio Transmitters and Receivers) I know a good quality radio when I see it. What you paid for the headphones and how great the sales folks said the are aside, I have reservations about the selectivity of the radio you a talking about. I have a radio that sits less that 1.5 Ft from my control box and there is no problem at all. Try some other radios close to the control box and see it they all have problems. Check the grounding of the machine and your electrical service.

Also does the control box bother any other radios, TV, cellphones, cordless phones, telephones, intercoms in the area? Does the machine operate the way is should or is it doing odd things?

If in fact your controler is radiating rf energy on or very close to the freq you are listening to and the station has a good strong signal at the receiver antenna and it is what your are hearing is this signal being demodulated then you have a bit of a job to do getting rid of it. It may well be radiating out on every lead comming out of the controler. Line and load reactor and EMI line filter are not going to do a lot for you.

If the problem is local and stays on site, as you say "within about 10 feet of it" it would seem less costly to see if you can find a radio that works than lots of time and money trying things that may have marginal effects.

Not sure what you have for a machine but my Alpha controler came in a NEMA metal box. From what I understand some of the others did not. So putting it in a NEMA enclosure could only help.

[patricktoomey]

Are you listening to AM or FM radio? On FM I have no trouble on any chanel with my Delta VFD and 5HP colombo. On AM as soon as you start the spindle you can hear it wind up on the radio and you can't pick up anything after that. This even happens on battery powered radios so I'm assuming as Ted said that it's radiating RF out the wires somewhere. It's enough to block all AM out to at least 20ft. Hopefully it doesn't extend much beyond my shop or the FCC may be paying me a visit :-)

[gus]

If you are trying to listen to a AM radio in a shop with brush type motors and devices using digital switching in operation, the only thing I can say is.

Don't!!!!!

I wouldn't want to risk getting a post deleted again.

[gerald_d]

Simon says he gets interference when he switches the control box on.....and I believe that to mean that he hasn't even started any motors yet, brushed or otherwise (stepper). Which must mean that his chopper (switch-mode) power supply is the culprit? Which means a faulty power supply?

[gus]

That is not the only thing that comes up when power comes on. Not a bad place to start looking. Just because your controler is not running a motor or some other extranious operation is is still very busy looking here and there at inputs and refreshing outputs waiting for you to tell it to do something real.

If you want to spend some money call the local 2-way radio shop and have them bring out a service monitor and take a look at what is radiating out of the controler. And not just on the freq the radio is listening to but around the first and second IF freq. My guess is there is no second IF in the receiver Simon is using. You can sniff around with the monitor and get some fair idea of where any RF is comming out of the controler enclosure and give you very direct feedback on what any changes you make in grounding or cable positioning is doing. When you think you have the EMI thing figured out start looking at intermodulation and some other nasty stuff. You can start here http://en.wikipedia.org/wiki/Electromagnetic_interference

Killer RF interfearance can come from a bad wire connection. Under some conditions that can be a rusty splice in a barb-wire fence.

[simon]

Hi

Answering the questions...
1. FM
2. PRT 96, 2003. It is well earthed (I Think. I guess you can never be well enough earthed.)
3. Even with MO, I still get interference.
4. When control box switched OFF, No interference. Flick the switch, you get interference.
5. Get about 10 feet away, no interference.
I think that is all.

[simon]

6. Box is metal

[davidp]

Simon,

You forgot that the water flows down the plug hole the wrong way around down here. Might also have some effect on the radio waves; -0

I have the same problem with my PrtAlapha. Turn it on forget about radio with in our factory and even in the neighbouring factories. We have tried lots of different filters etc.

Buy an Ipod or broadband internet and stream your radio.

regards,

David
Sydney

[mikejohn]

Or buy a portable CD player and the entire Rolf Harris music collection

..................Mike

[Brady Watson]

Just to add some feedback:

Another botter visited my shop a few months back and he mentioned that he couldn't listen to AM radio in his shop & that his neighbor couldn't either. We did a test in my shop and my PRT was quite a bit noisier than the Alpha on the AM band. There was absolutely no effect in the FM band. I used both a regular stereo hooked to speakers, and then we moved on to the sound reducing AM/FM headphones. You could really hear the difference on the PRT.

I think that stepper motors by nature cause some electrical interferrence of radio waves. It is not an issue for me, but I would be inclined to wrap the steppers with copper (or whatever is the proper material for suppression) as long as I felt that heat/cooling of the motors was not an issue. Might be worth a go.

-B

[gus]

You may want to look at the use of ferrites on leads. See here for some info:

http://www.ce-mag.com/archive/02/11/may.html

Or bypass caps. See this link:http://en.wikipedia.org/wiki/Radio_frequency_interference

The problem is more a issue of radiation from leads in and out of a device than directly out of a well grounded housing. Not to say it can't happen.

[bcammack]

www.allelectronics.com (http://www.allelectronics.com) has some ferrites at good prices. Capacitors, too.

[gerald_d]

A "drive" manufacturer said the following recently:

"I tried filtering the motor phase outputs with an LC low-pass filters. The results were so good I'm thinking of having a "low-EMI" version as well.

The filters are common-mode torroidal chokes on the phase outputs used with non-inductive film capacitors. The -3db cutoff frequency was set to 60kHz, meaning only the 2nd and 3rd harmonics are passed unattenuated (19.531 kHz switching freq). VHF post filtering is via ferrite beads. The measured dv/dt reduced from 3V/ns to 0.03V/ns.

Power supply input will have LC filtering as well to limit conducted noise. The magnetics will make the low-EMI drive 1.5" longer (2.5" by 4" by 0.85")"