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Author Topic: How do you measure the amount of RF emitted from a DC motor?  (Read 338 times)
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I had a project where I had to put caps along the power leads of an airsoft gun's motor to reduce the interference to the rest of the electronics within a several inches of it. The closer a hobby servo was to the airsoft gun's motor while it was firing, the more the servo twitched. I was told that the caps are reducing the 'noise' caused by the motor.
1. Is that noise in the wire itself, or in the air in the form of RF which is passing through the wire shielding and corrupting the flow of electricity?
2. Either way, how is that measured?
3. Is there a probe that can just be held at varying distances from the source of the interference to measure the strength of the interference?
« Last Edit: January 08, 2013, 08:51:58 am by David82 » Logged

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You can use an oscilloscope and measure the noise that way.
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« Last Edit: January 08, 2013, 10:29:42 am by HazardsMind » Logged

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Maybe I wasn't clear. The question isn't specific to servos. Servos were just an example. Any electronic board in range is affected by the interference caused by the airsoft gun motor. secondly, the caps were not placed on the servo, they were placed on the power leads going to the airsoft gun motor. They did reduce the negative effects considerably.
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1. Is that noise in the wire itself, or in the air in the form of RF which is passing through the wire shielding and corrupting the flow of electricity?

Probably both - things that spark usually radiate radio frequencies, and ordinary brushed motors typically spark at the commutator; the combination of an inductive load and frequent switching as the coils are turned on and off will induce voltages on the supply lines. That's how I see it , anyway. Voltage fluctuations on the motor supply lines could be measured with a scope. RF radiation could be measured using a specialised RF power meter (I've never seen one, but I'm sure they exist) or by testing how close your electronic equipment can be before it is affected. RF signal strength follows the inverse square law so you can estimate relative power from changes in the distance at which an effect is noticed. In any case I guess your goal is to get rid of the effects rather than put a number to the problem, so as long as you can make the symptoms go away that's probably all that matters.
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Its not just RF that get emitted from a motor, you also get EMF. Since all electric motors use coils, they can and do emit EMF. That EMF can cause voltage fluctuations to surrounding electronic devices that are not properly shielded. Simular to a magnet passing over a wire going from a battery to an LED. The magnet will either dim or brighten the LED depending on the direction and strenght of the magnet.

If your getting noise on your nearby electronics then you might want to look into adequate shielding. I wonder if those couplers you find on VGA cables will help shield your wires.
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Its not just RF that get emitted from a motor, you also get EMF. Since all electric motors use coils, they can and do emit EMF. That EMF can cause voltage fluctuations to surrounding electronic devices that are not properly shielded. Simular to a magnet passing over a wire going from a battery to an LED. The magnet will either dim or brighten the LED depending on the direction and strenght of the magnet.

If your getting noise on your nearby electronics then you might want to look into adequate shielding. I wonder if those couplers you find on VGA cables will help shield your wires.


RF is an EMF, there is no difference in the physics

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@retrolefty

Yea I was a little ify after I posted that, so I checked it out. Their is MF, EF, and RF, I was uncertain if they all were related or not, but I see that they are.
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This is a handy guide for testing RF emissions:-
http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CEsQFjAA&url=http%3A%2F%2Fwww.astat.com.pl%2Fdownload.php%3Ffile%3Dpliki%2Fschaffner_katalog_emission_gb.pdf&ei=NFHsUOXtKcel0AXw5oCoDw&usg=AFQjCNFd7f2jr1ndCzIRWAvcumBSuAmy6g&bvm=bv.1357316858,d.d2k

The equipment is not cheap and to correctly use it you need an RF dead chamber so that you only measure the thing you want to measure.
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