Reducing EMF on cooling fans

Capacitors across the motors? Ferrite beads near the motors?

Those are 'Brushless DC motor' type fans with an internal three phase generator. That is the source of the noise, and has to be filtered with a grounded screen shield that will pass the air, but not the RF.

Physical proximity... use a hose to the incubator, rather than having the motor be directly attached. Surround it with a sheet metal cage. It'll be quieter too, which is, in my view, a more likely culprit than emf.

Regards, Bob Monsen

The fans also blow fresh air over the babies, which the tape recorders don't. I'd suspect that the average baby notices hot/cold more than it notices weak magnetic fields.

What noise? Spikey PWM from your driver, or the low-frequency mag field from the fan?

John

My measurement device does not say. But the PWM is flat DC at 100% and since it is bit-banged by a 8051 is not too terribly fast either ;-) So I'm sure it is the mag field from the fans. easy to modify ? not likely .

Brushless DC fans draw spikey current and locally make AC magnetic fields. Both are low frequency, numbers roughly like 80 Hz. The current ripple can get into power supplies and bother low-level analog circuits, and may cause ground loop noise. The mag fields can induce low-level (microvolts, typically) voltages into loops on nearby circuit boards.

What is your "measurement device"?

John

OK don't laugh - (C) 1995 Connectware PCboard with 10 LEDs on it. It has a 43mm square antenna loop area (in to 18 mm at the center) it sports an LM 3914V and a TLE2114C IC. Powered by 9V battery, purchased from BG Micro about ten years ago. It lights up all the LEDs when I get to within 3 inches of the fans, tapering off to zero within 10 inches from the fans. It WAS sold as EMF detector. It seems to work as such. It is what I have. It is quite sensitive, a fraction of an inch makes a big difference in the display.

Why are you "tasked" to satisfy what is probably a crap, uncalibrated instrument?

John

The example of heart rate variability in newborns in incubators may be caused by 50-60 Hz EM fields, which is a dangerous frequency that can cause fibrillation (but usually because of conducted current through heart muscle). If your instrument shows zero levels at 10" from the fans, then why not maintain that distance? Also, see if there is a problem if you run the fan on a smooth DC signal. You can filter the PWM if that is the cause of the noise. And if your project is as sensitive and important as protecting infants in incubators, you really need to invest in some accurate and reliable equipment, and you probably should get data on the field strength that was measured in the experiments you cited in the article.

A simple magnetic field sensor can be made from an air core choke, and can be connected to a scope to see the frequency and intensity. The choke's axis can be oriented to determine the direction of the field. This will probably be more useful than your unknown instrument. You can probably produce a known field strength by passing a measured current through a toroidal transformer and placing the sensor in the hole.

Paul

Actually our incubator is for exotic bird eggs. The article prompted us to investigate. While there may be no evidence suggesting that EMF harms eggs we thought it wise to check into the EMF angle as far as our hardware is concerned, and perhaps claim superiority on the basis of "LOW EMF ENVIRONMENT IS GOOD FOR YOUR $10'000 EGGS" . What I have found so far: "our" EMF is not from line AC but from the fans, internally (not PWM). I will find out the frequency as per your suggestion above - thank you. Fritz

I think you mean 'EMI' - electromamgnetic interference.

I have found huge variations in the EMI of 80mm axial fans from one manufacturer to another. I'd suggest you try several different brands.

As you've discovered, not much of the noise is from the supply current but do put a largish cap (100uF perhaps) across the supply rails where the fan plugs in (plus a little series DCR or some L too). I expect the EMI's mostly from the internal magnetics and you can't see those but you can choose by manufacturer.

Graham

I use something very similar ! :~)

Graham