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| 03 May 2008 23:20:34 |
| Fritz Oppliger |
| Reducing EMF on cooling fans |
I am tasked with reducing "EMF" in the vicinity of computer type brushless 12VDC cooling fans. Problem similar to http://www.sciencedaily.com/releases/2008/04/080430201654.htm As I am controlling speed with PWM I suspected radiation from the feed wires. But even going 100% (no switching) the noise is there. Reducing duty cycle to about 20% reduces the noise but is not useable - it is near stalling speed and I DO need the fans to fan. Twisting the supply lines does not help much. What other quick fixes could I try? I suspect the motors themselves are noisy and all I can do is try different models / brands? ** Posted from http://www.teranews.com** |
| 04 May 2008 06:48:50 |
| Stephen J. Rush |
| Re: Reducing EMF on cooling fans |
On Sat, 03 May 2008 23:20:34 -0700, Fritz Oppliger wrote: > I am tasked with reducing "EMF" in the vicinity of computer type > brushless 12VDC cooling fans. Problem similar to > http://www.sciencedaily.com/releases/2008/04/080430201654.htmAs I am > controlling speed with PWM I suspected radiation from the feed wires. > But even going 100% (no switching) the noise is there. Reducing duty > cycle to about 20% reduces the noise but is not useable - it is near > stalling speed and I DO need the fans to fan. Twisting the supply lines > does not help much. What other quick fixes could I try? I suspect the > motors themselves are noisy and all I can do is try different models / > brands? > ** Posted from http://www.teranews.com** Capacitors across the motors? Ferrite beads near the motors? |
| 04 May 2008 10:34:11 |
| Michael A. Terrell |
| Re: Reducing EMF on cooling fans |
Fritz Oppliger wrote: > > I am tasked with reducing "EMF" in the vicinity of computer type > brushless 12VDC cooling fans. Problem similar to > http://www.sciencedaily.com/releases/2008/04/080430201654.htm > As I am controlling speed with PWM I suspected radiation from the feed > wires. But even going 100% (no switching) the noise is there. Reducing > duty cycle to about 20% reduces the noise but is not useable - it is near > stalling speed and I DO need the fans to fan. > Twisting the supply lines does not help much. What other quick fixes could > I try? I suspect the motors themselves are noisy and all I can do is try > different models / brands? > ** Posted from http://www.teranews.com** 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. -- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida |
| 04 May 2008 09:11:41 |
| Bob Monsen |
| Re: Reducing EMF on cooling fans |
"Fritz Oppliger" <fritz@fritzop.dot.com > wrote in message news:op.ual88kqny8zufa@tosh... > > I am tasked with reducing "EMF" in the vicinity of computer type > brushless 12VDC cooling fans. Problem similar to > http://www.sciencedaily.com/releases/2008/04/080430201654.htm > As I am controlling speed with PWM I suspected radiation from the feed > wires. But even going 100% (no switching) the noise is there. Reducing > duty cycle to about 20% reduces the noise but is not useable - it is near > stalling speed and I DO need the fans to fan. > Twisting the supply lines does not help much. What other quick fixes could > I try? I suspect the motors themselves are noisy and all I can do is try > different models / brands? > ** Posted from http://www.teranews.com** 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 |
| 04 May 2008 09:21:35 |
| John Larkin |
| Re: Reducing EMF on cooling fans |
On Sat, 03 May 2008 23:20:34 -0700, "Fritz Oppliger" <fritz@fritzop.dot.com > wrote: > >I am tasked with reducing "EMF" in the vicinity of computer type >brushless 12VDC cooling fans. Problem similar to >http://www.sciencedaily.com/releases/2008/04/080430201654.htm 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. >As I am controlling speed with PWM I suspected radiation from the feed >wires. But even going 100% (no switching) the noise is there. What noise? Spikey PWM from your driver, or the low-frequency mag field from the fan? John |
| 04 May 2008 10:49:49 |
| Fritz Oppliger |
| Re: Reducing EMF on cooling fans |
On Sun, 04 May 2008 09:21:35 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com > wrote: >> As I am controlling speed with PWM I suspected radiation from the feed >> wires. But even going 100% (no switching) the noise is there. > > What noise? Spikey PWM from your driver, or the low-frequency mag > field from the fan? > 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 . ** Posted from http://www.teranews.com** |
| 04 May 2008 15:40:38 |
| John Fields |
| Re: Reducing EMF on cooling fans |
On Sun, 04 May 2008 10:49:49 -0700, "Fritz Oppliger" <fritz@fritzop.dot.com > wrote: >On Sun, 04 May 2008 09:21:35 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > > >>> As I am controlling speed with PWM I suspected radiation from the feed >>> wires. But even going 100% (no switching) the noise is there. >> >> What noise? Spikey PWM from your driver, or the low-frequency mag >> field from the fan? >> >My measurement device does not say. --- What are you using for a "measurement device"? --- >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 . --- You posted earlier that twisting the leads to the fan helped somewhat, so you may want to try shielding the leads as well. JF |
| 04 May 2008 14:23:56 |
| John Larkin |
| Re: Reducing EMF on cooling fans |
On Sun, 04 May 2008 10:49:49 -0700, "Fritz Oppliger" <fritz@fritzop.dot.com > wrote: >On Sun, 04 May 2008 09:21:35 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > > >>> As I am controlling speed with PWM I suspected radiation from the feed >>> wires. But even going 100% (no switching) the noise is there. >> >> What noise? Spikey PWM from your driver, or the low-frequency mag >> field from the fan? >> >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 . > >** Posted from http://www.teranews.com** 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 |
| 04 May 2008 16:18:44 |
| Fritz Oppliger |
| Re: Reducing EMF on cooling fans |
On Sun, 04 May 2008 14:23:56 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com > wrote: > > 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"? > 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. ** Posted from http://www.teranews.com** |
| 05 May 2008 13:12:06 |
| John Larkin |
| Re: Reducing EMF on cooling fans |
On Sun, 04 May 2008 16:18:44 -0700, "Fritz Oppliger" <fritz@fritzop.dot.com > wrote: >On Sun, 04 May 2008 14:23:56 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >> 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"? >> >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. > >** Posted from http://www.teranews.com** Why are you "tasked" to satisfy what is probably a crap, uncalibrated instrument? John |
| 06 May 2008 20:31:05 |
| Paul E. Schoen |
| Re: Reducing EMF on cooling fans |
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com > wrote in message news:fequ141vjhdqon40lhh23ms22rilda2hjk@4ax.com... > On Sun, 04 May 2008 16:18:44 -0700, "Fritz Oppliger" > <fritz@fritzop.dot.com> wrote: > >>On Sun, 04 May 2008 14:23:56 -0700, John Larkin >><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>> >>> 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"? >>> >>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. >> >>** Posted from http://www.teranews.com** > > > > Why are you "tasked" to satisfy what is probably a crap, uncalibrated > instrument? 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 |
| 06 May 2008 19:43:08 |
| Fritz Oppliger |
| Re: Reducing EMF on cooling fans |
On Tue, 06 May 2008 17:31:05 -0700, Paul E. Schoen <pstech@smart.net > wrote: > > "John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote >> >> >> Why are you "tasked" to satisfy what is probably a crap, uncalibrated >> instrument? > > 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 ** Posted from http://www.teranews.com** |
| 07 May 2008 06:33:36 |
| Eeyore |
| Re: Reducing EMF on cooling fans |
Fritz Oppliger wrote: > I am tasked with reducing "EMF" in the vicinity of computer type > brushless 12VDC cooling fans. I think you mean 'EMI' - electromamgnetic interference. > Problem similar to > http://www.sciencedaily.com/releases/2008/04/080430201654.htm > As I am controlling speed with PWM I suspected radiation from the feed > wires. But even going 100% (no switching) the noise is there. Reducing > duty cycle to about 20% reduces the noise but is not useable - it is near > stalling speed and I DO need the fans to fan. > Twisting the supply lines does not help much. What other quick fixes could > I try? I suspect the motors themselves are noisy and all I can do is try > different models / brands? 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 |
| 07 May 2008 08:22:16 |
| John Fields |
| Re: Reducing EMF on cooling fans |
On Wed, 07 May 2008 06:33:36 +0100, Eeyore <rabbitsfriendsandrelations@hotmail.com > wrote: >I expect the EMI's mostly from >the internal magnetics and you can't see those but you can choose by >manufacturer. --- Ypu can certainly see the effect of the internal magnetics; just use one of these: news:aua3241i15s4ivvvtmd56isav38308u81o@4ax.com JF |
| 07 May 2008 16:52:29 |
| Eeyore |
| Re: Reducing EMF on cooling fans |
John Fields wrote: > Eeyore wrote: > > >I expect the EMI's mostly from > >the internal magnetics and you can't see those but you can choose by > >manufacturer. > > --- > Ypu can certainly see the effect of the internal magnetics; just use > one of these: > > news:aua3241i15s4ivvvtmd56isav38308u81o@4ax.com > JF I use something very similar ! :~) Graham |