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I'm looking to specify (well, suggest) a RC snubber network for someone over on diy.se who is having EMI problems with his ceiling fan knocking out his TV briefly on turn-off:

https://diy.stackexchange.com/questions/56205/why-would-turning-off-a-ceiling-fan-via-a-wall-switch-causes-3-seconds-of-tv-sig

Thing is, I cannot find any information on the winding resistance and inductance of a garden-variety ceiling fan motor. Is this something I'd have to measure myself, or has this ever been specified or measured for a typical US ceiling fan motor?

EDIT for those who haven't read the link: the fan turning off causes the TV screen to go blank (no picture) for a few seconds, but the TV stays powered all the while.

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ThreePhaseEel
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  • Does the fan have speed control? Maybe when you turn it off it uses the speed control, and that chops up the AC and puts a bunch of noise on the AC line. Maybe it is a conducted problem rather than a radiated emissions problem (as everyone seems to be assuming). Try a big choke on the TV power cord? – user57037 Jan 27 '15 at 07:51
  • Also, if it just started in the last two or three months, maybe something went bad in the fan. – user57037 Jan 27 '15 at 07:52
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    It would be truly astounding if the RL cutoff frequency of the fan itself was in the MHz range. Probably it's much lower, so it's hard to believe EMI is created. My guess is that changing the fan speed causes a transient wall power glitch, maybe on the millisecond scale. The glitch makes the cable box sad and it has to reset itself. – Dave Kielpinski Jan 27 '15 at 11:59
  • @mkeith -- that's a good point -- you'd think a fan of recent vintage would have interference suppression parts built-in -- perhaps the internal suppression network failed to an open-circuit? – ThreePhaseEel Jan 27 '15 at 12:37
  • @DaveKielpinski -- not only do brushed motors generate tons of broadband hash (arcs are *wonderful* broadband jammers!), *any* motor can generate an inductive kickback spike when turned off -- the latter is what I suspect the problem here is, considering he states very explicitly that the problem only occurs when he turns it *OFF*, not when he changes speeds. – ThreePhaseEel Jan 27 '15 at 12:39
  • @ThreePhaseEel: I thought this was an AC motor since it's a ceiling fan. Wouldn't that normally be brushless? I agree about the inductive spike. – Dave Kielpinski Jan 27 '15 at 13:53
  • Symptoms are important - knocking out a TV means nothing to me. – Andy aka Jan 27 '15 at 14:15
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    Maybe not everyone read the link. The TV signal is supplied by cable. The symptom is that the TV screen goes blank for several seconds and then comes back. TV's and anything that connects to them are supposed to pass a fast transient (noise injected on AC line) test, but maybe this fan shut off is creating a fast transient that is epic in size. I don't think this is a radiated phenomenon. I think it is conducted by the power lines. – user57037 Jan 27 '15 at 17:55

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I recommend you use a 1uf (or larger) 150v AC, capacitor across the motor power leads (no resistor needed). This should help suppress the electrical noise but since your problem started about 3 months ago, that tells me that something changed 3 months ago. Possible changes:
1 - new TV
2 - new cable box
3 - cable box moved and plugged into the same circuit as the fan switch
4 - fan motor grounding disconnected
5 - if fan has internal noise suppression, it went bad

Guill
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  • "150v AC" vs. "someone over on diy.se". I think the fan is running on 230V, the voltage rating of the capacitor would be to low in this case. – Roland Mieslinger Jan 30 '15 at 21:39
  • Also, beware that using only a cap without a resistor will lead to high inrush current when the fan is turned ON. Maybe not a problem in practice, but it would at least increase wear on the power switch used to turn the fan on. – avl_sweden Jul 22 '15 at 20:27