Interesting phenomenon

Ron wrote in news: snipped-for-privacy@bt.com:

Are you standing at the open door as you turn it on/off? I know the speed of our bathroom fan alters quite drastically if the door is open and slightly if there's a reasonable sized obstruction (e.g. me) in the doorway. Or perhaps there's some dodgy connection and the increased load of the light slows the fan a little?

Same issue as the door obstruction - differences in pressure so the fan works harder/less or runs slower/faster.

In message , Ron writes

The interaction with the wind suggests that it's some sort of resonance effect. Perhaps the wind blowing across the mouth of the duct or around the building is forming a Helmholtz resonator.

The interaction between the fan and the light suggests that the wind is only affecting the fan when the supply power is shared between the light and the fan. They aren't likely to be wired in series so the issue may be a voltage drop caused by excessive resistance in the supply cables or switches.

It's got to be those Damn Aliens again! They are the cause of all sorts of trouble.

Shaun

Darn rite.

I have a very similar situation; when cold our bathroom extractor fan squeals like mad for half a minute or so, then goes to a loudish hum (still rotating) then quietens down to normal operation. I put it down to temperature: either the spindle and bearing expanding at different rates, or maybe the lube-oil (as if) is changing viscosity with temperature.

No, it happens door open or closed, and once it`s started, it continues until the light is turned off.

A high resistance connection somewhere is probably the answer. Next time it happens I might ry removing the bulb with the light still on to see what effect that has.

On the times it`s working normally, you can hear the effects of the wind slowing the fan as it blows into the duct, but it`s not quite the same.

Ron

My thoughts, but I wouldn`t have thought a 60 watt bulb would have much effect on a syncronous motor, but who knows.

Ron

That`s caused by a shortage of cramolin I thnk

Why don't you hang a DVM across the leads of the fan to see what happens to the voltage when the light is switched on? I think your theory is probably correct.

Some sorta resonance, I guess.

Prolly whatever timer wossname keeps the fan on does so via summat, e.g. a triac, which runs it at a slightly different voltage or duty cycle or summat (might even be deliberate - it might be designed to run a bit slower when in light-off mode).

I guess it *might* involve some horrendous subtlety to do with the resistance of the bulb and reflections of electrical motor noise down yer wires, and a worn bearing/motor teetering on the edge of resonating, but my guess is that if you stick a 'scope across the motor, you'll find it's supplied with differently shaped or sized electrickery in the two modes, by enough of a margin to account for a different running speed, which would also account for hitting or not hitting a resonant mode. Yer wind will speed up or slow down the fan overall, I guess - mebbe completely still air allows it to cavitate and run faster?

The message from Ron contains these words:

Just supposing the length of the line to the bulb and the line to the motor are in such a proportion to each other that you get a resonance, cancelling/attenuating part of the emf to the motor.

Motor slows fractionally, cancellation/attenuation is interrupted, motor accelerates until...

On 2/28/2009 5:16 PM Rusty_Hinge spake thus:

Totally preposterous. You're reaching. No such inductance in house wiring would be strong enough to have any such effects here.

Preposterous but interesting reading. ;) I think that this problem is probably quite simple, and would more than likely be solved by a new fan. Have you tried unscrewing the 60w bulb? Does it still have trouble then? I'm a little unsure of how exactly this thing works after you turn the switch off, since I've never seen a fan like this. Your problem could be related to that cirucit somehow, depending on how it is implemented. I think the most likely problem is air pressure though and obstructions creating a vacume / stressing the fan like others have said.

My guess is

When the light is on the fan timer is constantly being re-triggered, which will cause the fan to run at its max power as the tirac gets triggered as soon as possible in each mains cycle.

When the light is off the fan timer is run only from the timer circuit which will cause the fan to run slightly slower as the tirac gets triggered a little later in each mains cycle.

It`s built into the wall. I can`t try removing the bulb as today is calm and the fan behaves perfectly.

Ron

Well, I was thinking maybe a slight alteration in waveform when the bulb is in circuit, but a 60 watt bulb? on 240 volts? nah! Hey I might try a lower and highter wattage lamp, just to see...

Ron

Well it`s not so much a problem, just as the header says, an interseting phenomenon. I`m neither going to lose any sleep over it or start tearing the plasterboard apart to find out. The wind doesn't blow steady, so I don't think it`s any kind of resonance effect.

One other thing, when it`s windy, to cut down draughts, we block the inside side of the fan with a small square of cardboard held on with velco. The hunting happens whether or not the fan is blocked.

Ron

That`s a plausible sounding answer, removing the bulb should support that theory I think.

Ron

Well there's your answer, no?

no