Interesting phenomenon

[Ron]

Here`s a thing, my bathroom fan - standard through wall 4.5 inch kinda
thing - makes a thrub thrub thrub noise when operating. It's a similar
sound to when a plain bearing is worn out, and you can see the blades
slowing and speeding up rythmicaly roughly once per second. The fan is
wired so that it starts when the bathroom light is turned on, and
continues for several minutes after the light is turned off.

but...

When I turn out the light - a single 60 watt incandescant bulb, the fan
behaves normally - no pulsing or noises - for the remainder of it`s time.

One other thing, this only happens when it`s windy outside, on a calm
day it behaves normally.

I have one or two theories of my own, but I wonder if there`s any
informed comment (or wild guesses) from the brains trust as to why this
should happen?

Ron(UK)

Crossposted to uk.rec.sheds cos it`s the kind of thing people in sheds
like to sagaciate about.

 

[Graeme Dods]

Ron <ron@lunevalleyaudio.com> wrote in
news:FLudnZqLNc8QnzXUnZ2dnUVZ8sPinZ2d@bt.com:

When I turn out the light - a single 60 watt incandescant bulb, the fan
behaves normally - no pulsing or noises - for the remainder of it`s time.

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?

One other thing, this only happens when it`s windy outside, on a calm
day it behaves normally.

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


--
Graeme

 

[Bernard Peek]

In message <FLudnZqLNc8QnzXUnZ2dnUVZ8sPinZ2d@bt.com>, Ron
<ron@lunevalleyaudio.com> writes

Here`s a thing, my bathroom fan - standard through wall 4.5 inch kinda
thing - makes a thrub thrub thrub noise when operating. It's a similar
sound to when a plain bearing is worn out, and you can see the blades
slowing and speeding up rythmicaly roughly once per second. The fan is
wired so that it starts when the bathroom light is turned on, and
continues for several minutes after the light is turned off.

but...

When I turn out the light - a single 60 watt incandescant bulb, the fan
behaves normally - no pulsing or noises - for the remainder of it`s
time.

One other thing, this only happens when it`s windy outside, on a calm
day it behaves normally.

I have one or two theories of my own, but I wonder if there`s any
informed comment (or wild guesses) from the brains trust as to why this
should happen?

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.



--
Bernard Peek

 

[Shaun]

"Ron" <ron@lunevalleyaudio.com> wrote in message
news:FLudnZqLNc8QnzXUnZ2dnUVZ8sPinZ2d@bt.com...

Here`s a thing, my bathroom fan - standard through wall 4.5 inch kinda
thing - makes a thrub thrub thrub noise when operating. It's a similar
sound to when a plain bearing is worn out, and you can see the blades
slowing and speeding up rythmicaly roughly once per second. The fan is
wired so that it starts when the bathroom light is turned on, and
continues for several minutes after the light is turned off.

but...

When I turn out the light - a single 60 watt incandescant bulb, the fan
behaves normally - no pulsing or noises - for the remainder of it`s time.

One other thing, this only happens when it`s windy outside, on a calm day
it behaves normally.

I have one or two theories of my own, but I wonder if there`s any
informed comment (or wild guesses) from the brains trust as to why this
should happen?

Ron(UK)

Crossposted to uk.rec.sheds cos it`s the kind of thing people in sheds
like to sagaciate about.

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

Shaun

 

[Sn!pe]

Ron <ron@lunevalleyaudio.com> wrote:

Here`s a thing, my bathroom fan - standard through wall 4.5 inch kinda
thing - makes a thrub thrub thrub noise when operating. It's a similar
sound to when a plain bearing is worn out, and you can see the blades
slowing and speeding up rythmicaly roughly once per second. The fan is
wired so that it starts when the bathroom light is turned on, and
continues for several minutes after the light is turned off.

but...

When I turn out the light - a single 60 watt incandescant bulb, the fan
behaves normally - no pulsing or noises - for the remainder of it`s time.

One other thing, this only happens when it`s windy outside, on a calm
day it behaves normally.

I have one or two theories of my own, but I wonder if there`s any
informed comment (or wild guesses) from the brains trust as to why this
should happen?

Ron(UK)

Crossposted to uk.rec.sheds cos it`s the kind of thing people in sheds
like to sagaciate about.

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.

--
Sn!pe ^Ď^ <http://snipeco.ath.cx/>

 

[nev young]

On 27/02/09 15:11, Ron wrote:

I have one or two theories of my own, but I wonder if there`s any
informed comment (or wild guesses) from the brains trust as to why this
should happen?

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.

--
Nev

 

[Ron]

Sleepalot wrote:

Ron <ron@lunevalleyaudio.com> wrote:

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

Well there's your answer, no?


no

 

[nev young]

On 01/03/09 12:16, Ron wrote:

nev young wrote:
On 27/02/09 15:11, Ron wrote:
I have one or two theories of my own, but I wonder if there`s any
informed comment (or wild guesses) from the brains trust as to why
this should happen?

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.


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

In so much as removing the bulb will have no effect on the phenomenon.
If removing the bulb does have an effect, then it supports the load/bad
contact/resistance of the supply cables etc.
--
Nev

 

[Carl LHS Williams]

In article <M5ydnaodDpID4TfUnZ2dnUVZ8umdnZ2d@bt.com>,
Ron <ron@lunevalleyaudio.com> wrote:

nev young wrote:
On 27/02/09 15:11, Ron wrote:
I have one or two theories of my own, but I wonder if there`s any
informed comment (or wild guesses) from the brains trust as to why
this should happen?

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.


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

I dunno that removing the bulb would prove anything either way, unless you
have a circuit diagram of the timer and suspect that the bulb load will
affect the motor supply in some way.

I would suspect a simple-and-cheap-as-possible circuit, TBH, and it's
prolly as straightforward as: Light on, fan supplied via connection
through a switch, by-passing triac; light off, fan supplied via triac
held on by timer circuit, which is prolly some extremely noddy
one-shot comprising a reasonable sized capacitor discharging through
a load (possibly the fan itself, or the triac, or something else very cheap).

At most, I'm guessing a 555 or similar. Either way, yer triac (or mebbe a
relay with dodgy contacts) is prolly shorted by the light switch, making
the motor go faster.