Hi all,

Recently I installed a 3 kW fan heater in the workshop. As with a great
deal of my stuff, it was scrounged, but I have no reason to suspect that
there is anything wrong with it. It heats well, but behaves a bit
weirdly when first switched on. Initially the fan spins slowly, only
creating a slight draught, and the elements make some twanging noises as
they heat up. After about a minute the elements become a very dull red
colour (barely noticeable as they are "black heat" elements), and
suddenly the fan accelerates to its top speed, producing a healthy
draught. Now I don't think this is an intentional feature of the design
- the heater is very simple. So I'm wondering why it happens and have
come up with a theory. The workshop is at the end of a long run of
cable, so suffers a noticeable voltage drop (more than it should,
really, but I didn't wire the place). I'm pretty sure the fan is driven
by a shaded pole induction motor. When the heater is switched on, the
elements are cold and have a low resistance. So the elements draw a high
current, resulting in a significant voltage drop. The reduced voltage
slows the shaded pole motor down. As the elements heat up, their
resistance rises, reducing the current drawn and causing the voltage to
rise again. The higher voltage causes the shaded pole motor to
accelerate. Well that's my theory, anyway. What do people think?

By the way, here's a picture of the heater, if it's of any interest:

http://www.mythic-beasts.com/~cdt22/fan_heater.jpg

Best wishes,

Chris

In article ,
Christopher Tidy wrote:

weirdly when first switched on. Initially the fan spins slowly, only
creating a slight draught, and the elements make some twanging noises as
they heat up. After about a minute the elements become a very dull red
colour (barely noticeable as they are "black heat" elements), and
suddenly the fan accelerates to its top speed, producing a healthy

Not a bad tale/explanation/guess, but I'd also suspect gummed up fan
bearings. To test, you'd want to switch the fan on with the heating
elements disconnected. If you are fully correct, fan will instantly go
to top speed. If I'm somewhat correct, the fan will still start slowly.

I've seen this behavior in many older fans not connected to a heater.
You might also simply try spinning the fan by hand when it's off, and
see if you notice any drag on it (and/or if you notice differing amounts
of drag when it's been off for overnight and when it's just switched
off).

Then again, if it works well enough, you might not really care about
diagnosing the exact issue it has.

--
Cats, coffee, chocolate...vices to live by

Gummed up fan bearings. The oil only lasts 5 to 10 years before it turns
to glue. Standard problem on "used" usnits. Disassemble clean the resin
off the shaft, figure out how to get some 20 wt oil into the bronze
bushing and it's oil resivoir, reassemble.

Christopher Tidy wrote:
Hi all,

Recently I installed a 3 kW fan heater in the workshop. As with a great
deal of my stuff, it was scrounged, but I have no reason to suspect that
there is anything wrong with it. It heats well, but behaves a bit
weirdly when first switched on. Initially the fan spins slowly, only
creating a slight draught, and the elements make some twanging noises as
they heat up. After about a minute the elements become a very dull red
colour (barely noticeable as they are "black heat" elements), and
suddenly the fan accelerates to its top speed, producing a healthy
draught. Now I don't think this is an intentional feature of the design
- the heater is very simple. So I'm wondering why it happens and have
come up with a theory. The workshop is at the end of a long run of
cable, so suffers a noticeable voltage drop (more than it should,
really, but I didn't wire the place). I'm pretty sure the fan is driven
by a shaded pole induction motor. When the heater is switched on, the
elements are cold and have a low resistance. So the elements draw a high
current, resulting in a significant voltage drop. The reduced voltage
slows the shaded pole motor down. As the elements heat up, their
resistance rises, reducing the current drawn and causing the voltage to
rise again. The higher voltage causes the shaded pole motor to
accelerate. Well that's my theory, anyway. What do people think?

By the way, here's a picture of the heater, if it's of any interest:

http://www.mythic-beasts.com/~cdt22/fan_heater.jpg

Best wishes,

Chris

Damn, Ecnerwal and Roy, you were quite right! I thought I'd discovered
something interesting here, but it seems it's just a mundane case of
gummed up bearings.

The fan behaves the same way with the elements switched off. If switched
off after a minute or two of running, then re-started, the fan reaches
its top speed almost immediately. It does this whether the elements are
on or off.

The voltage drop I have measured before. With just the lights on the
voltage is about 239 V. A 2 kW load reduces it to 219 V. My welding set
probably reduces it to about 200 V, but I've never measured it. It could
do with a new cable laying, but whether we'll get round to it I don't know.

I think I'll leave the heater for a short time and see if the bearings
are freed with use. If not I'll have to take it apart, which I've done
with these heaters before and it isn't the most enjoyable job. But at
least it isn't a plastic POS which you have to kneel on and lever with
two rulers simultaneously to put back together. Sometime I'll do it. By
the way, what is the equivalent of 20 wt oil in ISO terms?

Best wishes,

Chris

On Tue, 7 Mar 2006 19:18:13 +0000 (UTC), Christopher Tidy
wrote:

Hi all,

Recently I installed a 3 kW fan heater in the workshop. As with a great
deal of my stuff, it was scrounged, but I have no reason to suspect that
there is anything wrong with it. It heats well, but behaves a bit
weirdly when first switched on. Initially the fan spins slowly, only
creating a slight draught, and the elements make some twanging noises as
they heat up. After about a minute the elements become a very dull red
colour (barely noticeable as they are "black heat" elements), and
suddenly the fan accelerates to its top speed, producing a healthy
draught. Now I don't think this is an intentional feature of the design
- the heater is very simple. So I'm wondering why it happens and have
come up with a theory. The workshop is at the end of a long run of
cable, so suffers a noticeable voltage drop (more than it should,
really, but I didn't wire the place). I'm pretty sure the fan is driven
by a shaded pole induction motor. When the heater is switched on, the
elements are cold and have a low resistance. So the elements draw a high
current, resulting in a significant voltage drop. The reduced voltage
slows the shaded pole motor down. As the elements heat up, their
resistance rises, reducing the current drawn and causing the voltage to
rise again. The higher voltage causes the shaded pole motor to
accelerate. Well that's my theory, anyway. What do people think?

By the way, here's a picture of the heater, if it's of any interest:

http://www.mythic-beasts.com/~cdt22/fan_heater.jpg

Best wishes,

Chris
asy way to find out - set it up to start the fan first - then turn on
the heat. Once the fan is up to speed, the reduction in power won't
affect it as much. Could also be the bearings in the fan are a tad
stiff when cold - and when the element comes up to temperature, the
fan loosens up.
*** Free account sponsored by SecureIX.com ***
*** Encrypt your Internet usage with a free VPN account from http://www.SecureIX.com ***