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