Weird fan heater behaviour (electrical topic)

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

.

Relevant Pages

  • Re: AC comes on when it isnt supposed to
    ... Gas or Electric heat? ... It could be a high temp limit in the heater section going/gone bad. ... I got up and noticed the thermometer on the thermostat ... I turned the switch on the Tstat to Off (yes, the Fan is set to Auto, not ...
    (alt.home.repair)
  • Re: heater aggravation
    ... controlled the fan so as never to let the heater core get too hot ... found this assumed role for the resistor to be confusing, ... air stream helps control that heat. ...
    (alt.autos.toyota)
  • Re: Weird fan heater behaviour (electrical topic)
    ... Recently I installed a 3 kW fan heater in the workshop. ... they heat up. ... resulting in a significant voltage drop. ...
    (rec.crafts.metalworking)
  • Re: Looking for heater with decent fan
    ... in fact a purely radiant heater with no fan would likely be best. ... The perforated ceiling allows heat to escape. ...
    (alt.home.repair)
  • Re: Weird fan heater behaviour (electrical topic)
    ... 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. ... Recently I installed a 3 kW fan heater in the workshop. ... 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. ... The workshop is at the end of a long run of cable, so suffers a noticeable voltage drop. ...
    (rec.crafts.metalworking)