'Fan' dimmer query

[EdwardM]

A friend has decided to use a dimmmer on his bathroom lights and has discovered that there is an extractor fan in the same circuit. Are there any safety implications? My initial thoughts are that there may be a problem with radio interference but if the dimmer contains a inductive rated triac there shouldn't be any other problems. The communities thoughts would be appreciated

 

[Warpspeed]

It depends on the type of motor fitted to the fan.

If it is a brush type combination ac/dc motor (highly unlikely) the speed can be controlled by varying the average voltage through a Triac phase controller.

More likely the fan will be an induction motor of some type, probably a shaded pole motor. This type of motor works by creating a rotating magnetic field and the speed depends mostly on the mains frequency. Any attempt at seriously reducing the voltage will cause the motor to slow down, BUT the increased slip rate will cause massive motor rotor overheating. Definitely not a good idea.

It may appear to work for a short time, but be prepared to keep replacing cooked motors.

 

[Skogsgurra]

Actually, shaded pole motors are used in variable speed fans. A popular - I would say dominating - brand of bath-room fans in our part of the world (and probably all over the world) use shaded pole motors. And that brand has a variable speed option that works just as described, by stator voltage control with a "dimmer".

This is not against the laws of nature since a shaded pole motor of this size is such a poor motor that it already has a rather high slip and this slip is very voltage dependent. A speed range from 25 to 100 percent is a very real possibility and the resulting air flow goes with speed squared (I think). The non-linear speed/torque curve helps establish stable operating points depending on voltage applied.

There is no danger in reducing the stator voltage with regard to motor heating. The losses are around 70 to 80 percent at full load and reducing the voltage only reduces total power, i.e. mainly the losses.

The only thing that one should be careful with is that a triac has an unsymmetric characteristic and this can lead to firing on only positive (or negative) half-periods, which applies a DC component to the motor - which it cannot take. The usual reaction is letting out the magic black smoke and after that - silence.