Fan Back EMF to a dimmer module

[Charlie Crackle]

How do dimmer modules handle exhaust fans...

I am more refering to the back EMF they may generate when a customer has their evaporative cooling on high with all the doors and windows shut. All the air is rushing up the exhaust fans to roof cavity and the fan is spinning in this draft. (Dimmer in the off state) any chance for damage to dimmer ??

I thaught one solution might be to use the fans that the blade spring flat the fan is not being driven. This way the fan will not spin in the draft.

Other option is of course to wire on a relay module.

Charles

[rhamer]

I have used the old series 1 dimmer modules for exhaust fans and ceiling sweep fans with no problem.

The exhaust fans would sometimes free spin but with either of them when you switch them off they take a while to spin down, and I've had no issues in that regard.

The problem you are most likely to experience is noisy motors.

The harmonics in the AC waveform generated when the Triacs switch rapidly can make the motors very noisy, especially in ceiling sweep fans.

I battled with this in my previous house, and this time around I decided to keep the Capacitive speed controllers and replace the 3 speed switch with control from C-Bus.

Rohan

BTW running evaporative cooling with all the doors and windows shut, sort of defeats the purpose, but I'm guessing you told them that.

[UncleDick]

Good news from the World of fan motors, because the motors used in exhaust and sweep fan motors are normally split phase types they have no permanent magnet in them so when they are rotated they should not act like a generator and create a back emf. They may generate a blip of back emf at the instant they are turned off as the magnetic fields collaps but the dimmers are well able to withstand that.