Hello
I need to reduce the speed a fan rotates but I don't know what sort of motor powers it. I assume it it brushless and therefore possibly an induction motor because it is part of a refrigeration compressor unit and presumably designed for long maintanance free use.
Are there any clues to look for when trying to identify the motor type or is it possible to use a "fix-all" device that will slow all types of motor
my thanks in advance
Nick
If there are no brushes then it is an AC motor and if there is no big capacitor it is a shaded pole induction motor. This is the normal type of motor used for fans.
If it is a proper fan motor (quite likely) then the rotor resistance will be higher than normal, which allows speed control by reducing the supply volts.
Simple speed controllers for fans are available, either all-electronic or by tap-switching an auto transformer. The better controllers run the fan up on full voltage then drop back to the set voltage.
See Farnell's catalogue at
Most small fans can be slowed by fitting a series resistance. Capacitors are my favoured method, but if using a capacitor do make sure the motor runs slower with your chosen cap, if it runs faster or hotter, switch off. (Its possible to pick a C value that resonates with the motor L). Other options are an inductor or a high power light bulb in series with the motor.
Its not impossible to overheat a fan like this, but it doesnt normally happen.
NT
Is it a mains voltage motor, or is there a transformer near by? Could it be supplied through another piece of equipment in an appliance it is fitted to? The refrigeration unit may not take to kindly to having the fan being slowed down and reducing the air flow over the compressor unit, so you'll have to be aware of this. The need for a fan assisted air cooling system means the compressor runs hot enough to need this type of system.
Is the need to slow it down because it's noisy? It may just need new suppressor pads on its fixing points to stop it rattling against things if it is.
A mains voltage motor can be set to half speed with the use of simple diode in line with one supply conductor. The diode needs to able to withstand the repetitive reverse voltage of course, but it is the simplest way to make a motor run at half speed.
Yes, it's mains voltage and I've bought a Fan speed controller (off ebay).
The fan/compressor/evaporator unit is new and does need to be slowed down because it's too noisy. I've checked with my friendly refrigeration engineer who said that the compressor doesn't need ANY airflow as it is cooled by returning refrigerant.
Thje evaporator will be less effective with slower fan speeds but the air speed is so great at the moment that the air coming off it is indistinguishable from ambient. I'll monitor the duty cycle to make sure I don't slow the fan too much.
Thanks for all replies
Nick
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