Most desk fans I have come across have three speeds: Full Speed, Almost Full Speed, and Off – useless if you want just a gentle air movement, and far too noisy if you are trying to get to sleep (in your bedroom of course, not at your work desk!). The squirrel cage induction motors they use have switches to two or more windings – and possibly a capacitor – to reduce the drive current. But unless the drive frequency is also reduced, the torque and speed stability are poor, so minimal speed reduction is usually available on these fans. Using a triac to provide phase control of the voltage works poorly for the same reason, with the speed very sensitive to the triac firing phase angle and fan load, and has a tendency to stall.
Far better speed stability is obtained by lowering the drive frequency from the standard 50Hz (or 60Hz). The main problem is how to do this without spending more than the fan itself cost! The Design Idea shown in Figure 1 achieves this for a few dollars by using a triac to allow only every third half-cycle of the AC mains voltage through to the motor, as shown inFigure 2.
Although not a continuous sine wave, the fundamental frequency can be seen to be 1/3 of the original mains frequency (60ms per complete cycle rather than 20ms), and it drives the motor with smooth impulses of regular spacing and opposite polarity, which keep the motor’s internal magnetic field, and hence the rotor, rotating nicely. The result is stable fan operation at a nice low speed which is very much quieter. The usual fan speed control switch settings now have little effect, as the lower drive frequency is the dominant influence on speed.
For more detail: Sleep easy with this desk fan speed reducer
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