Fan motor control

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Is about a cheap room fan blower with a single phase motor and the difficulties to control it with an phase angle controller.When the speed is lowering more than the half the motor makes an audible noise and the heat of the magnetic core seems to increase somehow in few seconds.
My question is if a brushless motor can safely rotate in low speeds and if there is a simple way with a few components without sophisticated electronics and mainly a cheap controller to drive it.

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Induction motors, as you've discovered, don't deal well with phase angle control. The typical way of controlling the speed of a fan is to place a capacitor in series with the motor. Or have several different capacitors that can be switched in and out--that's how most household fans and those in-wall ceiling fan speed controllers do it.

The more complicated answer would be to build a variable frequency drive--essentially a sine wave inverter the frequency of which you can control in order to control the speed of the motor. But that's firmly in the realm of "sophisticated electronics".

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It's also possible to throttle motor speed with a series inductor, which is often called a 'reactor' in industrial applications. If you have a well-stocked junkbox, you might score iron core transformers with one or more windings that are suitable for this job.

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Using phase control you need to use current zerocrossing rather than voltage zero crossing. Even so, most induction motors arent design for speed control using this method.

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Quote:
Using phase control you need to use current zerocrossing rather than voltage zero crossing. Even so, most induction motors arent design for speed control using this method.

In addition, while fan and blower motors electrically and mechanically run just fine with phase control, they will run warmer because of lower cooling air flow across and through them. Further aggravating the situation is the fact that manufacturers will use an absolute minimum amount of iron in the rotors and stators to save money. An induction motor like this with a short "stack length" has a harder time getting rid of heat. All that being said, an induction motor can feel pretty hot to the touch without necessarily being "too hot".

Tom Pappano
Tulsa, Oklahoma