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'[EE]: What about 115 AC fan control? Was: Thanks D'
2002\03\11@014826 by

Hi Dale.

Can that circuit be used for 115 AC fan control? ;)
Sometimes I feel like my brain and body are overheated
(no kidding ;) Fan is helping alot but it only has two position:
very fast blow and medium blow. I can't stay under it for long
unless I want to catch a flue. So I'm thinking to put some
controller in series to it. Implemeting PWM code would be easy,
probably some indication (like one digit 0..9 is enough)
couple buttons (increase and decrease) what else I'm missing?

WBR Dmitry.

Dale Botkin wrote:
{Quote hidden}

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Dmitriy A. Kiryashov wrote:

>Hi Dale.
>
>Can that circuit be used for 115 AC fan control? ;)
>Sometimes I feel like my brain and body are overheated
>(no kidding ;) Fan is helping alot but it only has two position:
>very fast blow and medium blow. I can't stay under it for long
>unless I want to catch a flue. So I'm thinking to put some
>controller in series to it. Implemeting PWM code would be easy,
>probably some indication (like one digit 0..9 is enough)
>couple buttons (increase and decrease) what else I'm missing?
>
Most AC motor's speed are more dependent on the frequency of the AC than
the voltage.
Now, you CAN build a sine wave of variable frequency with PWM in a class
D amplifier fashion,
but it is non - trivial.

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On Mon, 11 Mar 2002, Dmitriy A. Kiryashov wrote:

> Hi Dale.
>
> Can that circuit be used for 115 AC fan control? ;)
> Sometimes I feel like my brain and body are overheated
> (no kidding ;) Fan is helping alot but it only has two position:
> very fast blow and medium blow. I can't stay under it for long
> unless I want to catch a flue. So I'm thinking to put some
> controller in series to it. Implemeting PWM code would be easy,
> probably some indication (like one digit 0..9 is enough)
> couple buttons (increase and decrease) what else I'm missing?

Well, that matches *exactly* another little project I just finished
testing...  simple little PWM controller with UP/DOWN speed buttons,
remembers the last speed set, etc.  Don't know how well it would work on
an AC fan, though.  I know the MOSFET switching would work, but not sure
if this would be a good way to control the fan speed, maybe someone more
familiar with AC motor speed control could tell you.

If you want to see the AC switching, look in the archives for "switching
low voltage AC" and look for Dave D.'s posts.

Dale

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Dmitry wrote...

>Can that circuit be used for 115 AC fan control? ;)

It might work, but probably not very well.

For one thing, MOSFETs rated for high voltage usually don't have very
low Rds(on) so they are likely to get rather hot.  The circuit works
well for low-voltage AC like with Dale's 12V yard lights, because you
can get MOSFETs with extremely low ON resistances; but the same isn't
true for high voltage.

The other problem is that I don't think AC induction motors do very well
having their speed controlled only by voltage reduction.  Without
reducing the AC frequency as well, they are forced to run in a high-slip
condition which causes them to overheat and have very low torque.  I'm
no motor-control expert, so that might not be correct; but I think
that's what happens.

Dave

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At 10:29 PM 3/11/02 +0000, you wrote:

>The other problem is that I don't think AC induction motors do very well
>having their speed controlled only by voltage reduction.  Without
>reducing the AC frequency as well, they are forced to run in a high-slip..

Yup, that's why "phase control" type speed controls (as you can get off
the shelf at Home Despot) only really work with motor loads such as fans-
where reducing the motor torque reduces the speed with little danger of
stalling- IOW, the load is mostly drag with little stick-slip friction.

If you want to vary the speed and maintain torque, you need to change the
frequency which is non-trivial.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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>I guess there should be more simple tecnique to control AC fan.
>Original manufacturers they using resistor in series I think
>(what else they can switch inside of cheap Holmes fan ? )
>Actually I didn't open a unit yet to prove that.

They have a tapped coil in the motor winding. The switch changes to the
other tap.

hope this helps,

Peter

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>Can that circuit be used for 115 AC fan control? ;)
>Sometimes I feel like my brain and body are overheated
>(no kidding ;) Fan is helping alot but it only has two position:
>very fast blow and medium blow. I can't stay under it for long
>unless I want to catch a flue. So I'm thinking to put some
>controller in series to it. Implemeting PWM code would be easy,
>probably some indication (like one digit 0..9 is enough)
>couple buttons (increase and decrease) what else I'm missing?

I believe that your fan has a synchronous motor. This cannot be PWM-ed
easily (see previous threads on controlling fan). However, I just had an
idea. A synchronous motor attached to a large inertial load (like a fan)
can be controlled in speed, by switching it on and off at a very *low*
rate. I.e. full wave mains switching using a triac, but a controller makes
it miss periods of the mains. You could do this with a PIC. Just count
semiperiods and do not fire the triac for every N out of M periods (you
have to fire the triac for two consecutive periods to avoid DC in the
motor).

One thing that must be checked is, whether the motor will not overheat in
this mode (it might).

Another is any noise that the fan might make in this mode.

good idea ? ;-) FYI it came when the desk fan developed a bad contact on
the mains plug and ran 'slow' and intermittently ...

In fact I will try this out myself because I also have a fan problem.

Peter

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