Ceiling Fan Speed Control:

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Re: Ceiling Fan Speed Control:

Sam,

With the right cap you could even have resonance which would yield maximum current, but I suspect that in a real motor, the capacitive reactance is the dominant factor in the impedance of the circuit.

Natheless, the motor speed falls as the applied voltage falls.

Your arbitrary numbers don't prove anything. Why don't you do some actual IV measurements with phase angles? I have only a foggy notion of the effective motor impedance. I only know that the current draw is in the order of one Ampere.
 
Re: Ceiling Fan Speed Control:

Sam, are you alluding to the fact that for some cap value, the impedance will be purely resistive. It is similar to PF correction.
 
Re: Ceiling Fan Speed Control:

Resonance is cool, but when I got to resonance I didn?t stop, I just kept on goin?.

The 5uf is more fun than that because (like in a good poker game) the capacitor sees the inductors bet of 264 ohms and raises it another 264 ohms. The same difference as before but you might say the table?s been turned. The vector arrow?s pointing in the opposite direction to the same degree.

Xc = 530.5 ohms for 5uf
The motor?s Xl = 264 ohms
R = 100 ohms

The series sum of the impedances is about 284.5. Almost the same as the vector sum of the motor?s R and Xl, 282 ohms.

Nothing happens. It works just like it did without the capacitor.

The 10 uf does resonance though. you really don?t even have to do the vector math.

Motor?s Xl = 264 ohms
10uf Xc = 265.25 ohms

That leaves the 100 ohms from R. That?s pretty cool too because the motor, if you left it alone, (or used a 5uf) would use .42 amps. But by adding the 10uf now the thing uses 1.2 amps. I know what you?re saying, you?re saying, Sam, come on, that?s crazy talk the motor?s supposed to go like three times faster now?

And you?d be right, because three times faster would be about what you?d get with 120 volts and 1.2 amps but the voltage has changed too so you don?t have to worry about that. Cause now the voltage is 338 volts.

The voltage drop across the capacitor is:

V=IZ or 265.25 X 1.2 = 318.3 volts

The motor is:

282 X 1.2 = 338.4 volts

So that must mean that with a 10uf capacitor the motor uses 338 X 1.2 = 406 VA, like half a horse power, instead of the 50 watts, about 1/15 hp, without the capacitor.

Of course this isn?t how it really works. The source is still only delivering 120 volts X 1.2 amps = 144 watts. But that?s what it ?appears? to use.

You have in fact tripled the current though. Only now it doesn't seem so bad because at least it's not half a horse power.
 
Re: Ceiling Fan Speed Control:

Sam, think about piezoelectric crystals. If you dope out the equivalent circuit, you will see kilovolts at resonance, and you probably have one on your arm. Watch out!

I won't check your math, but the maximum possible current is 1.2A and the maximum apparent power is 144VA no matter what. Bear in mind too that the equivalent circuit changes as the load changes.

Go on, measure the current on one of thes things at all speeds. I have already buttoned mine up and don't want to disturb it again.
 
Re: Ceiling Fan Speed Control:

You don't have to take your fan apart again rattus. You can measure it from the wall switch.

Bear in mind too that the equivalent circuit changes as the load changes.
Click to expand...
Yeah, how the inductance changes will depend on where the motor is in it's curve. This is interesting too. If the motor would draw more current with a bigger load on it, then you could make it draw less current instead by using the 5uf capacitor. (The one that flips it's impedance from inductive to capacitive)
 
Re: Ceiling Fan Speed Control:

So how does a variable speed control for a fan work? Is it a variable capacitor?

Steve
 
Re: Ceiling Fan Speed Control:

The one Rattus has is switching two capacitance values in series to change the impedance.
 
Re: Ceiling Fan Speed Control:

Originally posted by steve66:
So how does a variable speed control for a fan work? Is it a variable capacitor?

Steve
Click to expand...
The simple ones simply insert capacitive reactance in series with the motor which lowers the current and the motor voltage. A 3-speed fan requires two caps, and the switch sequence may be a bit more complex than you would expect. You must be sure you have the right switch before replacing one.

I would guess that the more sophisticated controls work like a dimmer perhaps with "catch" diodes to clamp the inductive spikes to the rails.
 
Re: Ceiling Fan Speed Control:

I guess I wasn't clear. I was thinking about the ones that have a dial or a slider, like a dimmer switch, only for a fan.

I saw some in a catalog yesterday. I think they are just variable resistors. I guess the down side of a resistor is that you waste real power, and you have to provide a heat sink to get rid of the power.

Steve
 
Re: Ceiling Fan Speed Control:

Steve,

A variable resistor (rheostat) would indeed dissipate too much heat for a wall mount and perhaps be too expensive. I have seen these used with universal motors for speed control.

The wall mount dimmers must be duty cycle modulated Triacs controlled by potentiometers. In addition, some method must be employed to clamp the inductive spikes which occur as the Triac switches off. Not sure how they do this--probably just a cap.
 
Re: Ceiling Fan Speed Control:

Yeah, dimmers don't get hot. :D

I think another problem with variable resistors (I've always disliked the word "rheostat", I don't know what it ever did to me :D ) is size.

They have to be big for two reasons. (1) To handle more power it needs a larger resistive material cross sectional area. (2) Heat dissipation needs surface area.

Another problem with variable resistors is that sometimes the resistive material is coiled to save space with larger cross sections. Those components can have lots of unwanted inductance.

That doesn't mean someone hasn't put a fan together that works that way. I've seen some pretty ingenious and simple solutions in designs before.

But Rattus is right about the wasted power. You're just dumping power into something to make heat, that you don't want, it's actually destructive.

[ August 24, 2005, 05:52 PM: Message edited by: physis ]
 
Re: Ceiling Fan Speed Control:

Sam,

Dimmers do generate some heat but not like a rheostat. I would prefer to mount them in a metal box with a metal plate to help dissipate this heat. However, I would guess that max dissipation at 600W would be only about 6W--about the same as a night light.
 
Re: Ceiling Fan Speed Control:

I don't have my data manuals anymore, The device dissipation's in there. I wouldn't accept an off hand comparitive guess at it. You might be surprised at what you end with.

And you also need to watch where you are in the dissipation curve with both a resistor and a semiconductor.

The outcome isn't always a slam dunk.

Edit: Ok, maybe it is with a resistor.


Edit: Error B

[ August 24, 2005, 07:46 PM: Message edited by: physis ]
 
Re: Ceiling Fan Speed Control:

I just picked up on this thread (only 2 years too late), but the correct circuit that uses 2-5uf capacitors has one capacitor connected directly to the fan motor. Also connected is the center pole of a SPDT switch. The other two connections go to the other capacitor and the power wire, respectively. The other ends of the capacitors are connected to the power wire. It works, but no off.
 
Re: Ceiling Fan Speed Control:

Here's how they work:
fan%20switch%20diagram%203.gif
 
Re: Ceiling Fan Speed Control:

Hunk's diagram demonstrates the point that the switch sequence must be considered when making a replacement.
 
Re: Ceiling Fan Speed Control:

I say, I wonder what may have happen to that ole' "Physics Chap" that so frequented the many pages of this forum.

You may remeber him....

The stately gentleman always sharing his vast intellectual knowledge, and the spreading of his joy and light-hearted hummor with this forum. What name did he go by.....Why wasn't it dave? ;)
 
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