# Simple fan control

• posted

I know just enough about electricity to ask stupid questions. This seems like a very friendly group, so let me try.

I bought a simple, cheap ceiling fan here in SE Asia; it has a speed control unit (Off-1-2-3). The speed control has four external wires: two to fan, two to house electricity (phase and neutral).

After a lightning strike, fan had maximum speed (3) when minimum speed (1) was selected! I replaced the little speed control box; problem solved.

I'd wondered how such a speed control worked, and got my chance to find out when replacing it -- I could see the circuit, as well as a written diagram. It was very simple: Inductors were placed in series at lower speeds. (There was also a high-valued resistor.)

I decided (wrongly?) that the inductors, in effect, delayed the current wave-form, so that at low-speed the fan was still getting the same volts and amperes as before, but fewer watts because of phase difference. Does this make sense?

I did Google searches like "fan variable power inductor" and saw many ways to slow down a fan, but none of them seemed to be this way. (They spoke of \$40 solutions, much more expensive than mine.)

The little puzzle got me thinking and Googling. I learned how "watt" and "volt-ampere" have different definitions. I guess the power company consumes watts but bills me for volt-amperes because they're easier to measure. The fans don't cost much to run, but I guess I'm billed at a higher rate (volt-amperes) than I actually consume (watts), right? If other appliances run concurrently, perhaps that would somehow "average" the current phase and minimize volt-amperes wasted???

I'm afraid I suffer from serious misconceptions and this whole post will seem silly....

Sam

• posted

The old electro-mechanical induction watt-hour meters register only the real component of power consumption, so domestic users are not billed for the consequences of poor power-factor with these meters. The new electronic power meters monitor much more than just the active power consumption, though I'm not sure off the top of my head whether the domestic billing policy has changed, to take account of this and charge domestic users for the V*A product.

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• posted

The inductors have a frequency dependant impedance associated with them, which is calculatable from the formula Impedance (Z) =

2*pi*Freq*L. Consequently, the inductors can be used as part of a voltage divider circuit to reduce the voltage applied to the motor and hence reduce the speed. One of the advantages of the inductors is that they don't have the power or rather heat dissipation like a resistor would as the engergy is stored in the magentic field and released. Real inductors, do have a certain amount of resistance and this does have loss associated with it, though it is less than a resistor of comparable impedance.
• posted

"Singburi Sam" schreef in bericht news: snipped-for-privacy@e3g2000cwe.googlegroups.com...

The series inductor changes both voltage and current to the fan. A series inductor acts like an AC-resistor, so the fan is slowed down due to the reduced current. The overall voltage will not be changed, but voltage across fan and inductor added will not result in the overall voltage. (So when you measure them separately and add the results.) Conventional TL-tube ballasts function the same way.

AFAIK power companies all over the world bill for real power. You do not use anything else. But inductive loads cause losses in the network so the companies claim measures to be made by their customers to reduce the so called blind power. That's why that ballasts are often accompanied by capacitors. Especially power users will be checked and have to pay a fine when they cause to much blind power. Lately I've heard that some power companies simply bill their customers for the blind power. But I can't imagine they can claim the same rate as for real power.

petrus bitbyter

• posted

How big are the inductors in this "little" speed control box? It would work that way, just as you said (introducing a delay, which is increasing the inductive reactance) but inductors that size would probably cost more than a simple triac phase controller.

Say it runs full speed at 1 amp (no series inductors) and medium speed at 1/2 amp. For a 120Vac line, that 120 ohms of inductive reactance. At 50 or 60 hz you are going to need a 300 MILLI-henry inductor, rated for half an amp. That's going to be a couple two or three cubic inches for each inductor, I think.

Sure there isn't a triac in there someplace? Would make a lot more sense with the lightning knocking it out, too.

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