Variable Switch for Wood Burning Stove

One possible link: 240V LED Bulbs and Dimmer Switches - Lighting.

You will need to check if the motor is of a type that can be so controlled, and sufficient watt rating of the switch.

I see no reason why you can't use a variable resistor (potentiameter) It will need to be big enough to handle the power loss from the resistance. Look at the tag on the motor for the current it uses and then figure that you want to drop the voltage from your 240 and use ohms law to see how much resistance and the wattage dissipated when you're dropping most of the voltage across the resistance. If you're "crafty" you can make your own variable resistor by using thin iron wire (do they have baleing wire there?) Wind a stick of something that won't burn with many turns and put it in series with the motor. You can add or subtract turns for the desired speed with a slider.

Alternatively, you can probably use a lamp dimmer.

That may work OK for a Universal type motor, but not for an inductive motor, which most fans use. The universal motor's speed is determined by the voltage/current supplied to it, and speed may be varied by controlling the applied power; this motor also uses brushes which tend to wear out and also to cause rapid wear to the motor when they are used in an application such as you have described (that is, running much of the time, rather than occasional short "bursts" followed by periods of "rest"). An inductive motor uses the frequency of the applied AC to determine speed. If you put a variable resistor in series with an inductive motor, it will not control the speed, since it would not change the frequency of the applied AC power, but it may prevent the motor from starting and cause other problems, such as would be caused by putting an inductor in series with the motor windings, since the variable resistor would act something like an inductor at higher power levels.... The best thing to do would be to either (A) use a universal motor, which will run on AC or DC and can be controlled by a simple "lamp dimmer" type of controller (heavy duty, of course, to match the motor's power draw), which will also be MUCH cheaper than a variable resistor of sufficient size and power rating to handle the relatively high VA rating of the motor (assuming it would even work, of course!), or, (B) Buy or make a fan using an inductive motor with speed taps already available, which you can wire to a switch assembly to select the speed desired. Inductive motors used in fans, for example, are usually made with several different windings in the motor's stator, allowing a switch to be used to select windings in a way which allows the speed of the motor to be set by how the windings are wired, etc. This is not a trivial project for someone who is not very familiar with these issues in motor speed control; in fact, it is not trivial for ANYONE, though a good electrical engineer could do it in a couple of hours if he had experience doing it often in the past. There isn't room enough here to expand beyond what I have said. If you check around with salvage companies which take in fans or motors you may find something fairly inexpensive which would work for you. Of course, if you are determined to build it yourself, I'd suggest you go to a library and look for books which contain instructions on how to build the motor or controller or whatever part you wish to make. Again, I don't recommend this for a novice; I am an electrical engineer, but I do not design motors or power controllers for a living, and I would need to refer back to my college textbooks and notes to be able to design a simple motor, which would then need to be prototyped, tested, designed again, etc. As I said, it is not a trivial design project! Oh, I just thought of a way to make a very simple fan: you purchase a dozen or so small d.c. motors and a toy train power pack to drive them. Put small fans on each motor and mount them in a round metal duct compatible with your system. Wire the motors in parallel so they all turn the same way when power is applied. Then, hook them up to the transformer, which you can use to vary their speed just as it would be used to control a toy trains speed. There, now THAT is a simple design! Of course, it would be noisy and bulky and probably not cheaper than simply buying a new fan with built-in speed control, but, at least it would work to some extent! All kidding aside, it is my opinion that you would be better off to purchase a fan assembly and speed control for it, than to try to design and develop it is a way which would work and would be safe. The cost of a new or perhaps a used fan unit, already designed to work as you wish, would probably be less than the ultimate cost of doing it yourself. Perhaps someone else with more experience will add to this, or correct me if I made any errors??.... good luck! Larry

Theorietically, you're right BUT with less voltage, the motor will run slower!

True if you try to start the motor at minimum voltage it may not be enough to get it going so it's NOT idiot proof!

True, a big pot will cost some but I'm presuming that as a Brit, He's still got the ability to cobble something up out of bits laying about--as opposed to we dumbed down amurikuns~~

Phillips of Eindhoven developed a wood burning stove with feed back control of the fan using electronics powered by the heat of the stove itself. I have seen the stove itself when they tried to introduce this into rural India. The article on page 28 here gives some info: http://www.research.philips.com/password/download/password_28.pdf

It is not a good idea to use an induction motor, and control the speed by using a light dimmer. They are constant speed motors, and you cannot lower the speed more than % 15. The motor blocks and burns.

However dimmer is the chiepest and effective solution, don't give up the idea of using a dimmer.

Therefore the selection is at the fan motor type, of which can be effectively controlled by dimmer.

They are Universal motors. Use an universal motor to power your fan. Then you can effectively control the speed from 0 to maximum.

The example is the drill motors, of which the speed can be controlled from the trigger.

This is the problem with asking an engineer to solve a problem: This Bloke is just ECONOMICALLY trying to control the air flow, not design another system for production.

How about JUST blocking the cold air input with a piece of cardboard over the input grid. With some paper clips you can arrange a number of positions: heat fast, medium and slow.

Michael Rogers

Sr. Space shuttle Engr. RTD.