Fan Motor

PWM is the usual way. Some good discussion at

Just wire it to a lamp dimmer.

Thanks guys. I'll look for a fan speed controller, but I guess they usually only come cheap with fans?

I've read that a transformer would be OK. I see a cheapie 36V that would suit. What speed would the fan likely do (fraction of full speed) at 36V? Perhaps around half speed, if the curve I/rpm is logarithmic?

Then someone suggested a diode in one lead. Is that liable to damage the motor? Perhaps a tranny is safer.

I don't really need a variable speed, just roughly halve the full rate.

Cheers, jack

Could someone check my calculation, please?

The motor draws 125mA at 240V. So at 36V, it should draw roughly 36/240 x 125 = 18.75 mA The VA (~Watts) here will be 18.75 x 36/1000 = 0.675 W Which will cost me ~ $1 per annum to run.

What would be the approximate efficiency of the transformer?

Cheers, jack

At some point the voltage will be to low to start the motor, it will run if pushed, it just won't start. My favorite solution is to use a series capacitor (non polarized, voltage appropriate) to drop some of the voltage. The capacitor is smaller than a transformer and produces much less heat than a resistor. I don't have the math ability to calculate the proper capacitance and you don't know what voltage you want on the fan. So use a transformer, resistor, or variac to find the proper speed then measure the voltage across the motor. With the wanted voltage known, someone on here can calculate the proper capacitance. I would try putting the two fans in series that may be the simplest solution. Anyone know how to model that motor in LTspice. Is it as simple as an inductor? (or as complicated :-))

Mike

You're assuming that the current draw is linear over a wide range of source voltage.

This is a mistake.

It depends on what kind of motor. Different types use different methods of controlling the speed. Most of them don't like just reducing the voltage to them. Do you have any more information about the motor, like a brand name, part number, maybe a nameplate with some information?

Thanks, Rich

Thanks for your help Rich.

The fan is a Sirocco brand. Model YX2517 Ball bearing AC230/240V 50Hz

That was from the label on the fan.

The cattledog says: Air volume 3 m^3/min rpm 3000 Input Watts 17 Current 125 mA Noise 47 dBA

Maybe a diagnostic feature is that it freewheels without any cogging for a long time after you switch it off. I guess that means there are no permanent magnets in it.

I realise that it would be the ideal to buy another fan with less current draw, but these are what I've got, and I can't see anything more suitable. These were spot on, on paper, just that the acid test shows them to be noisy little buggers -- I should have investigated that 47 dB :)

Cheers, jack

Thanks, Rich, I've done some reading and it seems that small fans on mains power are generally large-slip motors which are amenable to speed control with voltage, as opposed to most induction motors that approach synchronous speeds and not much away from that.

Now, I calculated that this fan has an "effective resistance" of around 3.3 kOhms. I happen to have a 3.3 kOhm 10W resistor handy and so I connected it in series with this fan.

The fan ran magnificently. Just about right, maybe a little faster would be better, but perfectly quiet and acceptable.

Unfortunately, after about 3 minutes, the resistor was so hot it could have burned me. I know that wirewound cement resistors can run really hot, but this appeared to be absorbing more Watts than my calculation would have led me to believe.

The motor says it draws 0.125 A but the catalogue says "input Watts

I suppose the effective resistance (reactance, impedance and all that) of the motor can only be found by measurement, so it's possible that the resistor was absorbing anything up to about 15W in the circuit that I tried it.

Anyways, that is going to be too expensive to run 24/7. I can't seem to find a transformer that would drop 240VAC to around

Perhaps stripping out the secondary of an existing tranny and rewiring to suit? Are big transformers less efficient than one just above specs? Do you know of an easier way to efficiently drop the mains voltage to two thirds?

Disclaimer : All advice taken without any implied responsibility of the adviser. Rest assured that I treat electricity with all the paranoia it deserves. No touchy when plugged in!

Cheers, and thanks, jack