How to Reduce a Fan Motor Speed

How about just blanking off part of the fan with a cardboard ring, leaving the center open to maintain air flow over the motor?

OK mask it at the back with an annulus of thin polycarbonate.

I have experienced problems with restrictions in the intake of blower fans. The motors burned out in an unusually short time. We measured the current through leads of the replacement OEM motor and found the current to be above the rated FLA. When we opened up the intake vents, the current went down to normal.

The manufacturer said that they wouldn't replace, under warranty, the blower fans again if the restriction remained in place.

"I have experienced problems with restrictions in the intake of blower fans. The motors burned out in an unusually short time. We measured the current through leads of the replacement OEM motor and found the current to be above the rated FLA. When we opened up the intake vents, the current went down to normal."

Probably was not a centrifugal blower, in which case you pulled a vacuum and overloaded the motor.

With most fans that I have seen, restricting the air in UNLOADS the motor and it speeds up. Try it with any vacuum cleaner.

Restricting the exhaust loads the motor up......

Did you get the position of your restriction wrong?

As the power of a fan varies with something like the cube of the tip speed, the motor is not likely to suffer too much if the speed is reduced to, say, 1/3rd. I say, hook-up a lighting dimmer to it and try it out.

Is your icon from the Pink Panther movies; or from where (if you don't mind)? I am pretty sure I've seen that (actor) image before.

Don't use a dimmer switch that is made for lights, it will burn up the motor. You need a fan control switch that you can wire inline between the fan and the wall.

We use dimmers for lights and motors. We havent seen any motor burnt out by this dimmer.

Good for you.

I suppose the OP should probably just roll the dice and try his luck based on your post. Might work. He'll be smelling smoke if he accidentally turns it down too low though.

http://www.fancollectors.org/info/speed.htm

As you have read so far there are a lot of options.

You can impede the airflow circulation in front or at the back of the fan.

You can use a dimmer switch only for brushed universal motors. like the power drill or blender ones. Or a transformer. Or with a diode to use half of the sine wave.

If the motor is an induction motor you need frequency converter to lower the frequency. Or you can shut down powerplants so that the grid frequency drops.

Last method I think of is shortening the blades. smaller fan diameter means smaller airflow for the same pitch. be careful to keep the rotor balanced.

To my mind, the first decent post reading down from the top (sorry Guys!).

Therefore GA.

I wonder if anyone has a decent DIY circuit for a small VFD? I would love to have a copy if possible.....I am sure I am not alone....

I agree. GA on your GA.

...or direct the fan at a wall and rely on the turbulence generated reducing the force of the wind.

Reverse the rotation.

Use a rheostat rated for 220 volts and 250 watts (I would size for 300 watts). This will give you 0-max fan speed control and will not damage the fan motor.

I'll bet you that buying a different fan that moves less air will be much cheaper than any modification you do to this floor standing fan. I'm all for being frugal with my money but this seems silly and potentially dangerous to me to capriciously reduce the airflow of the whole fan.

If my only choice was to modify this fan or live with the airflow, I'd first live with the airflow or turn it OFF. If my work required using only this fan to make a diffuse, gentle airflow I'd consider changing the blade or cowling design in some fashion to reduce the airflow. I would consider ducting the whole airflow to then take out just some part of the flow from the initial duct.

It will be so much easier to just buy a smaller fan.

Great answer.

You also recognize that the OP probably couldn't tell the difference between any of these electric motors unless a label from the motor manufacturer was still present on the motor case. Way too many people on this forum don't understand the difference between the various types of AC motors.

What stuns me even more is how many forum contributers still fall for and present anecdotal fallacies in their answers.

You could reduce the diameter of the fan blades as suggested earlier or make the blades narrower.

Is the fan one piece or blades fixed to a hub?

There have been motor speed controller kits/projects in Silicon Chip magazine. A kit for this is available from Jaycar Electronics, it costs $A100. Cheaper to pick up a fan from the roadside garbage.

Jim

Cut the fan blades back with a pair of scissors.

Variable Speed Drives do exist for that .... on top of it they may pay for themselves on the electricity savings .

If this is a single phase room fan or wall ventilator there are available radio remote control sockets which have on/off and speed or dimmer control. So no wiring involved just plug one into the power point you wish to control and register it with the hand control and the job is done.

I use one for personal cooling and the control is from zero to full speed with no overheating problems so far.

This type is not as common as straight on/off types have a look on google, here is a starter link.

http://www.dlf-data.org.uk/products.php?groupid=2993&sortname

As mentioned before, can't see why you could not use a dimmer-type speed control.

It's not a small fan, so it probably uses a permanent capacitor (easy to check), not a shaded-pole motor. No need for a VFD, though.

We use a lot of ceiling fans with dimmer down here.

Transformers or rheostats are expensive, cumbersome and... hot.

Half wave operation (diode) will damage some motors.

Shortening the blades is not so easy because of balancing.

Brgds

Snel

The only way a half wave (diode) method may damage a fan motor is if the motor overheats, nothing else will be damaged.

Over heating can be easily checked out with say checking every 5 minutes for an hour.

In spite of using this method many times over the last 30 odd years, I have never ever seen a fan motor overheat, quite the reverse, hot motors often run cooler!!!

I expect that somewhere out there is a type of motor that overheats, which is why I never say never, which is also why you need to make sure before using the method for a long time unwatched, but I don't think they would be used for fans though.....

My present caravan I have had for 12 years, I built in such fans in the first year. They run continuously on-site (when mains available) to keep the back of the fridge and the oven cool. Usually on low speed!!! Never had a problem.

Its not as though I am speaking from a position of completely no experience either.

Please would you be so kind as to explain your base of knowledge/experience and detail for us all here which fan motor types will be damaged and how they will be damaged.

I am keen to learn more from an expert.

Hi Andy,

you are right, the only damage mechanism is motor overheating by the additional DC at lower rpm. Overheating is less likely with a fan than with other loads.

I have seen at least one fan motor damaged by DC. Ceiling fan motors are "inverted" (fixed shaft, rotating housing) with many pole pairs for slower turning. The windings have not so many turns each and have some empty space from the core, meaning good cooling. A capacitor is used for phase shifting. A friend of mine connected the motor by mistake to a 3-position diode switch which was supposed for light dimming. It worked, but it burned out after some time.

A standing fan will have a different motor. A diode is an option, but even a triac circuit should not go to "zero" as in lamp dimmers, or else your motor may stop and burn out.

"Lowest setting" means the motor probably has a tapped winding, which cannot be easily expanded.

Brgds

Snel

Then we are in agreement, as long as he takes the time to make sure that the motor doesn't warm up, he could try using the diode method, cheap and cheerful!! (My family originated in Scotland, is it not obvious?)

Thanks for your post.

I don't know why there is all the beating about the bush on this matter, I have given him a perfectly good and simple option. It is just a 250 Watt fan not a very big unit.

The controllers are Triac controlled so the average current will decrease with decreased voltage so there will be no overheating.

Why does eveybody want to complicate the matter?

You are right Garth, its a perfectly simple matter.

Some others here like poking their fingers in where they have no experience and less knowledge....

Have a great day.

From time to time, some here go a little hard on those who offer anecdotal answers to incomplete questions. Paying customers usually get the most complete advice...excuse me for being curt.

If part of the appeal of this fan is the aesthetics of it, why not just leave it turned off, and put a small low volume fan behind it out of sight.

Thanks' all your contributions just proved to me that I am not the only one ever thought about this.

I had the most entertaining weekend reading and evaluating your comments. I am going to get a smaller fan for the time being but will tackle this problem as soon as I have some free time on my hand. Really would like to try the diode method of Andy Germany , and the dimmer way as well.

What I have done in the meantime is I changed the cutting edge by slipping a spilt open nylon air tube over it. It increased the speed of the fan a bit but reduced the airflow by I would say 25%

Once again thanks for the contributions.

renewing discussion.

i have floor fan and i would like slowing it down. :) im newbie on electronic and motors. have soldering skills thats it :)

for simplicity adding photos of fan and switch

looking for simplest solution to slow it down. if need buy something please drop link or img example. my English is not native and sometimes misunderstand explanations. ty.

You have your answer in the thread, above your post.

PS: Most likely the fan has an induction motor so you need a VFD (variable frequency drive) to alter the rpm.