AC Fan motor speed control

Has anyone tried controlling the speed of an AC fan motor (induction?) with a picaxe.
I've thought of using a domestic fan speed controller and turning it with a servo but the pot requires about 300 degrees and the servo only gives me about 200 degrees. Any ideas?
This has been covered several times.
Basically, if it's an induction motor it's not an easy project.
If it's a 'universal' motor then it's easier.

Do a SEARCH on "induction motor" selecting both words together. You will find several threads covering this but, bottom line, if induction then it's usually done by F/V control (thats how my mill works) and it's not easy/cheap.


Senior Member
The full range of a pot doesn't have to be used and still achieve full range of control. It's quite common for those triac based fan controllers to be useless at the bottom end for example.

If you already have a speed control that works with the motor you intend to control, and you only want to physically turn the pot on the control it may be possible with the servo you have. There may be a mechanical stop in(side) the servo that can be removed - the servo pot will still have a stop at something like 270 degrees.

Simple voltage controllers for induction motors won't work well, but a lot depends on the motor - fan motors are one exception. They do use voltage control on some ceiling fans for instance, in that case the fan already has a lot of slip and the control doesn't really hurt the motor. Same goes for small shaded pole motor powered fans (3-10" sizes) ("fans" not "blowers")

Controlling induction motor speed is a topic that takes a lot more research - unless you really need this, it probably isn't worth the effort.
Or you could buy a controller.
I've just seen a nice 1phase inverter frequency contoller. Only £330.
If it's that cheap then I guess it must be quite simple - not.
Just gear up the servo, pulleys and string, wrapping the string around both pulleys a few times for better grip and a small spring to tension the ends together.
They have been using this method in radio tuners since day dot.
I have never seen a commercial single phase speed controller advertised. If the big boys don't do it then its a fair assumtion that it can't be done. The only way is to vary the frequency.

Even these will be single phase input, 3 phase output.

I once stayed in a hotel in Singapore that had a wall control for a ceiling fan. Chair on bed, look at fan motor. 0.22kW 400v 3 phase. MCB for fan in wardrobe, 1 phase.
I was looking at wrong page re price.

Try this jaka
<A href='' Target=_Blank>External Web Link</a>

Page 11.
Dippy, some interesting stuff in there, but I can't see any reference to the type of motor, on page 6 it does mention a 3ph motor is required.

Interesting is the part about auto tranformers. I once carried out some tests on a 0.75 kW 1 phase motor using a variac. I found that the speed could be controlled from 2950 RPM down to about 700. Had to connect the variac to the run winding but keep the capacitor winding at 240V to get it to run. Also I had to start it at near full speed, otherwise it wouldn't get away. This was a pump application so starting torque was not a problem.

Could get a small picaxe controlled DC motor to operate the variac, but not for the faint hearted as auto transformer/variac use is not permitted in the UK wiring regs.
Thanks all for your replies. I'm going to double check the motor. I was told this controller;catid=0&amp;id=1943 (which uses phase control) would work well and for under $50NZ it's not worth spending an hour and a few parts trying to make one.
Like the idea of gearing the servo a lot. Thanks all again for your input.
NZgeek, Looks like this device uses zero voltage detection and chops the start of the sign wave, posibly with a triac.

It would appear to work with shaded pole and split phase induction motors, but not a standard 1 phase induction motor, that will have a capacitor, wich is probably what everyone thinks you have.

Spit phase is only a normal motor that has a centre tap, these ar'nt too common, be interested to know what you have.
Jaka, page 11: &quot;230V 1Ph 50Hz&quot; and &quot;designed for speed controllable single phase motors up to
10 amps&quot;.
Personally, before buying, I would want more details fo clarification.
But as this is someone else's thread I'm not going to spend any more time on it.


Senior Member
I'm guessing that &quot;designed for speed controllable AC motors&quot; would preclude its use on common shaded-pole motors that seem to be the device of interest lately.

I think that a single-phase shaded-pole speed controller would look something like the sketch below.

The need for two husky power transformers in adddition to the other circuitry would lead me to believe that the speed controller would be neither simple nor cheap. The good news is, however, that the switching speeds are well within the capabilities of a Picaxe. And, even with the two husky transformers, the speed controller would only be suitable for small motors.


<code><pre><font size=2 face='Courier'>

Voltage and Frequency Speed Controller

+-------+ mosfet1 ------
| | -----o------------&gt;
| | xfmr xfmr | to
| PS |--- pass xsistr ---------------- sec pri | motor
| | CT -------------o----&gt;
| | | |
+-------+ mosfet2 ------ | |
sec2 | |
HV monitor

| | +--------------+ +VDC
| Power Supply |-----&gt;---| Pass Xsistor |-----------&gt; to
| 15 VDC, 3+ amps | +--------------+ center tap
| unregulated | | of
| | | transformer secondary
+-------------------+ |
| Note: transformer is
PWM -------&gt;------+ 240 VAC to 25.2 VCT
at 3 amps min

Dig1 ------&gt;----------- to mosfet1,
connected between sec1 and gnd

Dig2 ------&gt;----------- to mosfet2,
connected between sec2 and gnd

Note: Dig1 and Dig2 are inverse-polarity drives at
approx 20 to 50 Hz

| |&lt;---------
ADC &lt;------------| Rectifier, filter, | from
| voltage divider | transformer primary
HV monitor | |&lt;---------


Edited by - Tom2000 on 24/08/2007 10:24:07


Senior Member
On second thought, I think it might be possible to accomplish this without transformers at all. The sketch I posted came off the top of my head; I didn't give it enough thought.

I think that you can rectify and filter the 240 VAC line directly, then use optoisolators to switch the pass transistor and the inverter transistors. (The inverter would have to be configured in an H-bridge arrangement.)

But, someone who's good with this stuff could probably build a voltage and frequency controller that can be used with single-phase shaded pole motors of just about any size, without incurring a great deal of expense or a large physical size.

And it might even be possible to bring this in with an 08M.


Edited by - Tom2000 on 24/08/2007 10:42:11
After reading this thread, I realised my education regarding motors was sadly lacking. Others may have realised something similar.

Here is an informative, and quite amusing, series of articles courtesy of John Storey from the University of New South Wales.

<A href='' Target=_Blank>External Web Link</a>



Senior Member
I tried my hand at a transformerless speed control design, and have posted the schematic on my page: <A href='' Target=_Blank>External Web Link</a>

Stuff like this is well outside my recent experience. I wonder if one of you kind souls who's experienced with high-voltage switching circuitry might take a peek at it and tell me where I went wrong.

If this circuit is anywhere near functional, it looks like a V/F speed control isn't all that difficult or expensive after all.

The bad news, though, is that I had to spec an 18X rather than an 08M. The 8 had enough pins, but they didn't all point in the right directions.

Thanks in advance,


PS - Oops! Another brain fade. With the addition of one more transistor (serving as a logic inverter), it looks like the 08M will not only do the job, but will have a free input for... well, right now I don't know what for, but a spare line is always useful!

Edited by - Tom2000 on 24/08/2007 19:36:31
Tom, my knowledge is limlted to bigger motors and a little electronics, but that could well work.

If it does then this could be used to control a tropical fish tank pump which would answer a lot of peoples problems. These must be shaded pole motors.

Could even cotrol an old record deck motor (remember them).


Senior Member
<i>&quot;What happens when out4 &amp; out5 are both high with PWM at maximum?&quot; </i>

You test the veracity of one of the corollaries of Murphy's law: &quot;When a $25 transistor is protected by a 25 cent fuse, the $25 transistor will invariably blow to protect the 25 cent fuse.&quot; :)



Senior Member
OK. Back to the 08M. The revised schematic is in the same place: <A href='' Target=_Blank>External Web Link</a>

Pre-amble: I am not an AC motor Guru OK.

I don't feel comfortable about this circuit if trying to control a single phase Ac induction motor. These usually have starter/running capacitors and banging in a high voltage square wave in doesn't sound a good idea.

Maybe it'll work on some motors I simply don't know. All I do know is that I'll let somone else try it first.

Tom, are you brave enough to try it? If it works then you'll have invented a real low-cost solution. My bet is that you've invented a complicated detonator. I suggest switching it on from the next room :)


Senior Member
Dippy, the motor I'm addressing is the common single-phase shaded pole induction motor, commonly found in small appliances such as fans, small aquarium pumps, portable heaters, etc. These don't have separate start circuitry.

You bring up a good point, though. Before applying a circuit such as this, it's important for the user to verify the type of motor that's used in the appliance. This isn't, by any means, a universal speed controller.

On another front, I've done some Googling, and have found that the voltage derates linearly with frequency:

<b>V = design V * target freq / design freq </b>

That makes programming simpler.

Thanks for your input!



Senior Member
I was keen on the idea of using a picaxe to variably control a 240VAC, 50W aquarium pump. After a bit of research I put it to one side... AC pumps don't like to be PWMed even when I tried to sync the zero-crossing. Lots of shuddering. It would run about 80-100% (i.e. you could vary it between these figures) but below that shudder-jerk-shudder-rattle-etc-so-forth.

If anyone cracks this nut it would be very useful.
I'm hoping Tom tries his circuit - and lives to tell the tale. And even if it works for HIS type of motor it won't be sutiable for all.

NBW: simple pwm for induction motors was discussed here ages ago as a no-go and it was realised that F/V is what the big boys do.
My fingers are crossed for a cheap solution, but I'm sure if it were the case then an enterprising Chinese company would be flooding our markets right now.


Senior Member
Actually, Dippy, if I do try it, it will be a while. I just received an order from Mouser; it will be a while before I'm ready to place another. I have everything I need on hand except for the filter caps and the MOSFETs, and just a couple of caps and a few transistors is too small an order.



Senior Member
Dippy, actually, I don't have a compelling need for a speed controller. While I'd like to see if the circuit works or, more likely, debug it to see why it doesn't and fix it until it does, this is way down on my priority list. I have other irons in the fire.

I was hoping that Flooby, or someone else with HV switching experience, would take a peek at my circuit and give me some advice, but nothing so far.

I'll probably get to it on of these days, but it isn't going to be soon.

I bet wilf_nv could give us some pointers on this subject - he's a HV guy I think. Seems not to be frequenting this forum lately, though. Must have other stuff on his plate.