controlling the speed on a 240volt single phase extractor fan

[m3vuv]

Hi all,i need to cobble together a speed controller for this,i thought of a light dimmer but as its an induction motor thought better of it,the motor is about 1/4hp as far as i can see,any idea how i can do it?,cheers m3vuv

[Zero999]

Presumably it has a shaded pole motor? In that case, just use a phase controller (lamp dimmer) circuit.  It's good idea to grossly over-rate it, so use one designed for 400W and add a snubber circuit across it, or the motor.

[Microdoser]

Cheap fan controllers, like light dimmers, can cause buzzing from the fan but if you are happy with the noise then they work fine. A variac, or a transformer that can supply various voltages will be much quieter, although you will still use the same power even when it is slowing down the fan.

There are digital fan controllers that are a bit more costly available, I did see one a year or two back that was efficient and quiet but I can't find a link at the moment.

[poorchava]

Triacs will buzz at low rpm. I would use an AC-buck topology. Basically a half bridge, but instead of mosfets you have diode bridges with an IGBT between + and - output. Requires a floating drive of some description, but it's relatively easy to do. It produces a HF chopped mains voltage at the output. HF may be anything as long as it's not audible (40kHz ish?) The motor is the inductor, so the main argument to drive frequency high is void. And lower frequency means lower switching losses.

I've done that commercially exactly for controlling fans (up to 300W). Obviously needs snubbers as well as some line filtering if you want to pass emissions and some MOVs for power line surges

This can (and i have done it) be cobbled together on a perfboard. Needs 2 igbts, 8 fast diodes, and whatever random ferrite core without gap you can get to either build 2 floating power supplies for isolated gate drivers it just use it as a GDT fed by a dual fet driver in full bridge drive mode (ie feed complementary PWMs into 2 channels of a dual driver - makes adjusting dead band very easy). Plus whatever microcontroller that can output a complementary PWM. I used STM32F0 on some Chinese module.

I literally built it out of stuff from scrap pile. IGBTs came from a partially dead induction hob IIRC.

[bill_c]

If you just want to reduce flow (and power consumed) then restrict the inlet.

[m3vuv]

its not as simple as that as it will be mounted outdoors in the air,its to go over chip friers in a pub kitchen.

[bdunham7]

Does it need to have infinitely variable control or would 3 or 4 steps work?  If so, a multi-position switch with appropriately sized capacitors will work.

[m3vuv]

yes 3 or 4 steps would be fine,cheers .

[Zero999]

Does it need to have infinitely variable control or would 3 or 4 steps work?  If so, a multi-position switch with appropriately sized capacitors will work.

That will work, but be careful, connecting capacitors in series won't necessarily reduce the speed. One of the first mains experiments I did with a shaded pole motor stator was to connect a capacitor in series with it and the current actually increased, rather than decreased. The capacitance can cancelled out the inductance of the motor, plugging it closer to resonance, hence more current. It started to get hot and if I'd left it for too long, it would have overheated.

To reduce the speed, the capacitance should always dominate the overall impedance, i.e. X_C>>X_L, meaning the capacitor's value should be lower, than what you'd expect for a typical capacitive dropper circuit. If you don't know the motor's inductance and lack the equipement to measure it, start with low value capacitors and increase from there.

To eliminate the possibility of resonance, I'd recommend an inductor, try a fluorescent lamp ballast, or high powered resistors, rather than capacitors.

[bdunham7]

That will work, but be careful, connecting capacitors in series won't necessarily reduce the speed.
To eliminate the possibility of resonance, I'd recommend an inductor, try a fluorescent lamp ballast, or high powered resistors, rather than capacitors.

If it is a hood extraction fan, it is likely a PSC motor.  I doubt they use a 1/4HP shaded pole motor.  Shaded pole might take some experimentation, but PSC motors are commonly speed controlled with capacitors and multiposition switches.  I'd recommend the OP determine the size of the run capacitor and start with capacitors that size, half that size and twice that size.  Capacitors are used because they dissipate minimal heat when used this way.

OP, do you have a photo of the motor and label,  or can you tell us if it has a run capacitor?