Designing a mains AC-AC converter that shall adjust the output up to 230Vrms?

[David_]

Hello.

I have a mains(230V_RMS 50Hz) driven fan and it's speed is controlled by a digital fan-speed controller that implements phase control with a Triac, the problem is that this speed-control technique is creating a lot of noise in the motor. I am not bothered by the noise that the air moving through the fan is creating but that buzzing weird noise that the phase control is creating is a big problem and I want to get rid of it. Others have told me that the only option to a digital fan-speed controller is to use a Variac which I simply refuse to believe to be true,sure I could get a Variac and mounting on a stepper motor to turn the shaft or something but that doesn't make a lot of sense.

I have read a lot online but no I can't find more than loosely described schemes but it sounds to me as I should be able to use a controlled rectifier(or even the same circuit that is used in a Triac phase control) together with a capacitor/filter to create a adjustable crude DC rail that would adjustable between pretty much 300V_DC and 0V_DC(probably not 0V but some very low voltage).

Then I want to feed that DC voltage to an inverter that would chop it up into AC and create a sinewave such as a pure sine-wave inverter or even modified sine-wave inverter though I really would prefer a pure sine-wave inverter since maybe a modified sine-wave inverters output waveform could possibly also produce some noise?
I don't know a lot about inverters but if I had a inverter with a output that was classified as a modified sine-wave, then I could use a filter to filter it into a 50Hz "pure sine-wave" right?

But I can't find any clues as to how to construct a inverter who's output amplitude is dependent simply on the input DC voltage amplitude.

The fan in question a rated for: 230V_RMS @ 0,13A or 30W, so the inverter current output doesn't have to be very high but it would of course be a good idea to over-dimensions the circuit to maybe 0,5A or something(depending upon what that would require in terms of circuit & components), 0,15A would be fair enough.

Question 1:
Would a simple single Triac + filter circuit connected to the 230V_RMS mains be sufficient to generate an adjustable(actually programmable) <=300V_DC or do I need to use something like 4 SCR's in a rectifier bridge configuration?

Question 2:
Can you imagine a DC-AC inverter that features an output amplitude that's dependant upon the input DC voltage amplitude?
If so can anyone suggest to me which topology I should look into first or any other directions to give me?

If anyone have a better idea(simpler/easier to implement) for achieving an adjustable sine-wave voltage that can be programmed as high as or close too 230V_RMS?

Even though I know very little about inverters I have some knowlagde about them although my interest are mainly focused around AC-DC or DC-DC SMPS, a single Triac phase control circuit is easy to make and program with a microcontroller and from what I have seen a controlled rectifier isn't that much of a leap from there, but I would very much like to get some thoughts about this idea before I dive into the prototyping. I'm sure I am capable to figure this out with some advice.

I hope you all are having/have had/going to have a very nice day.

[strawberry]

use LC low pass filter to filter out all triac made  noise

[IanB]

use LC low pass filter to filter out all triac made  noise

This seems most promising. I can't see using a VFD on a 30 W fan.

[beanflying]

We have run into the same issues on small coffee roasters and the control of the drum fans generally 100W or less.

Single phase with single wound motors aren't that happy being speed controlled in my experience they run hot and suffer premature death. Better option is three phase or even DC motors as you can get much greater variations in speed.

My personal preference is full speed and use a mechanical gate to restrict airflow.

Not the answer you want but a few alternates to consider

[NiHaoMike]

For a little 30W fan, it should be easy to replace it with some 12V BLDC fans.

[David Hess]

Another way to do it at low powers is to drive a step up transformer with a power audio amplifier driven by a variable amplitude 50Hz sine wave oscillator.  This can be pretty efficient if the audio amplifier is class-D but a linear amplifier can work also.

[Nauris]

Others have told me that the only option to a digital fan-speed controller is to use a Variac which I simply refuse to believe to be true,sure I could get a Variac and mounting on a stepper motor to turn the shaft or something but that doesn't make a lot of sense.

They mean autotransformer with multiple taps, not variac and stepper... As that is the usual way of adjusting those small fans. Use relays to switch taps if you need to control it from some controller, else use multi-pole switch.