kitchen fan speed control

[gerry maddenParticipant @gerrymadden53711]

Hi All, I'm in the process of building hopefully a quiet kitchen extractor fan. After completing the basic structure yesterday I ran a few tests. At high speed most of the noise is aero so I need to work on this. But when I try to run at very slow speed I get a lot of electro/mechanical noise. The problem appears to be when the motor voltage is between 20 and 65V. Initially this will kick the rotor into gentle motion but this isn't maintained and it will slowly come to rest again. At the same time there is a strong mains hum which modulates with each turn of the rotor.

Interestingly if I flick the power on and off a few times without adjusting the controller setting, occasionally it will stop humming and start accelerating to around 1000 RPM.

I would like to be able to run quietly at speeds less than 1000RPM. So what may be wrong ? Is it the small 1A speed controller is not compatible ? Do I need a better one, or even a better motor ?

Another thing is the 2uF/450V capacitor – it has a big M8 stud for mounting which 'seems' to be electrically isolated from the usual connections. Do I need to use this stud or can I mount the capacitor in a plastic tool-clip instead ?

And finally, I have noticed that on full power, the controller puts out 260V into the motor. This is above mains! So is this normal or some kind of measuring error ?

[gerry maddenParticipant @gerrymadden53711]

[Martin WParticipant @martinw]

I suspect that the fan motor is split winding synchronous motor and I as far as I am aware you can't use a triac speed controller with this type of motor. The rotor will want to rotate in sync with the rotating field generated by the field coils and while they can be run with a certain amount of slippage it is not ideal. Hopefully another member of this forum who is more knowledgeable regarding motors and control will give a better explanation and if I am wrong will correct any error.

[JourneymanParticipant @journeyman]

As far as I can see from the *** Blauberg *** site it is designed for fixed speed to provide known airflow depending upon the ductwork resistance.

John

[gerry maddenParticipant @gerrymadden53711]

Hi Martin and Journeyman, thanks for your comments. I had a look at the website – thanks for the link – and it seems to suggest that this motor is an asynchronous design. And somewhere else I think I saw a statement that said something along the lines that 'all our motors are designed to be controllable'. Certainly from 1000 all the way to 2530 it behaves well which makes me think its the cheap controller that not good for slow speeds.

I would still be interested to know if the 260V output is realistic and the capacitor mounting if any one else has any thoughts ?

Gerry

[John OlsenParticipant @johnolsen79199]

Actually when driving a fan you sometimes can use those triac type of controllers to control motor speed. This is partly because of the nature of the load from a fan. Dropping the effective voltage reduces the torque available, but since the torque required is a function of the speed, the torque required drops very rapidly as speed is reduced, so it tends to find a lower speed where it runs OK. The motor will be running at a higher slip, but that does not matter too much. My late father used a triac controller to reduce the speed of a small shaded pole motor driving a fan with reasonable results.

However it tends not to be all that satisfactory. One potential problem is that the waveform from a triac controller at lower powers is not always symmetrical, which means that you are putting a DC component into the motor, which it will not like.

The voltmeter may not like the spiky waveform from the triac. Unless it is a true RMS device it will not read accurately on anything other than a sine wave.

A DC motor might be a better approach, being smoothly controllable over a wide speed range. To reduce the aerodynamic noise, the best approach is to use the largest possible fan running at the lowest speed that will move sufficient air.

John

[John HaineParticipant @johnhaine32865]

It's almost certainly a capacitor start and run induction motor, of the "torque motor " variety used for fans. These run below synchronous speed and have a degree of controllability, but at low speeds I suspect your controller is interacting with the odd motor speed/torque characteristics giving odd behaviour. If you really want smooth control down to low speeds you will need either a variable frequency drive, or a dc motor as John Olsen says. There was a long thread here a couple of years back on this, someone trying to fix a cooker hood fan.

John is correct about the waveform affecting the observed voltage. Probably made worse by the switching spikes interacting with the motor inductance.

It looks like the capacitor case is plastic, there should be no problem using a plastic or indeed a metal tool clip. Even with the metal cased motor caps the case is usually isolated.

[Wout MoermanParticipant @woutmoerman25063]

For silencing a rather loud ventilation fan I wanted it to turn at a lower RPM. I wired a 230 volts light bulb in series with the fan. By changing the lightbulb to a different wattage I can adjust the speed. A higher wattage means a higher RPM. Not a continiusly adjustable speed and some heat lost in the lamp, which faintly glows. But it is an easy method.

[SillyOldDufferModerator @sillyoldduffer]

Posted by gerry madden on 29/07/2018 16:20:47:

Hi All, I'm in the process of building hopefully a quiet kitchen extractor fan.

…

 

I would like to be able to run quietly at speeds less than 1000RPM.

…

 

Before getting too entangled in getting the fan to spin slowly and quietly, I suggest testing the air-flow at slow speeds to confirm that slow speeds are worth the effort.

The efficiency of that type of fan reduces rapidly as the blade speed drops. Below a certain minimum speed the fan is completely ineffective, and performance stays poor until they are travelling fast enough. Small fans have to spin quickly. Low rpm fans like a ceiling fan have much bigger blades, so that the outer part of the blade has enough velocity.

An easy way to indicate airflow is with a lit joss stick. See from how far away from the fan the smoke is pulled through it at full speed, then repeat with the fan idling. You should find that the fan at low speed stirs air rather than extracting it, i.e at low speeds it's only wasting electricity and making a racket. Using the same technique it should be possible to find the fan's minimum effective rpm, that is the speed at which smoke is obviously pulled through the fan. With luck that'll be fast enough to avoid your very low speed control and noise problems.

Dave

 

 

Edited By SillyOldDuffer on 02/08/2018 09:36:24

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