OK, at last fitted the fan speed controller for the downstairs loo, and it causes the fan to buzz. Not surprising I suppose as it is bound to be a switching device. Well, the result is crap and doesn't sound good for the fan either. Anyone know of a voltage dropping fan speed controller that will fit in a single backbox ? Or a non-continuous voltage dropper that can turn the fan down in steps. Although I might decide I like a bit of noise to mask to "other" noises ! Cheers, Simon.
That's the one I've got ! It doesn't buzz but it causes the fan to buzz unless it is on full speed. Since I got a slightly overpowered fan to turn it down and make it even quieter, the buzzing is no good at all. I have an S&P fan, so they are designed to be compatible. I think its just the nature of switching fan controllers. Simon.
Interestingly, the technical manual (pdf) indicates its use with _induction_ motors (I thought it might be limited to universal ac/dc motor types only).
I've always known that speed control of induction motors required both frequency and voltage control to avoid overheating of the rotor (but that is in the case where you need to maintain the same torque output from the motor in question).
I suppose the reduced drag from the fan blades modifies the situation somewhat, allowing simple (effective) voltage control without commensurate frequency changes to avoid rotor overheating.
From the dim and not too reliable past, I understood that the loading varied as the fan speed cubed. The uk diy fan speed control wiki is very good and as has been said you should be able to knock up a 3 speed capacitive unit fairly easily. Most ac fan motors are shaded pole, very resistive and need only voltage control so switched capacitors are very satisfactory. The unit you're using is a triac system I guess, these will tend to rattle if the motor coil is a bit loose on the stampings.
I like to use large capacitive 3 speed US ceiling fans for air stirring as they are very cheap and run somewhat slower than 50HZ units on the low setting (more slip?) and are very, very quiet. Yes, supplying 120V is a pain but you only need a few watts. Obviously your requirement is different.
You shouldn't use phase control (dimmer) type speed controllers with shaded pole motors (unless it was specifically designed to handle this). The harmonics cause significant eddy current heating in the cores, combined with reduced cooling due to lower speed.
The sound (buzzing) is due to these harmonics causing some part of the motor to vibrate.
+1
A series impedance is the way they are slowed (unless they have a tapped winding for dual speed operation). Don't try slowing it too much, because again there is the possibility of overheating - the airflow cooling drops very much faster than the power dissipation. Above all, if it can be started at the lower speed, make sure there is still enough starting torque to ensure there's no chance of it sitting there powered up with a stalled rotor.
If the motor doesn't have an overtemp cutout, you might consider adding one whilst you are doing this. I do so when reconditioning some types of fan.
Are these in line extractor fans for bathrooms/kitchens?
The ones we have are like bloody great rugby balls and fit in the ceiling void.
Haven't noticed any buzz - in fact the non-speed-controlled one in the loft is virtually silent and the noise comes from the air intake in the bathroom.
Small ones, anyway. Shaded pole motors are generally limited to around 50W max (at which point you should change to a more efficient motor design).
They use an induction motor design with very many poles. Some have a choice of windings which alternatively combines or spilts ajacent poles to change the effective number of poles, but I think most of the speed control is done with capacitor switching to under-power the motor and generate excessive slip, which works because the torque required also drops dramatically as the speed drops.
The motors are a kind of inverted design compared with more conventional induction motors, with the stationary stator in the middle, and the armature/rotor around the outside of it, so that it's the outside of the motor which goes round as the centre is fixed to the mounting, and can pass wires through for stationary lights underneath.
There's a series on one of the freeview channels about how things are made, and one of the issues covered how these motors are made.
Well its a known effect of triac type controllers that send the fan a chopped up waveform. I'd describe it as a hum/buzz and probably more like a hum to you if its above the ceiling. Anyway, the point is, does the fan motor noise increase when then controller is turned down from maximum ? At full power all I can hear is air noise. Anyway it looks like capacitor voltage droppers is the answer so I shall use the wiki procedure to sort it out - when I eventually get a round tuit. Simon.
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