I have an old Woolworths desk fan that is quite noisy.
Because of the recent heatwave(s) I wanted to be able to make the fan quieter so that I could use it at night.
I happened to have three old 'Zenith' adjustable Triac power controllers stored away in my garage.
I decided to wire one up to a mains socket so I could plug in the fan and run it at a lower speed.
It worked for about 1 minute.
It then failed in spectacular fashion, foul smelling smoke coming from the case. The 5A fuse in the plug remained intact.
When I opened it up I discovered a Rifa PME 0.1uF 250V capacitor connected directly across the mains input had failed.
It left black soot and residue all over the PCB and case.
When I looked it up I discovered failures in these capacitors are well known. See:-
https://w140.com/tekwiki/wiki/RIFA_capacitors.
I replaced it with a Red 0.1uF Wima capacitor rated at 630V.
It seems to be working ok at the moment.
Is it safe?

 

I would think so.

 

The explanation depends on the fan motor type. My expnation resumes that the fan motor is an AC only induction motor.
NEXT, understand how a common triac adjustable Triac power controllers function: A triac speed controller reduces the speed by reducing the delivered power. That reduction is achieved by cutting off the leading edge of the AC sine wave on both the positive and negative parts of the incoming sine wave. The result is a distorted waveform, which includes quite a few harmonics, and thus a fair amount of harmonic energy.
When that waveform with all those harmonics is applied across a capacitor , the current will rise because the impedance of a capacitor decreases as the frequency increases.
As the current in the capacitor rises, the losses inside the capacitor also rise. Those losses produce heat, which can then cause a temperature rise, which can lead to failure.
THAT is the explanation without any math at all!!

 

If that is the only capacitor on the motor, it could well be a shaded pole motor.
The cap across the power input is not for phase shifting . Especially at 0.1uF
If it is the Wima style I think it is, it is suitable for using across the supply.
. The original make are notorious for cracking, which lets in moisture and eventually causes them to fail.

 

These capacitors are not any type of capacitor. They MUST BE safety-rated capacitors. And indeed, RIFA and to some extent WIMA also, had issues even though they had the X or Y rating.

Rather than writing a couple hundred words describing the why and the how, please have a read of the following article:
https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/
I sincerely hope that you DON’T IGNORE my advice.

 

@brianmk where are you located?

 

I have an old Woolworths desk fan that is quite noisy.
Because of the recent heatwave(s) I wanted to be able to make the fan quieter so that I could use it at night.
I happened to have three old 'Zenith' adjustable Triac power controllers stored away in my garage.
I decided to wire one up to a mains socket so I could plug in the fan and run it at a lower speed.
It worked for about 1 minute.
It then failed in spectacular fashion, foul smelling smoke coming from the case. The 5A fuse in the plug remained intact.
When I opened it up I discovered a Rifa PME 0.1uF 250V capacitor connected directly across the mains input had failed.
It left black soot and residue all over the PCB and case.
When I looked it up I discovered failures in these capacitors are well known. See:-
https://w140.com/tekwiki/wiki/RIFA_capacitors.
I replaced it with a Red 0.1uF Wima capacitor rated at 630V.
It seems to be working ok at the moment.
Is it safe?

What does the output of these controllers look like on a scope? at say a 50% setting.

 

I have an old Woolworths desk fan that is quite noisy.
Because of the recent heatwave(s) I wanted to be able to make the fan quieter so that I could use it at night.
I happened to have three old 'Zenith' adjustable Triac power controllers stored away in my garage.
I decided to wire one up to a mains socket so I could plug in the fan and run it at a lower speed.
It worked for about 1 minute.
It then failed in spectacular fashion, foul smelling smoke coming from the case. The 5A fuse in the plug remained intact.
When I opened it up I discovered a Rifa PME 0.1uF 250V capacitor connected directly across the mains input had failed.
It left black soot and residue all over the PCB and case.
When I looked it up I discovered failures in these capacitors are well known. See:-
https://w140.com/tekwiki/wiki/RIFA_capacitors.
I replaced it with a Red 0.1uF Wima capacitor rated at 630V.
It seems to be working ok at the moment.
Is it safe?

Can you post a picture of your WIMA capacitor so that everyone can see the safety rating?

 

Usually the output of an actual triac speed controller looks like a sine wave with the rising edge cut off and replaced with a delayed vertical rise. So the leading portions of the sine wave are missing. This is serious harmonic distortion.

 

I should have made it clear that the failed RIFA capacitor is inside the Triac power controller unit and not in the fan itself.

The temporary replacement I used is a 630Vdc Wima PET capacitor. These are not really designed for connection directly across 230V AC mains so they do not carry an X or Y safety rating. They are really intended for DC applications, however they are rated at 400V AC.

I have now ordered some TDK 'X2 rated' Metallised PP safety capacitors as a more permanent
solution.

As far as the original failed RIFA PME capacitor is concerned, it was X rated. However, that did not prevent it from exploding. The fact that the 5A mains fuse remained intact suggests that at least it eventually failed open circuit rather than short circuit. Despite the alarming crackling noises and smoke I don't think it actually caught fire.

It was interesting to watch the video by Jerry Walker who carried out tests on hundreds of these capacitors in an attempt to understand why they fail.
See:-

Before watching the video I did wonder if moisture could have been the cause since the power controllers were stored in an unheated garage for some years. However, his findings indicate that moisture is not the root cause of the failure. He found that these RIFA capacitors failed even in sealed environments where they were potted with wax or tar to keep moisture out.

 

I have an old Woolworths desk fan that is quite noisy.
Because of the recent heatwave(s) I wanted to be able to make the fan quieter so that I could use it at night.
I happened to have three old 'Zenith' adjustable Triac power controllers stored away in my garage.
I decided to wire one up to a mains socket so I could plug in the fan and run it at a lower speed.
It worked for about 1 minute.
It then failed in spectacular fashion, foul smelling smoke coming from the case. The 5A fuse in the plug remained intact.
When I opened it up I discovered a Rifa PME 0.1uF 250V capacitor connected directly across the mains input had failed.
It left black soot and residue all over the PCB and case.
When I looked it up I discovered failures in these capacitors are well known. See:-
https://w140.com/tekwiki/wiki/RIFA_capacitors.
I replaced it with a Red 0.1uF Wima capacitor rated at 630V.
It seems to be working ok at the moment.
Is it safe?

The replacement is better, but make sure the capacitor is a proper X2-rated mains safety capacitor. A normal 630V film capacitor may not be suitable for direct connection across the mains. The controller may be fine, but check for soot, carbon tracking, or damaged PCB areas. For safety, replace it with a 0.1µF X2 275VAC (or higher) capacitor.