Failure of polypropylene motor-run capacitors

Over the years I've had a couple of motor-run capacitors fail. A plastic cased one had some grey gunk come out of it. Recently I had an aluminium cased polypropylene film capacitor fail, and drop to <10% of its rated capacitance. When I opened it up, there was what looked like lumps of silver grey plastic that had melted, flowed to the bottom of the casing and solidified.

I was just curious as to what is actually going wrong in these capacitors.

Also, the latest one was on the refrigerant compressor of a heat-pump tumble dryer. Would I get better capacitor life and / or lower power consumption if I arranged the the capacitor disconnected after the heat pump started?

Modern capacitors are made by sputtering metal vapor onto thin polymer film (physical vapor deposition in a vacuum chamber), cutting to size, adding connectors to left and right edges, rolling them tightly, sliding them into their case and sealing the case.

The polymer molecules in the film do move, undulate, flow slightly as they are thermal-cycled with each on/off cycle. The metal is in the 10-100nm range and film is in the 1 to 5 micrometer range.

Eventually, the very thin metal coating tends to flow across the surface of the plastic and agglomerate. The metal doesn't bond to the polymer very well. The polypropylene softens and moves at much lower temps than other options but it can be rolled (extruded) to much thinner sheets than other polymers. PC is a higher temp material but much more expensive and the film is thicker so it's difficult to get the same capacitance in the same space as your current device (or, a very difficult and expensive process to make thin PC is required and Bayer was the only company to do it and still couldn't compete with PP so they quit making the film more than 25 years ago.

Ultimately, there is no way to prevent it other than making larger capacitors that can dissipate heat better.

From what you have said, I might fit extension wires to the capacitor so that I can put is somewhere where it is going to run cooler, which might help, and be easier to change, which will help if running cooler doesn't help.

This tumble dryer has a base assembly that is about 20 cm tall, which contains the heat pump (compressor, evaporator and condenser), both fans and the water pump. The drum and the whole casing has to be removed from this base unit to change the capacitor. I don't want to have to do that lot again any time soon.

Motor-run caps are required to be attached continuously to the motor as long as it is running.

As such, these are designed to be very robust but occasionally still fail. The key word is occasionally.

Like everything else in the consumer World, nowadays there is a significant push to reduce costs. Which eventually means cutting corners.

Case in point, something that I have personally seen in high-ripple rated capacitors: the good practice is to stitch the terminals to the metal foil, with dozens of stitches for a solid electrical connection. Cost-reduced capacitors use far fewer stitches or even a conductive epoxy to create the electrical connection instead.
These poor connections have a reduced operating life.

Generally speaking polypropylene capacitors are very high rated, especially the WIMA versions.
If the cap is polypropylene it must be a smaller motor?
One manufacturer I use for start caps is CDE, they make high grade oil filled paper versions for motor run conditions.

MaxHeadRoom78 said:

Generally speaking polypropylene capacitors are very high rated, especially the WIMA versions.
If the cap is polypropylene it must be a smaller motor?
One manufacturer I use for start caps is CDE, they make high grade oil filled paper versions for motor run conditions.

Click to expand...

Historically Wima were the most unreliable capacitors available

'Back in the day' if you had a faulty amplifier, tape recorder etc. the first thing you did was to look if there were any Wima capacitors fitted - then blanket change them, and see if the fault was cured. Most of the time, that solved the repair

No point in fault finding while there were still Wima capacitors fitted!.

I'd like to think that they are more reliable now, after all the decades.

Interesting!
The bottom line for me, is to use run capacitors that are designed specifically for this purpose, CDE etc,
Definitely NOT start caps of Chinese origin. !