Faulty motor run capacitor. - UK Vintage Radio Repair and Restoration Discussion Forum

ukcol · 12th Oct 2018, 1:03 pm

I have just ordered a replacement 6uF motor run capacitor for my Zanussi tumble dryer.

The original component had gone down in value to 3.5uF.

The mechanism of loss of capacitance in class X capacitors is well known, but do motor run capacitors use a similar technology to class X types and loose capacitance by a similar mechanism or does something quite different cause loss of capacitance in motor run types?

I realize that "motor run" is a description of the of the capacitor's application and not necessarily its construction technology. There may be different technologies used for them and therefore different modes of failure.

MotorBikeLes · 12th Oct 2018, 8:02 pm

I have a large number of motor run, 450v AC rated capacitors, and checking against the nominal value on the cans, I find very little difference between claimed and measured. I also have some 240v AC rated ones, with unusual values eg, 120UFd - 160UFd. All measured somewhere within their stated ranges.
The only failures I have had have been very loud exploding failures. As I am using these in connection with rotary converter developments, I now double all values with two in series. No exploders since. I do have a few smaller block like ones in the 1UFd to 5UFd range, will check them sometimes. I don't think I have used any of them yet.
Les.

Sideband · 12th Oct 2018, 8:11 pm

My experience of motor-run capacitors is that they lose value. My old tumble dryer used an 8uF 450VAC capacitor and the first symptoms were reluctance of the drum to turn usually accompanied by a low humming noise. The first one I removed had gone down to around 3.5uF. That replacement was replaced about 5 years later and the value had dropped to around 4uF.

ukcol · 12th Oct 2018, 8:50 pm

Quote:

Originally Posted by Sideband

.....and the first symptoms were reluctance of the drum to turn usually accompanied by a low humming noise.....

That is exactly the same symptom that my dryer had.

I was just interested if anyone knew the mechanism of failure that caused the capacitor to loose capacitance.

In class X capacitors it is the "self repairing" mechanism that causes gradual loss of capacitance. Perhaps its the same with motor run types?

McMurdo · 12th Oct 2018, 8:57 pm

well..the spec sheets call them Metallised Polypropylene, as are most x caps.
These days they usually have a safety release that cuts off the power if the vent cap lifts

ukcol · 12th Oct 2018, 10:20 pm

Quote:

Originally Posted by McMurdo

well..the spec sheets call them Metallised Polypropylene, as are most x caps.

Thanks Kevin, that explains it.

Maarten · 12th Oct 2018, 11:11 pm

And before that, most were metallised paper sporting the same self healing properties.

Richard · 13th Oct 2018, 5:21 pm

Motor run capacitors used to be very reliable, and held value well, I have equipment that's 40 years old and the caps measure fine.

Then 10 years or so ago they suddenly became unreliable, loosing value very quickly.
When I worked on commercial dishwashers, glasswashers etc we used to fit new ones at every annual full service, they are that bad.

I now work on whitegoods, including tumble driers etc and regularly change them on machines that are only 2 or 3 years old.

Richard

ukcol · 13th Oct 2018, 5:45 pm

That's an interesting observation Richard. I wonder what's going on there?

A reduction in the quality of new capacitors?

An increase in the amount of noise and spikes on the mains supply?

Both?

dave walsh · 13th Oct 2018, 6:06 pm

I had the Engineer round for a previous T Dryer. The 8uF had failed under warranty. He consulted the service details on his gadget and said "That's funny it shows a 16uF replacement!" I said "Maybe this is why?" He agreed, it never failed again and as the back was all metal it was very safe anyway!

Dave W

slidertogrid · 13th Oct 2018, 8:50 pm

I recently changed one on a mate's compressor, about 4 years old but to be fair it is used all day 5 days a week. the cap was strapped to the motor body against the cooling fins in the airflow of the cooling fan. I put the failure down to the hot summer and in my opinion bad siting of the capacitor.. But reading this maybe not...
Rich.

Refugee · 13th Oct 2018, 10:40 pm

We have a local scrappy and he had a little compressor of the kind sold by pretty well all the DIY sheds.
It was humming and he was patting the plastic cover.
I told him to whip the cover off and look for a capacitor with its guts spilled out.
We found one with a split in it. We got it before it had all spilled out.

12th Oct 2018, 1:03 pm	#1
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Faulty motor run capacitor. I have just ordered a replacement 6uF motor run capacitor for my Zanussi tumble dryer. The original component had gone down in value to 3.5uF. The mechanism of loss of capacitance in class X capacitors is well known, but do motor run capacitors use a similar technology to class X types and loose capacitance by a similar mechanism or does something quite different cause loss of capacitance in motor run types? I realize that "motor run" is a description of the of the capacitor's application and not necessarily its construction technology. There may be different technologies used for them and therefore different modes of failure.

12th Oct 2018, 8:02 pm	#2
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Faulty motor run capacitor. I have a large number of motor run, 450v AC rated capacitors, and checking against the nominal value on the cans, I find very little difference between claimed and measured. I also have some 240v AC rated ones, with unusual values eg, 120UFd - 160UFd. All measured somewhere within their stated ranges. The only failures I have had have been very loud exploding failures. As I am using these in connection with rotary converter developments, I now double all values with two in series. No exploders since. I do have a few smaller block like ones in the 1UFd to 5UFd range, will check them sometimes. I don't think I have used any of them yet. Les.

12th Oct 2018, 8:11 pm	#3
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,571	Re: Faulty motor run capacitor. My experience of motor-run capacitors is that they lose value. My old tumble dryer used an 8uF 450VAC capacitor and the first symptoms were reluctance of the drum to turn usually accompanied by a low humming noise. The first one I removed had gone down to around 3.5uF. That replacement was replaced about 5 years later and the value had dropped to around 4uF. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

12th Oct 2018, 8:57 pm	#5
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,271	Re: Faulty motor run capacitor. well..the spec sheets call them Metallised Polypropylene, as are most x caps. These days they usually have a safety release that cuts off the power if the vent cap lifts __________________ Kevin

12th Oct 2018, 11:11 pm	#7
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,199	Re: Faulty motor run capacitor. And before that, most were metallised paper sporting the same self healing properties.

13th Oct 2018, 5:21 pm	#8
Richard Heptode Join Date: Feb 2003 Location: Garnant, near Ammanford, South Wales, UK. Posts: 657	Re: Faulty motor run capacitor. Motor run capacitors used to be very reliable, and held value well, I have equipment that's 40 years old and the caps measure fine. Then 10 years or so ago they suddenly became unreliable, loosing value very quickly. When I worked on commercial dishwashers, glasswashers etc we used to fit new ones at every annual full service, they are that bad. I now work on whitegoods, including tumble driers etc and regularly change them on machines that are only 2 or 3 years old. Richard __________________ BVWS member.

13th Oct 2018, 5:45 pm	#9
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Faulty motor run capacitor. That's an interesting observation Richard. I wonder what's going on there? A reduction in the quality of new capacitors? An increase in the amount of noise and spikes on the mains supply? Both?

13th Oct 2018, 6:06 pm	#10
dave walsh Dekatron Join Date: Jan 2006 Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex. Posts: 5,817	Re: Faulty motor run capacitor. I had the Engineer round for a previous T Dryer. The 8uF had failed under warranty. He consulted the service details on his gadget and said "That's funny it shows a 16uF replacement!" I said "Maybe this is why?" He agreed, it never failed again and as the back was all metal it was very safe anyway! Dave W

13th Oct 2018, 8:50 pm	#11
slidertogrid Octode Join Date: Apr 2005 Location: Peterborough, Cambridgeshire, UK. Posts: 1,898	Re: Faulty motor run capacitor. I recently changed one on a mate's compressor, about 4 years old but to be fair it is used all day 5 days a week. the cap was strapped to the motor body against the cooling fins in the airflow of the cooling fan. I put the failure down to the hot summer and in my opinion bad siting of the capacitor.. But reading this maybe not... Rich.

13th Oct 2018, 10:40 pm	#12
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Faulty motor run capacitor. We have a local scrappy and he had a little compressor of the kind sold by pretty well all the DIY sheds. It was humming and he was patting the plastic cover. I told him to whip the cover off and look for a capacitor with its guts spilled out. We found one with a split in it. We got it before it had all spilled out.