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Power-User

Join Date: May 2010
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Start Capacitor Failure

08/06/2011 1:21 AM

A friend has a number of rental properties which present him with some 14 air conditioning units (heat pumps.) He finds that the average lifetime for a capacitor on the axial fan motor (located out-of-doors as distinct from the in-house fan-coil unit) is 15 months. He is located in Texas and the current heat wave has very much shortened the life span of these capacitors. His A/C contractor ascribes this to the elevated temperature and says that he is now carrying a box of these capacitors in the service truck as capacitor failure seems to be reaching epidemic proportions.

First of all, 15 months seems to me to be an unreasonably short lifespan for a starting capacitor and secondly it seems unreasonable that a capacitor thus sensitive to high ambient temperature would be selected by the A/C manufacturer.

Can anyone shed light on this apparant mystery?

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Guru

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#1

Re: start capacitor failure

08/06/2011 1:31 AM

Just like in any electronic device, capacitors (especially e-cap) have always been one of the weakest link in reliability .

Most so for capacitors directly across mains AC line as they are subject to much continuous voltage spikes.

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Power-User

Join Date: Jun 2009
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#2

Re: start capacitor failure

08/06/2011 1:57 AM

anything can happen in the tx summer . . . was 105 when i was there last month . . . please provide voltage of capacitor (from label) and system voltage (as supplied by electricity supply company) . . . and farad rating if available

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Active Contributor

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#3

Re: start capacitor failure

08/06/2011 3:26 AM

The poor quality of the capacitors. You should use a capacitor of a reputable manufacturer, not just cheap capacitors.

Zlatkodo

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Guru
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#4

Re: Start Capacitor Failure

08/06/2011 9:10 AM

If nothing works ask him to pull long wires with the cooling coils, and hang all capacitors indoors in the air-conditioned environment.

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#19
In reply to #4

Re: Start Capacitor Failure

08/08/2011 12:56 PM

But then when the cap blows it will send the smell into the conditioned space.

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Guru

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#5

Re: Start Capacitor Failure

08/06/2011 12:06 PM

This does not sound like much of an issue to me.

Switch to a capacitors over to ones with a far higher voltage rating but of the same uF rating it will have a larger body of course. That will greatly reduce the applied internal thermal load placed on the capacitor thus greatly increasing the working life of them in high ambient temps.

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#6

Re: Start Capacitor Failure

08/06/2011 12:22 PM

15 months in Texas heat is probably 15 years anywhere else.

Elevated temperatures can drastically reduce component life. It seems clear that the operating temperature is higher than expected for much longer periods than expected and this is causing the capacitor failure. I'd contact the manufacturer and ask if they can recommend/supply a capacitor designed to handle abnormally high temperatures. You could try to find one yourself, but getting the manufacturer to do the work and supply a better cap for free would be preferred.

The post #4 suggestion to get the capacitor out of the "heat" could work, but you will need to calculate the cost/benefit ratio of running extra wiring vs. the maintenance/repair trips. If you do attempt to relocate the capacitor, follow all applicable codes to prevent possible injury and/or fire damage.

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#7

Re: Start Capacitor Failure

08/06/2011 12:44 PM

The main reason really why these starting capacitors give up at high ambient temperature is that, these capacitors are coupled to supply 3phase motor of the compressor. At high ambient temperatures, condeser(located outside) pressure is significantly above normal, making the compressor motor stressful thus to the current and phasing of this capacitors. That's is why these capacitors often give up at hot season, much more if there are voltage drop/variation of the entire power supply system

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Power-User

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#8

Re: Start Capacitor Failure

08/06/2011 12:53 PM

where is baffled? if baffled answers some queries raised in this thread then a specific solution can be offered instead of generalizations and conjectures . . .

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#9

Re: Start Capacitor Failure

08/06/2011 1:31 PM

No one should be baffled. This is a simple problem with a simple solution.

* residential A/C (and heat pumps) are normally single phase 220-240Vac here in US
* motor "starting" capacitor also indicates single phase because three-phase systems don't need starting capacitors
* capacitor self-heating during start-up will be small because duty cycle is small

duty_cycle = start_time/(start_time + run_time + off_time)

* capacitor is typically mounted on the "axial fan" motor
* heat from the motor + heat from the condenser + excessive & prolonged ambient temperatures will cause premature capacitor failure

Find a higher temperature rated capacitor or reduce the temperature of the existing one. Best wishes.

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Power-User

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#10
In reply to #9

Re: Start Capacitor Failure

08/06/2011 2:05 PM

I give mjb a GA. He didn't suggest that the problem could be a function of capacitance and pointed out that the presence of a capacitor indicates single phase. The start capacitor is not located on the fan motor in the hot slipsteam of the condenser but rather in a small enclosure within the pump enclosure.

I didn't know that these capacitors came in different temperatire ratings - I will check it out.

I wonder if, as the temperature rises and with it, the demand, as the grid reaches maximum capacity, the product gets dirtier. Is there any reason that that might happen? If that were so then the problem would be only indirectly related to temperature.

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#11
In reply to #10

Re: Start Capacitor Failure

08/06/2011 3:16 PM

There should be two capacitors, one for the compressor and one for the axial fan motor. Which one is actually failing?

I suspect these capacitors are more likely to be motor start+run capacitors. That means the internal temperature of the capacitor will be even higher than the case temperature. Where the excessive heat comes from, internal or external to the capacitor, doesn't really matter in this case. I still think your problem boils (pun intended) down to excessive temperature. The links below may help.

http://www.illinoiscapacitor.com/tech-center/life-calculators.aspx

http://www.aerovox.com/Portals/0/PDFs/Hid-Lighting-Apps.pdf

While a higher voltage rated capacitor may last longer, the main problem is still temperature. A higher temperature rating, like the HID caps in the above link, or moving the capacitor to a cooler location are your best options for solving the problem.

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Power-User

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#12
In reply to #11

Re: Start Capacitor Failure

08/06/2011 10:39 PM

You could mount it / thermally couple it to the compressor intake line - which should be sufficiently cool to make a difference.

Test -

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Guru

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#13
In reply to #12

Re: Start Capacitor Failure

08/07/2011 5:10 AM

Bad idea.

If the capacitor, for any reason, blows up (likely possibility), you will also have a damaged copper inlet pipe and gas los etc. Leave it simple and put it anywhere less exposed to extra heat from the hot air being moved out by the fan. That should be enough (otherwise the problem will probably be in the capacitor selection...).

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Power-User

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#14
In reply to #13

Re: Start Capacitor Failure

08/07/2011 9:16 AM

Chances are low that a cap (paper & foil) blowing would penetrate the pipe. Also could put cap in an aluminium clip which would guarantee no pipe damage.

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#15

Re: Start Capacitor Failure

08/07/2011 9:26 AM

Two years ago, we had many failures of the run capacitor on the cooling blowers for one of our product. They had been fairly reliable for many years prior to this.

First reaction was to switch to a higher voltage since the ambiant temperature was reasonable. It didn't help.

We then changed the manufacturer. Same result.

By then, we had accumulated a douzen failures and found a commonality. MADE IN CHINA. We switched to Cornell Dublier made in Mexico and haven't had one fail in a year. http://www.cde.com/catalog/motorrun/ I even placed a few representatives of the three different units in an accelerated test at 20% over voltage. The chinese caps all degraded badly while the mexican's were barely affected.

This is another example that the quality controls in chinese manufacturing has a problem. Those capacitors had been OK for more than ten years. Why the suddent high failure rate from different manufacturers? I tried to contact them but was lost in the maze of distributors and the language barrier.

N.B. We had looked for European or NA made caps. The European were not compatible and no more capacitors seem to be wound in Canada or US. I hope that the Mexican will not offshore to China.

As a manufacturer, we are becoming more and more concerned about the origin of the material we use. We are now in the process of rejecting the few chinese made components we use at the first sign of failure. I also avoid them in my new designs if I have a choice. I tell the sales people "If you want to sell to me, don't offshore production to China". I don't believe that they will listen to me but if enough of their customers say it and back it up with a change of suppliers when it happens, we could reverse the trend. Then, it is up to the rest of us to provide quality components at reasonable price.

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Power-User

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#16
In reply to #15

Re: Start Capacitor Failure

08/07/2011 9:30 AM

Sounds like the computer caps problem all over again.

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#17
In reply to #16

Re: Start Capacitor Failure

08/07/2011 9:59 AM

Yes, It is easy to copy something. They just reproduce the original component in all its details. They later on try to cut cost. Since they don't understand the reasons why it is done that way, they eventually cut into something important and the failures start popping up. That is my simple explanation for this dropping quality.

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#18
In reply to #15

Re: Start Capacitor Failure

08/08/2011 11:32 AM

It took 38 years of use for the 4.7 uF run capacitor for the blower on my A/C unit to fail. It was a GE cap. I vote with Marcot to be choosy about where you buy your caps. These oil-filled foil caps are generally indestructible and live inside the "corners" of my outside unit. These caps regularly see temps well over 130 F. Don't buy the generic $2.39 cap from the local supply house. Go to Grainger or other reputable supply house and get a name brand cap, like Proline (Mex.) or Dayton(China). You can even buy run caps from Digi-Key. And they sell brand names like Cornell-Dubilier.

http://www.cde.com/catalogs/MPF.pdf

http://media.digikey.com/pdf/Data%20Sheets/Epcos%20PDFs/B33364%20Series.pdf

http://media.digikey.com/pdf/Data%20Sheets/Panasonic%20Capacitors%20PDFs/AC%20Film%20Caps.pdf

You will pay 2x the amount of the generic capacitor but it will in all likelihood last A LOT longer.

IMHO, I would go through and replace all the caps with a quality-made capacitor and be done with this nightmare once and for all.

Good luck with your quest.

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Baffled (1); Brave Sir Robin (1); bravo88 (1); LAA_Lucke (1); marcot (2); mjb1962853 (3); Noudge79 (1); rakesh_semwal (1); soebfatehi (2); Stedou73ish (1); Stuart21 (3); tcmtech (1); Zlatkodo (1)

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