what kills capacitors?

[operator]

What is the main cause of death for ac dual caps, or caps in general?

Found three blistered yesterday, systems in spec

[tecman]

Capacitors deteroriate over time. There are 3 general factors in order of significance:

1. Age
2. Temperature
3. Overvoltage

Any of these factors will accelerate failure. Most failures are a result of the dielectric material (an insulator) in the cap starting to leak some current. This results in internal heating, and either a short circuit in the cap will occur, or the current conduction and heat can sometimes burn out a conduction path in the capacitor, so it becomes an open circuit (no capacitance) or occasionally the excess internal heat causes it to vent (most have a built in vent device) or in a worst case explode.

paul

[operator]

thanks tec, i figured age was the culprit on these but thought id ask the question anyway

[cem-bsee]

voltage spikes

[Swampfox]

Mexico

Ive cut a few of the swollen ones open, many of them the dielectric fluid had hardened up and caused the cap to overheat and swell, GE caps suck...period

[DeltaT]

Two words for me...Mexico and China.

[bb]

And York capacitors in Lennox HS29's. Go figure.

[mark beiser]

Most times I find a bad run cap, the condensor coil is dirty. Heat kills them, plugged up condensor coil causes the unit to run hotter, and the higeher amperage also causes the run cap to generate more heat internally.

All these run caps coming in units these days are very small, wich gives them less surface area to reject heat from, and less mass to heat up during the on cycles, so they run hotter. Toss in any condition that gives them less than ideal ability to cool off and *POP* goes the top.

I find the run caps with the plastic housings to be the most reliable by far as a replacement cap.

Some units with scroll compressors are using 60 and 80 µf run caps that are very small. I like to replace them with two 30µf or two 40µf run caps in parallel.
This splits the heat generation between 2 caps, each of wich is usually physically larger than the origonal, giving much more reliable operation.

I replace far more run capacitors every year than I replace any other single type of part.

[oilman52]

That was my call today. HO called and said AC not working. Run cap looked like it was ready to pop. What run caps do you guys carry in your trucks?

[AllTemp]

Originally posted by oilman52
That was my call today. HO called and said AC not working. Run cap looked like it was ready to pop. What run caps do you guys carry in your trucks?

Planned part replacements...

[Irascible]

Originally posted by oilman52
What run caps do you guys carry in your trucks?One 4, four 5's, two of everything from 7.5 through 60, one 80 and two rather expensive but handy universals. I HATE running to the wholesaler for parts that cost nothing.

[markco]

[i] I like to replace them with two 30µf or two 40µf run caps in parallel.[/B]Don't you mean in series, or have I been doing in wrong.:)

[HVAC/Stud]

You been doing in wrong. Caps in parallel add. NATE question # 56.

STUD

[greenhorngreg]

M BEISLER just commented about using 2 capacitors instead of one when replacing bad caps with hi ratings and little surface area. is this the reason why Trane does this on some of their rooftops. I posted a question couple of days ago bout this but no responses. on one of these caps there is a bleed resistor. curious to find out what thats in there for. bossman says they do this to save money, but don't sound right to me

[mark beiser]

Some Trane and older GE units, use 2 run capacitors, one with a resistor across its terminals. Those units also use a single pole contactor, or if they have a double pole contactor, one pole breaks run to the compressor, the other is for the fan motor, and there is a jumper so that the start winding is powered all the time.

Common to the compressor is energized at all times so that a small current will be running through the start winding at all times to keep it warm. The "trickle current" through the start winding and the resistor on one of the run caps acts as a crankcase heater. Some of the older 1 ton and 1.5 ton condensors even have one of those 3.2A red cap fuses in line with the start winding.
If you replace the run cap, install the resistor on the new cap, or the compressor will have no crankcase heater. Same for the contactor.
You can use a regular 2 pole contactor in place of the relay that is there, but look closely at how the electrical connections work on the existing relay and make sure you wire up the contactor so that everything is electricly the same. Miswiring it will eliminate the crancase heat at best, or let the smoke out of something at worst.

I have seen some older Magic Chef condensors that had a similar setup, but they used a 25/15 dual run cap with a resistor across the 15µf side.

What funney is Trane kept wiring a lot of units the same old way for years fater they stopped using the trickle current for crankcase heat. Just no jumper to power the start winding, or cap with a resistor. I always rewire the contactor like a normal setup when I see one of the units like that. I guess it was simpler to just erase a couple of lines on the wiring diagram than to redraw one from scratch and retrain assemblers, lol. Nothing wrong with the way they were wired, it was just wierd. ;)

[Edited by mark beiser on 06-09-2005 at 10:17 PM]

[greenhorngreg]

thanks for the knowledge MBEISLER, makes perfect sense

[Swampfox]

Originally posted by mark beiser
Some Trane and older GE units, use 2 run capacitors, one with a resistor across its terminals. Those units also use a single pole contactor, or if they have a double pole contactor, one pole breaks run to the compressor, the other is for the fan motor, and there is a jumper so that the start winding is powered all the time.

Common to the compressor is energized at all times so that a small current will be running through the start winding at all times to keep it warm. The "trickle current" through the start winding and the resistor on one of the run caps acts as a crankcase heater. Some of the older 1 ton and 1.5 ton condensors even have one of those 3.2A red cap fuses in line with the start winding.


Lots of those around here, sometimes if they loose all the charge the compressor will run with the contactor open, talk about blowing your mind the first time you see that :eek:

Ive found a lot of those trickle circuit capacitors failed also, Im sure lots of compressors have been condemned as a result, repaired one on a second opinion call once and the homeowner thought I was trying to rip him off because the first tech said the compressor was bad, some people :rolleyes:

[mark beiser]

100% of the 2nd opinion calls I have ever run for a "bad compressor" on a Trane unit turned out to be a bad run cap or a shorted crankcase heater.

Actually, probably better than 90% of the total 2nd opinion calls I have ever run for a "bad compressor" in any brand of unit was really a bad run cap.

I have been told at my favorite supply house that I buy more run capacitors than anyone they know.
The last company I worked for had 12 service techs. I found only a fraction of the "bad compressors" that most of the other techs found, and usually my followups on other techs there turned out to not be a problem with the compressor...

Whats worse, a crook or an honest idiot?

[Mr Bill]

Originally posted by mark beiser
Whats worse, a crook or an honest idiot?

Um how about an honest idiot thats a crook. :D

[bb]

It's amazing how many units I run across that had boosters installed when the problem most likely was that the run caps were open.

[HVAC Pro]

First 3 calls today had bad run caps. One older Trane system had 3 of them. The next two had cases swollen almost to the point of hurling. :eek:

[Irascible]

Originally posted by mark beiser
Whats worse, a crook or an honest idiot?They're one in the same. I contend that the two combined comprise 80% of HVAC tradesmen. But some say I'm too harsh. Go figure.

[phosgene]

the original question mentioned "dual caps", to this I agree, I find more bulged dual caps than single run and starts combined.

[-80guru]

For those of you who stock caps on your trucks, stock the higher voltage caps the 460s. They can be used in place of the 370s and are more durable. This way you don,t have to stock both voltage run caps.

[nc hvacman]

I think that a lot of the problems is that the dielectric fluid is not as good as the PCB filled ones that used to be used. They lasted forever. But, gave rats cancer so its NOT used anymore. Kind of like how termites USED to be killed easily with I can't think of the name of it but its illegal now to. We do have some sacrifices in the name of safety but who doesn't enjoy seeing that exploded or swelled capacitor when you got to a NO COOL call. Thats some easy money in my book.

[SandShark]

...that I signed on specifically to do a search for "chinese" or "China" because of the last "call-back" I just went on; another leaking and swollen "Made in China" run capacitor. My boss changed a condenser fan motor a month or so ago and installed the capacitor, even though I've been harping on this issue for a year or more. I understand all of the factors that would cause a run capacitor to go "bad", but when a vast majority of capacitor problems I've seen in the last couple of years have "Made in China" on the label, I start to wonder. Sure, other "Made in ______" capacitors fail, but I'd venture to say a large majority of capacitor related calls I've been on in the last year or more have a connection to China.

[Yuma]

Originally posted by markco

[i] I like to replace them with two 30µf or two 40µf run caps in parallel.Don't you mean in series, or have I been doing in wrong.:) [/B]

In parallel you add MFD+MFD= Total of both.
In series you go MFDxMFD÷MFD+MFD= lower total
So you can wire them both ways, just what do you want to do?


Yuma,

[markco]

Originally posted by Yuma

Originally posted by markco

[i] I like to replace them with two 30µf or two 40µf run caps in parallel.Don't you mean in series, or have I been doing in wrong.:)

In parallel you add MFD+MFD= Total of both.
In series you go MFDxMFD÷MFD+MFD= lower total
So you can wire them both ways, just what do you want to do?


Yuma,
[/B]I have been doing it MFD+MFD, but to me, that is wiring in series. As an example,if you wire a 10 ohm resistor end to end with another 10 ohm resistor you get 20 ohms. That is wiring in series. If you wire the resistor leads together pigtailed and measure the resistance, you get 5 ohms. This is parallel. Unless they have changed the laws of physics since I went to school for electronics.

[mark beiser]

The rule for adding capacitance is the opposite of the rule for adding resistance.

Resistors add in series, capacitors add in prallel.

[dan wong]

Originally posted by Irascible

Originally posted by mark beiser
Whats worse, a crook or an honest idiot?They're one in the same. I contend that the two combined comprise 80% of HVAC tradesmen. But some say I'm too harsh. Go figure.

I have to agreed with you, I wish it was different.

[dan wong]

Originally posted by SandShark
...that I signed on specifically to do a search for "chinese" or "China" because of the last "call-back" I just went on; another leaking and swollen "Made in China" run capacitor. My boss changed a condenser fan motor a month or so ago and installed the capacitor, even though I've been harping on this issue for a year or more. I understand all of the factors that would cause a run capacitor to go "bad", but when a vast majority of capacitor problems I've seen in the last couple of years have "Made in China" on the label, I start to wonder. Sure, other "Made in ______" capacitors fail, but I'd venture to say a large majority of capacitor related calls I've been on in the last year or more have a connection to China.

I am having the same problem here ( both made in China and Mexico). I can't seen to find one made in USA any more, do they still make them here in the USA? I am willing to pay a higer price for quality part.

[deanoac]

Please let's not forget about reason #4 tech's using their mfd meters to determine the capactiors reliability. Thank GOD I have finally that all my tech's to use the formulas to check caps, it has saved my a ton of recalls.

[zzyzzx]

What formula do you use to check a capacitor? I thought the way you checked them was to charge them up with a DC power Supply, then discharge (with an old screwdriver shorting across the terminals).

[deanoac]

MFD = AMPS/VOLTS x 2653 IN OVER 20 PLUS YEARS THIS HAS NEVER FAILED ME. THIS TESTS THE RUN CAPACITORS UNDER A TRUE LOAD NOT JUST WITH A 9V BATTERY THAT COULD BE WEAK IN A METER. AND YOU DONT HAVE TO DISCONNECT ANY WIRES AND IT CAN BE DONE WHILE UNIT IS RUNNING.

[Irascible]

Sheesh. Such a simple formula and this is the first I've heard of it. I'm hanging my head low in shame. :^)

So the voltage and amperage is being measured where exactly?

[danj]

Amps of the start winding---Voltage --back EMF-- between start and run.