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Time to change that Run Capacitor!
This has been a topic of debate in our shop (and others that I've worked over the past twenty years). When is the time you replace a run capacitor? The past couple of years have been very hard on run capacitors here in North East Texas. I have been very deliberate about checking every capacitor I come across. I carry a large stock on my truck as well as the Turbo's so having the part to replace is usually not an issue. But when do y'all suggest to a customer that today is the day to replace that worn cap? I have seen tolerances from 5% to 20% from the manufacturer. I will usually let the residential 3mfd through 80mfd go to 10% unless specified for more. The commercial ones I maintain the manufacturers spec because the business just wants to keep the equipment running and is less likely to argue aver a small dollar part during an inspection or service call. I have guys in my shop who will wait until 50-80% or even total failure. I don't understand their mentality on this, but I wonder what you guys are doing.
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I go no more then 10%.
I don't use 370vac anymore. Strictly 440
If they are leaking, they get replaced.
I also check caps under load. (start winding amps) x 2650 / voltage read at capacitor
http://web.fscj.edu/Mark.Bowman/hand...der%20Load.pdf
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I have been 10% as well, beats the heck out if doing a summer check and going back in a month for a bad run cap
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Originally Posted by
gravity
Same here.
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Considering they are all junk now I stick tightly to the 10% tolerance and give the customer the option with a dose of do it now or pay me another service call and capacitor later . I keep a huge stock on my van but it never fails I will always get caught in the sticks or after hours lacking that one odd ball cap that I might otherwise not use twice in a year. Only keep one turbo cap on the truck and don't even offer it unless it's a business or deep pocket client. Which reminds me I don't have one on the truck right now lol.
"I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it."
Benjamin Franklin, 1766
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In replacing motors there's always be a capacitor in it as per designed,i based my multiplier / tolerance on its marking for refference,its just me anyway.
Any other techniques and approaches to the costumers i should learn from you hardcore and experience guys is highly appreciated,don't make hard on me i'm still learning on the trade.
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Ditto on the 10%, and checking them every time I'm working on a unit.
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Originally Posted by
onnuke84
In replacing motors there's always be a capacitor in it as per designed,i based my multiplier / tolerance on its marking for refference,its just me anyway.
Any other techniques and approaches to the costumers i should learn from you hardcore and experience guys is highly appreciated,don't make hard on me i'm still learning on the trade.
So are you saying that a capacitor listed as being within 6% of it's rating is bad if you measure it to be over 6% lower than its rating? If that isn't what you meant then you can disregard the following. If it is what you meant, or if any readers use the listed tolerace as a guide, then the following is for you/them.
If your net meter error is 3-5%, then you may be replacing perfectly good capacitors using those rules. Let's look at the Fluke 902, a popular meter among hvac techs. The stated accuracy of the capacitance function is 1.9% plus 2 counts. The resolution from 0 to 100 mfd is listed as 0.1 mfd. So let's break this down. Suppose you're checking a 10 mfd blower motor run capacitor with a listed tolerance of +/- 6%. According to the manufacturer of the capacitor the true mfd value of the capacitor can range between 9.4 and 10.6 mfd (+/- 6%) and be GOOD. So let's say this particular capacitor has an actual value of 9.8 mfd, which I think you'll agree is a GOOD capacitor. You come along and test it with your Fluke 902 that happens to be running right at the low end of it's specified accuracy range, that is, it's reading 1.9% plus 2 counts low compared to NIST standards. Well 1.9% of 9.8 is 0.1862. That means that it'll read that capacitor as having a value of 9.6. But wait, what about the "2 counts" mentioned. Well that means that the last digit can be off by as much as 2 in addition to the percentage error, so the meter in this case is actually going to register a value of 9.4 mfd. That's 6% under the nominal value (almost all meter error BTW), so with your rule of thumb you would consider that capacitor to be borderline. If it's actual value was even slightly lower than 9.8 mfd, which would still be well within specs, then you would measure it as being more than 6% under it's nominal rating, and you would condemn that capacitor as reading too low even though it is GOOD according to the manufacturer. Keep in mind that a weak battery or a worn meter can throw that reading off even further, and also keep in mind that some meters have even worse specs than the Fluke 902.
This doesn't mean that your particular meter is that inaccurate, but unless you know your meter is super accurate you should treat it as though it is not. There's a whole science devoted to this subject. It's called metrology.
http://en.wikipedia.org/wiki/Metrology
If every Fluke came off the line with better specs than listed in the manual, then the specs listed would be tighter than the ones listed. But they aren't, so that's telling you that there are indeed Fluke 902's out there that will be measuring anywhere within that accuracy range. (And don't let the percentage in the accuracy specs fool you. When you take into account the "counts" or "digits" the actual percentage error can be quite high when reading lower values) There are even a small number of meters that are allowed to be shipped that fall outside the listed specs, so don't assume that you accidentally got a more perfect meter than the next guy. You'd just be guessing. Allow 10% error on your mfd readings. In most cases this will cover both the allowable capacitor manufacturer's error and possible meter error.
+/- 10% is also an (ac motor) industry standard for run capacitors.
Last edited by hvacrmedic; 06-03-2013 at 12:50 AM.
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Nice explanation hvacrmedic, I agree I have always ignored the 6% and gone by strictly a 10% rule no matter what cap says.
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These points and facts you post is an added knowledge to me and a big contribution to my future excellence in this trade,i'm a sponge who needs more water to absorb.
Thanks,
Rod
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Originally Posted by
onnuke84
These points and facts you post is an added knowledge to me and a big contribution to my future excellence in this trade,i'm a sponge who needs more water to absorb.
Thanks,
Rod
You're very welcome. But wait, there's more. If you run a live test on a capacitor, using the formula
mfd = amps x 2653/ volts, then there are two components of meter error to account for. The voltage and amperage error bars must be added together to get the net possible error in your reading. Using the Fluke 902 again, the amperage range has specs of 2% plus 5 counts. That's already worse than the mfd function specs, much worse. Let's assume that the actual voltage across the capacitor is 352 volts and the actual current through the start winding is 1.3 amps. Using the formula this would give us a capacitance of 9.8 mfd as before. Now let's account for meter error. The voltage range has specs of 1% plus 5 counts. Resolution is 0.1 in both amperage and voltage ranges. Applying the maximum error in those specs we could potentially measure/calculate a mfd value using the Fluke 902 of
mfd = .8 x 2653 / 356
= 5.96 mfd = REALLY BAD
So make sure your actual results using this method are comparable to the reading using a capacitance test function on a good DMM before deciding to run live tests on your capacitors on a regular basis. Preferably your amp clamp will have a low amp range with a resolution of at least 0.01 amps. The high resolution pretty much eliminates the "count" portion of the meter error from your calculation. A 0.05 amp error won't significantly alter the results of the calculation. If the 902 had a low amp range with resolution 0.01 amp, and the same 2% uncertainty plus 5 counts, then the results of the calculation, worst case, would have been
mfd = 1.22 x 2653 / 356
= 9.09 mfd, which is less than 10% under, or GOOD. Huge difference due to meter selection alone. The Fluke 322, which does have a low amp range and better accuracy than the 902 (The 322 specs at 1.8%), would be a better meter for running the live capacitor test.
As you might have guessed, I'm not a big fan of the Fluke 902. It has poor specs all around, and limited functionality for hvac work. But I digress.
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Ok, not that it makes a significant difference, but this makes the 3rd different number I've seen used for the constant in that capacitor formula.
Previously I've seen 2654 and 2652, now hvacmedic posts 2653!
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Originally Posted by
mark beiser
Ok, not that it makes a significant difference, but this makes the 3rd different number I've seen used for the constant in that capacitor formula.
Previously I've seen 2654 and 2652, now hvacmedic posts 2653!
Both hvacmedic and Mgenious have discussed how this formula is derived in previous threads. Trying to find those old threads is a pain, so i will attempt to recite what they said.
The formula "Amps x 2653 / Volts" is derived from ohms law.
E = I * Xc
Capacitive Reactance (Xc) is calculated like this:
Xc = 1 / (2 x pi x hz x farad)
Xc = 1 / (2 x 3.1415 (pi) x 60 (hz) x farad)
Xc = 0.0026527 x farad
farad = 1,000,000 microfarads
So 0.0026527 must be multiplied by 1,000,000
Xc = 0.0026527 ✕ 1,000,000
Xc = 2652.7
E = I x 2653
Formula :
amps x 2653 / volts
example - 4.8 amps x 2653 / 363 volts = 35.08 mfd
Changing the 2653 constant to 2652 or 2654 is insignificant, as it will only change the resulting mfd calculation by a 100th of a mfd (35.07 and 35.09 mfd respectively).
Instead of learning the tricks of the trade, learn the trade.
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Originally Posted by
mark beiser
Ditto on the 10%, and checking them every time I'm working on a unit.
Mark, were are yaw finding that there is a 10% tolerance allowed by any mfg. on motors or compressors? I have never seen or read this anywhere. I have read were the -10% or +10% is the tolerance rating you might have when you buy a new capacitor, it's right on the side of most caps., in other words a 50 mfd new could be a 45mfd or 55mfd right off the shelves.
__________________________________________________ _______________________
“Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law
"It's what you learn after you know it all that counts." ~ John Wooden
"When the teachers become unteachable we're all in trouble" ~ Mr. Bill
"Remember "Pro" is only a name, it's not always a mindset determined to do everything correctly" ~ Mr. Bill
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nevermind already got answered.
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Originally Posted by
hvacrmedic
+/- 10% is also an (ac motor) industry standard for run capacitors.
The couple motor manufactures I have talked to in the past, say they engineer their motors around a +/- 6% capacitor.
Of course neither could provide me a publication that stated that fact.
Instead of learning the tricks of the trade, learn the trade.
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Originally Posted by
rundawg
Of course neither could provide me a publication that stated that fact.
See, that's what I am talking about.
__________________________________________________ _______________________
“Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law
"It's what you learn after you know it all that counts." ~ John Wooden
"When the teachers become unteachable we're all in trouble" ~ Mr. Bill
"Remember "Pro" is only a name, it's not always a mindset determined to do everything correctly" ~ Mr. Bill
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Originally Posted by
hvacrmedic
+/- 10% is also an (ac motor) industry standard for run capacitors.
If you could get a fact sheet on this, I seriously would love to have one.
__________________________________________________ _______________________
“Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law
"It's what you learn after you know it all that counts." ~ John Wooden
"When the teachers become unteachable we're all in trouble" ~ Mr. Bill
"Remember "Pro" is only a name, it's not always a mindset determined to do everything correctly" ~ Mr. Bill
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I do the 6% if it states that, or I go 8% for general checking.
A meter 2% off could mean a cap reading 5% could actually be 7%- or it could be 3%, but caps aren't that expensive and a weak cap is a slow killer.
I check new caps before I install them and most are within 1.5%, so I know I'm close.
I check them all. I've had blowers running fine, yet the cap tested, for example, 4mfd when it was a 7.5mfd. Go figure.
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Originally Posted by
Mr Bill
Mark, were are yaw finding that there is a 10% tolerance allowed by any mfg. on motors or compressors? I have never seen or read this anywhere. I have read were the -10% or +10% is the tolerance rating you might have when you buy a new capacitor, it's right on the side of most caps., in other words a 50 mfd new could be a 45mfd or 55mfd right off the shelves.
From Carrier Catalog No.24-25-3SM, page 12
"2. Capacitance testers are available which will read value of capacitor. If value is not within ±10 percent value stated on capacitor, it should be replaced. If capacitor is not open or shorted, the capacitance value is calculated by measuring voltage across capacitor and current it draws. Use following formula to calculate capacitance: Capacitance (mfd)= (2650 X amps)/volts
3. Remove any capacitor that shows signs of bulging, dents, or leaking. Do not apply power to a defective capacitor as it may explode."
Last edited by hvacrmedic; 06-03-2013 at 11:39 AM.