Testing (Start/Run) Capacitors

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JFletcher

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Ive been asked to look at some PTAC units that are not running. These are all 208V 1ph 8-9kBTU Carrier and Amana models. There is a start capacitor which I think serves both the fan(s) and the compressor. How do I tell if these are good or not? I found a video on it:

https://www.youtube.com/watch?v=JV7GthtBjlY

Is it that simple? Any other tips when working on them (capacitors)? Havent had much if any need until now to learn about these..
 
Thank you Tom. At first look it was a bit imposing, 3 sets of terminals with 10 wires (5/3/2), but I guess it is easier/cheaper to make the units using the cap as a terminal block instead of using separate blocks with one wire each to the cap. Thanks for the tips on the bulges/leaks.

What values would I look for as far as % within spec? For example, say I'm testing a 40ufd capacitor - what would be the lower and upper maximum range?

Once power is secured, is 45 seconds enough time for them to discharge on their own?

As luck would have it, my current meter lacks a capacitance test function, so I'll be off to get a new one. I think the CL700 Klein will do nicely, has capacitance, amp clamp, frequency, and leads that also test temperature, tho an IR thermometer is pretty inexpensive on its own and can find more things than leads can.
 
40 years ago, we would take a Simpson 260, set it to ohms. When connected to a good capacitor, the needle would fire towards zero and slowly go back towards infinity as it charged. Bad ones were either shorted, open or stabilized at some non-infinite reading. It was a simple qualitative test that worked 95% of the time.
Back then, multimeters did not have capacitance test and a stand alone capacitor tester was expensive and bulky.
 
its technically not ez to test a cap in the field unless one is dragging their testing bench with them.

the basics tests are:
1) not bulged/leaking as mentioned
2) it actually takes a charge
3) then actual F test using a LCR (aka LCZ) meter (remove all wires and short it before testing). one should have a decent/dedicated LCZ meter.

if all 3 come out ok, then you leave it. however, these 3 tests are not indicators as to the health of the cap. the innards could be very dry and ready to short at any sec, etc.

JFletcher said:
What values would I look for as far as % within spec? For example, say I'm testing a 40ufd capacitor - what would be the lower and upper maximum range?

Once power is secured, is 45 seconds enough time for them to discharge on their own?
Click to expand...


hard to say exactly what % deviation is acceptable, this is a design metric of the system. perhaps those types of systems have a generic %, but call manufacturer and ask. there may even be a spec sticker on the unit somewhere.

45min to discharge (with wires still attached)? maybe maybe not. most AC guys (w/o power) simply short it with a screwdriver. this is technically a bad practice. a "short" with a ~1k resistor in series for about 15sec or longer (get a alligator clipped jumper wire and splice in a 1k resistor in series), then followed by a direct short with screwdriver or std alligator clipped is a better method to take that cap down to zero, remove the wires, short it one last time before do tests.
 
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retirede said:
40 years ago, we would take a Simpson 260, set it to ohms. When connected to a good capacitor, the needle would fire towards zero and slowly go back towards infinity as it charged. Bad ones were either shorted, open or stabilized at some non-infinite reading. It was a simple qualitative test that worked 95% of the time.
Back then, multimeters did not have capacitance test and a stand alone capacitor tester was expensive and bulky.
Click to expand...
Yes, that test is fairly effective as most of the time the failure of the capacitor is going to result in shorted or open condition. Meter deflection results in test current flowing from your meter into the capacitor, it eventually "charges" a good capacitor and then the current stops flowing. Reversing test leads will result in a more rapid initial deflection, discharge of the capacitor, then recharging to the opposite polarity, then infinity reading on your meter again.

If you should encounter a capacitor with a charge already stored in it - it is going to be good as far as a test for shorted or open capacitor goes.
 
FionaZuppa said:
its technically not ez to test a cap in the field unless one is dragging their testing bench with them.

the basics tests are:
1) not bulged/leaking as mentioned
2) it actually takes a charge
3) than actual F test using an LCR (aka LCZ) meter (remove all wires and short it before testing). one should have a decent/dedicated LCZ meter.
Click to expand...

I don't know about all that. Bulging caps are sure sign of a bad one, but as far as testing my old Fluke 12 does a fine job.
 
kwired said:
If you should encounter a capacitor with a charge already stored in it - it is going to be good as far as a test for shorted or open capacitor goes.
Click to expand...

A charged cap blew the AGC1 fuse in my Simpson once....from that point on, I always discharged the cap before testing!
 
retirede said:
A charged cap blew the AGC1 fuse in my Simpson once....from that point on, I always discharged the cap before testing!
Click to expand...
But the fact it held a charge should have told you it was not open or short circuited, still a reasonably effective test. :happyyes:
 
kwired said:
If you should encounter a capacitor with a charge already stored in it - it is going to be good as far as a test for shorted or open capacitor goes.
Click to expand...
partially yes. lets say i remove one wire from the cap with my linesman needle nose, and then i measure the voltage on the cap with my nifty/fancy fluke dmm, it reads 92.6v, but 5min later it reads 64.3v. is that a good or bad cap? that 1st v test alone is not a good indicator.

the caps are fairly small F, a good test starting from a 0v cap is to measure v across the cap using 9v batt and say a 1M resistor in series. you should obtain the std charging profile. you should see that same profile if you place that same 1M across the cap terminals while measuring the voltage. this is why a handheld recording scope meter is good to have, you can actually see the profile curve, etc. 9v w/ 1M on 1uF will charge about 5v in about 1s, and nearly 9v @ ~4s. w/ 60Uf 5v is at ~1min, full charge in about 4min.

imho, the fast/quicky in/out tests that hvac folks do, is crap.

https://www.allaboutcircuits.com/tools/capacitor-Charge-and-time-constant-calculator/
 
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If you have a Megger set it to 250V and charge the capacitors. The reading will be low to start with but will climb if the capacitor is good.

PS, discharge it after the test as it will bite you otherwise. You’ll also need a good set of batteries, I got through 40+ testing a PFC capacitor bank.
 
JFletcher said:
... Once power is secured, is 45 seconds enough time for them to discharge on their own? ...
Click to expand...
NO!

"Maybe, maybe not" doesn't cut it when your cardiac rhythm is at risk.

Motor capacitors don't usually have internal discharge resistors and can hold a charge for weeks if they're in good condition.

JFletcher said:
... What values would I look for as far as % within spec? For example, say I'm testing a 40ufd capacitor - what would be the lower and upper maximum range? ...
Click to expand...
99% of the time, they're either OK or open. Motor caps usually have an internal fuse, which is the most-common failure mode. (and a good reason for not discharging them with a screwdriver)

With modern process control, I would expect them to leave the factory within ±5% of spec or better. If all is well -- the line voltage isn't low and the bearings aren't dry, gummed up or worn out -- a cap that's 20% below nominal should work. Greater-than-labeled capacitance is OK for a start cap -- that's precisely what a "Hard-Start Kit" is -- and 50%-100% over should be OK for a run cap.
 
drcampbell said:
NO!

"Maybe, maybe not" doesn't cut it when your cardiac rhythm is at risk.
Click to expand...

so, maybe maybe-not is a valid answer. "NO" is a safe assumption though.
just like i said in the other thread: perhaps yes, and perhaps no, we suspect yes, but we dont know
that is a 50/50 conundrum

if one knows how the ckt's of the system are wired and how they work, one might know that 45min is enough time.
certainly we know that is not the best approach here. a simply $2 meter from HF takes care of that rattler for at least knowing.

its the same thing as "ok to cut this wire? kwired said he flipped the breaker", right? we know kwired, hes a good guy, but we trust our test tools and own eyes more ;)

every apprentice should learn asap, the journeyman or master is not the final word, dipwad master Joe said "cut the wire", but Mikey the apprentice said "hold 1sec, let me see if this wire is still hot". :D
 
kwired said:
But the fact it held a charge should have told you it was not open or short circuited, still a reasonably effective test. :happyyes:
Click to expand...

Yeah, but it's a pain lugging around all those spare meters...
 
gadfly56 said:
Yeah, but it's a pain lugging around all those spare meters...
Click to expand...
pain or dead, i prefer the 1st ;)

hobbled-old-man-with-back-pain.jpg



dead-karnataka_thinkstock_759.jpg
 
FionaZuppa said:
partially yes. lets say i remove one wire from the cap with my linesman needle nose, and then i measure the voltage on the cap with my nifty/fancy fluke dmm, it reads 92.6v, but 5min later it reads 64.3v. is that a good or bad cap? that 1st v test alone is not a good indicator.

the caps are fairly small F, a good test starting from a 0v cap is to measure v across the cap using 9v batt and say a 1M resistor in series. you should obtain the std charging profile. you should see that same profile if you place that same 1M across the cap terminals while measuring the voltage. this is why a handheld recording scope meter is good to have, you can actually see the profile curve, etc. 9v w/ 1M on 1uF will charge about 5v in about 1s, and nearly 9v @ ~4s. w/ 60Uf 5v is at ~1min, full charge in about 4min.

imho, the fast/quicky in/out tests that hvac folks do, is crap.

https://www.allaboutcircuits.com/tools/capacitor-Charge-and-time-constant-calculator/
Click to expand...

Sure, that's what I want is an hvac guy, at a $125 an hour standing around with a nine volt battery, some resistors, and a thousand dollars hand held scope checking out the charging profile of a twelve dollar starting cap.
 
Thanks all for the replies. I read somewhere on the 'net today that simply connecting a DMM across the cap will discharge it since there are caps in the DMM - sound legit? I dont want to short out a good unit with a screwdriver, tho it seems many do it that way, usually with no ill effect on the cap.


ActionDave said:
Sure, that's what I want is an hvac guy, at a $125 an hour standing around with a nine volt battery, some resistors, and a thousand dollars hand held scope checking out the charging profile of a twelve dollar starting cap.
Click to expand...

Haha true. Normally replacing parts willy-nilly is a good way to spend a lot of money w/o fixing the problem, tho given the parts and labor rates here it makes sense to replace it than to test it. I've run into that with switch problems, esp Decora style, I just replace from the rip since they are just a few $ ea.
 
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