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- Placerville, CA, USA
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- Retired PV System Designer
Testing (Start/Run) Capacitors
- Thread starter JFletcher
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kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Exactly, the smaller capacitors for single phase motors are not that expensive to begin with. HVAC guys as well as some of us that have to service such motors frequently usually have a variety of capacitors available. If for some reason you don't have a capacitor tester or setting on a DMM, simple continuity test as well as watch to see that it holds a charge is often all you need. Not very often have I run into one that just simply lost it's rating (a 10 MFD turned into a 7.5) they are either shorted, open or within marked tolerance. On top of that probably 75% or so of the bad ones are physically obvious that they are bad - burned off terminals, bulging case, oil leaked out of them, start capacitors that stayed in the circuit too long just spit their guts out all over the place quite often.
The "fuse" in metal case run capacitors is not an overcurrent device, it is a mechanical fuse link that pulls open when the case expands from too much internal pressure.
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
if he has handheld fluke scope then why would he have 9v and resistors? even those handhelds may not have enough input impedance to diagnose proper cap profile, may still need to add a meg in there
JDB3
Senior Member
- Location
- San Antonio, Texas
Excuse me for being lazy, but I did not read all of the replies. An A/C buddy of mine, shorts them out, then tests them. His rule of thumb is, if they read within 3% of the name plate, they should be fine. Side note, the had installed a name brand cap. and was still having problems with a unit (outside air conditioner (heat pump). The fan would run in different directions at different times. Cap. was defective.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
HID lighting capacitors need to be within 3% or the light may not work properly. Motor capacitors I believe are typically manufactured to 10% tolerances and is one reason you can't use a motor capacitor on HID lighting.
drcampbell
Senior Member
- Location
- The Motor City, Michigan USA
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- Registered Professional Engineer
True, but you'll need to wait a long time; the impedance of a DMM is usually more than 10⁷Ω.
The low-Z function on many meters might not be much help, either. Were it a continuous load, the meter would be dissipating dozens of watts while measuring hundreds of volts. I suspect the circuitry applies the low-Z load briefly, measures the voltage quickly, then removes the low-Z load. (I'm speculating here and would welcome someone with either design knowledge or experimental data chiming in to confirm or deny)
ptonsparky
Tom
- Occupation
- EC - retired
GGM, I hope the OP has some sense in not to use the OHM scale while trying to check for voltage.
The CAPS will have a voltage on the side, use that as the minimum scale setting and it is going to be DC, if it is charged yet. Runs are connected full time to the motors and will bleed off any charge and if there is still a charge, look further down the food chain for the problem.
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
Fluke has dual Z meters, take a look
http://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/electrical/dualimpedance
- Occupation
- Licensed Electrician
My fluke 12 has an automatic low z. It flashes DISC if I'm testing caps and one is holding a.charge.
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