AC Motor Run Capacitor Troubleshooting

[pizza]

i'm troubleshooting a dishwasher, and i'm wondering how to test the circulation pump run cap.

physically/visually, it's in good condition.

my fluke 289 reads correct capacitance (value within spec... 13.9uF on a 14uF±5%).

is there any possible way this cap could be bad?
it's 18 years old, so maybe i should just preemptively replace it anyways. but i'm curious what the correct procedure is.

what's the actual, authoritative test? do i have to hook it up to a scope and measure its characteristics under real load? lol

the pump runs, but i'm getting indications that it's possibly not performing as well as it needs to sometimes.

 

[Jim greengo]

Anything is possible,how hard is it to get at the cap?

 

[pizza]

easy to get at the cap. i already looked at it and tested it (to the extent i know how).

i'll replace it... just curious if there's a more correct, in-depth way to test them.

 

[dogdog]

I don't think it's the problem with your cap though.

 

[Jim greengo]

easy to get at the cap. i already looked at it and tested it (to the extent i know how).

i'll replace it... just curious if there's a more correct, in-depth way to test them.

That's the way I check them.

 

[nadogail]

I use an ohmmeter and just check to see that it’s not shorted and appears to charge.

Beyond that I can’t really test a capacitor.

 

[Wrench97]

If the fluke meter read the correct capacitance value why do you think it's bad?

 

[PoorUB]

If the fluke meter read the correct capacitance value why do you think it's bad?

Exactly! I have tested hundreds of capacitors over the years. IMO, they are good, or bad, there is not much in between.

 

[pizza]

I don't think it's the problem with your cap though.

that makes sense. i guess i'd need to refer to the datasheet and either use a purpose-built instrument like that or build a test circuit and hook it up to a scope. or scope it under real operation in the device.

not quite worth the effort though when a replacement is $10-$35, but maybe i'll look at it for fun.

also, i don't actually think it's bad. i was just looking for confirmation.

 

[BukitCase]

theoretical troubleshooting and wishful thinking is ALWAYS more fun than biting the bullet and finding the partial plugging (AKA "gunk") you'd rather NOT hafta get into ... Steve

Sorry, couldn't resist...

Seriously tho, in almost ANYTHING that has plumbing, some types of "gunk" (like leaves, partially consumed hot dogs, etc) I've seen this type of stuff act like a check valve in one direction and be almost perfect in the opposite direction...

 

[pizza]

theoretical troubleshooting and wishful thinking is ALWAYS more fun than biting the bullet and finding the partial plugging (AKA "gunk") you'd rather NOT hafta get into ... Steve

Sorry, couldn't resist...

Seriously tho, in almost ANYTHING that has plumbing, some types of "gunk" (like leaves, partially consumed hot dogs, etc) I've seen this type of stuff act like a check valve in one direction and be almost perfect in the opposite direction...

yeah. aside from the issue of authoritatively determining a run cap's condition...

i do want to take the pump apart and inspect the impeller and stuff, but a new gasket for it is 75 bucks from miele (only sold as a kit that includes a new impeller and stuff), and i can't find another source. the pump's 18 years old. i hesitate to take it apart and get it leaking. new pump is $550.

 

[fitter30]

yeah. aside from the issue of authoritatively determining a run cap's condition...

i do want to take the pump apart and inspect the impeller and stuff, but a new gasket for it is 75 bucks from miele (only sold as a kit that includes a new impeller and stuff), and i can't find another source. the pump's 18 years old. i hesitate to take it apart and get it leaking. new pump is $550.

Have purchased parts from the UK for a clothes washers from ebay. https://www.google.com/url?q=https:...0QFnoECAUQAg&usg=AOvVaw0mkhF24xcjoCqjZn1mbpNZ
Purchased a fill valve told me it was 50 cycles came it was 50/60 cycles. Was going to take the chance anyway for a simple solenoid valve.

 

[pizza]

lol @ that link...

aye, i have checked ebay.

 

[bonneyman]

I didn't check run capacitors to see if they were serviceable per se. I tested them as assurance for the expensive motor they were running. A weak capacitor puts electrical strain on the motor windings, and a simple check and replacement of a borderline cap is alot cheaper than a motor or compressor.

I'd do a charge/no charge test as has been described, and then I'd do a static microfarad check with a good multimeter. If those two steps panned out I'd do an operating capacitance check. Started finding alot of weak caps under load that tested fine when unpowered
The formula used is measured amp draw times 2650, then divided by applied voltage. (More recent calcs use starting amp draw and 2652 for the constant)

Testing the Run Capacitor While the System is Running

Many companies are beginning to integrate capacitor testing into their regular diagnoses and maintenance practices, and I applaud this; however, it does lead to a debate on how to do it.

www.achrnews.com

Here's a simpler diagram on how to do it. You can make a copy, laminate it, and carry it in your toolbox till you get comfortable doing it.

https://hvacrschool.com/wp-content/uploads/2017/06/How-To-Test-a-Run-Capacitor-Under-Load-1.pdf

 

[dogdog]

that makes sense. i guess i'd need to refer to the datasheet and either use a purpose-built instrument like that or build a test circuit and hook it up to a scope. or scope it under real operation in the device.

not quite worth the effort though when a replacement is $10-$35, but maybe i'll look at it for fun.

also, i don't actually think it's bad. i was just looking for confirmation.

I have the cheap $10 hobby LCR measuring from eBay works fine. EEVblog guy did a review on it. You can also do it with an oscilloscope, a freq generator and a know resistance setup. Plenty of ham guy can explain that to you. That was a popular topic when that Samsung g tv caps were massive recall. Measuring the ESR.

As far as the other methods. Your motor have to be running.

Your problem might be something simple like a micro switch on the bowl float or what ever that water thing is. I have that problem when it is defective, prevented the water from being drained on the dish washer.

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Something like these with case is $20 worth it if you do lots of cap resistor inductor testing at hobby scale.

 

[pizza]

that's nifty. if i get a hobby grade lcr meter, i'll probably get one of the tweezer types for ease of in-circuit testing. i have a lot of old test equipment to troubleshoot. lots of bad caps i'm sure.

 

[Innovate1]

To avoid taking the pump apart can you hook up a hose and back flush it? I would keep the flow fairly low as it's not designed for a lot of pressure. But I think many of them somehow reverse flow normally to avoid plugging - not sure how that works. If the motor runs and the cap measures the proper value you have pretty well checked that it's not the cap. I have seen cases where the cap value is low and the motor slows down but if the cap is the right value I think it's good. Can you get at the rotor to check how freely it spins? Many appliance motors are open frame. Might be a bad bearing.