# Testing Motor Capacitors



## Big John (May 23, 2010)

Just curious if anyone has an accurate test for motor capacitors.

I had a motor recently that was acting for all the world like it had a bad cap. because it was struggling with startup torque. But when I tested the cap. with my Fluke it read the proper capacitance, and I had many megaohms of resistance between the terminals, so from my perspective the capacitor looked good.

I couldn't find anything else wrong, so I replaced the cap. and it solved the problem. Just curious if there's a more reliable way to test them?

-John


----------



## Wirenuting (Sep 12, 2010)

I have a capacitor tester that works great. About $80 in todays prices. Free for me as an old AC guy gave it to me when he retired a few years ago.

It's a Supco brand. 0-10 & 10-10,000 ranges.


----------



## 76nemo (Aug 13, 2008)

The capacitance function of a DMM won't tell you squat about the condition of the cap itself. Even if it reads close to it's correct value, it can still drift and fail under load.

If you have another cap of nearly the same value you can charge them with a megger and compare the time it takes for them to charge.

If the suspect cap charges VERY quickly, then it's most likely open internally. If the readings shoot up and down and travel all over the place, the cap is arcing internally. If it's shorted than................

Use a range closest to the caps rating. Any megger would do, but an analog is alot easier to witness if the cap is arcing.

So again, if ya' think your DMM will tell you much of anything about the quality of life for a cap,................well, good luck, but it won't be yes or no


----------



## Big John (May 23, 2010)

76nemo said:


> ...So again, if ya' think your DMM will tell you much of anything about the quality of life for a cap... well, good luck, but it won't be yes or no


 I kinda figured the DMM just didn't have the nuts to be able to test it properly, sorta like trying to check insulation resistance with an ohm meter. I thought about megging it, but it was a 120V cap and my megger doesn't go that low.

It's pretty rare that I play with motor caps, so I don't really want to shell out for a dedicated meter, I was just wondering if there was any other tricks to testing them. 

-John


----------



## Electric_Light (Apr 6, 2010)

You can assume line is 60.0Hz. 
Measure the voltage across the line. 
Then measure the current when placed across the line. 

With known E, F and C values(as stated on capacitor), you can calculate the I. 

If the measured current after factoring in capacity tolerance isn't even close to calculated current, then you know its bad.


----------



## Big John (May 23, 2010)

Electric_Light said:


> You can assume line is 60.0Hz.
> Measure the voltage across the line.
> Then measure the current when placed across the line.
> 
> ...


 So if I understand you correctly: When attempting to start the motor, I'd measure the voltage across the capacitor, simultaneously measure the current through the capacitor, then figure the reactance, and then use that to figure the actual capacitance, and compare it to the name plate?

Man. Maybe I will buy that capacitor tester... :jester:

-John


----------



## HARRY304E (Sep 15, 2010)

Big John said:


> So if I understand you correctly: When attempting to start the motor, I'd measure the voltage across the capacitor, simultaneously measure the current through the capacitor, then figure the reactance, and then use that to figure the actual capacitance, and compare it to the name plate?
> 
> Man. Maybe I will buy that capacitor tester... :jester:
> 
> -John



Take a look at this John..


----------



## 76nemo (Aug 13, 2008)

HARRY304E said:


> Take a look at this John..


 

B-U-L-L-S-H-*-T!!!!!!!!!! U-T-T-E-R BS:jester:


John already did that Harry. A capacitance meter or the capacitance function of a DMM will identify an unknown cap, and not much else. If you're talking about properly measuring electrolytics of that size, than the proper meter is called an ESR meter, (equivalent series resistance). That can show if a cap is leaky, they're fail safe, but big money. They can test in circuit so there is no need to remove the cap in the first place. I thought you were speaking of testing much larger capacitors.


http://en.wikipedia.org/wiki/ESR_meter



That guy and his Fieldpiece can take a hike. (Sorry Roadhouse)


----------



## Electric_Light (Apr 6, 2010)

Big John said:


> So if I understand you correctly: When attempting to start the motor, I'd measure the voltage across the capacitor, simultaneously measure the current through the capacitor, then figure the reactance, and then use that to figure the actual capacitance, and compare it to the name plate?
> 
> Man. Maybe I will buy that capacitor tester... :jester:
> 
> -John


The whole point is that just because DMM reports within a range, its not an automatic go. The video someone posted in response to you, simply shows how to use the DMM.

When you know the capacitance and frequency (60.00Hz), the impedance is known. http://www.allaboutcircuits.com/vol_2/chpt_4/2.html

If the current going through capacitor at measured voltage @ 60.00 Hz doesn't correspond to what it should be for the rated capacitance, something is wrong. This is an actual load test unlike DMM test.


----------



## Big John (May 23, 2010)

Well, it turns out this is all for nothing anyway:

Motor started acting up again today even with the new capacitor, so I probably got a correct reading because the old cap was actually good.

It's a reversing single-phase motor and sometimes even with no load, it won't start, it'll just hum. If you spin the shaft, it'll run like a champ. 

Not too familiar with single phase motors. Gonna have to do a little reading.

-John


----------



## Electric_Light (Apr 6, 2010)

Big John said:


> Well, it turns out this is all for nothing anyway:
> 
> Motor started acting up again today even with the new capacitor, so I probably got a correct reading because the old cap was actually good.
> 
> ...


Could be a failing centrifugal switch that isn't closing. The start winding is normally connected, then once the shaft reaches speed, it cuts it out. 
No start without load indicates starting winding isn't getting power which could be intermittent capacitor, intermittent start winding or centrifugal switch issue.


----------



## Big John (May 23, 2010)

Electric_Light said:


> Could be a failing centrifugal switch that isn't closing....


 That's the only other thing I could think of, because I'm still convinced that capacitor isn't doing the job it's supposed to. Since the damn thing don't work regardless, I have nothing to lose by yanking off the end bell tomorrow. Maybe the switch just needs grease?

-John


----------



## HARRY304E (Sep 15, 2010)

76nemo said:


> B-U-L-L-S-H-*-T!!!!!!!!!! U-T-T-E-R BS:jester:
> 
> 
> John already did that Harry. A capacitance meter or the capacitance function of a DMM will identify an unknown cap, and not much else. If you're talking about properly measuring electrolytics of that size, than the proper meter is called an ESR meter, (equivalent series resistance). That can show if a cap is leaky, they're fail safe, but big money. They can test in circuit so there is no need to remove the cap in the first place. I thought you were speaking of testing much larger capacitors.
> ...


Thanks 76nemo I was just trying to help out the OP..

Next time there is a thread about all the bikering that you were crying about..read your post over and over again.Here is a copy. http://www.electriciantalk.com/f28/testing-motor-capacitors-22260/#post411029..:thumbsup:.."Hypocrisy".It's ok when you do it.....:laughing::laughing:


----------



## 76nemo (Aug 13, 2008)

HARRY304E said:


> Thanks 76nemo I was just trying to help out the OP..
> 
> Next time there is a thread about all the bikering that you were crying about..read your post over and over again.Here is a copy. http://www.electriciantalk.com/f28/testing-motor-capacitors-22260/#post411029..:thumbsup:.."Hypocrisy".It's ok when you do it.....:laughing::laughing:


Then post something useful on the subject Harry. I wasn't knocking you, nor ever would. Don't jump on my back. A DMM won't show you squat about the quality of life for a cap.

Don't take me for one of these people who like to argue. I was trying to be helpful as well. Cut me a little slack......

I'm not an azzhole, I was just stating what I have learned first hand. 

Even testing caps viewing Lissajous figures in sweep mode proves nothing, I use to rely on that, then I was proven wrong.

Don't assume me to be an azz on auto mode.

I wasn't bucking you, I was slamming the video you posted a link for. Towards the end of thee video, I believe he states if the value measured is close to the caps value, than it is probably good. That is utter bologna.

I didn't say you stated it, so ease up a little........


----------



## HARRY304E (Sep 15, 2010)

76nemo said:


> Then post something useful on the subject Harry.


 
Thanks 76nemo....

The fact is i felt it was useful how about that..:thumbsup:

If you know somthing about the subject then thats good by me and go right ahead and help the "OP"

You don't need to slam me or roadhouse to take part in the thread..
In fact Roadhouse does not have one post on this thread .
So what are you slaming him for...:no: 
What do you think no one is going to call you out for your pompous know it all attitude sorry but that is how you came accross..

And you don't need to use cuss words to prove your point that is unprofessional:thumbup:


----------



## Roadhouse (Oct 16, 2010)

76nemo said:


> B-U-L-L-S-H-*-T!!!!!!!!!! U-T-T-E-R BS:jester:
> 
> 
> John already did that Harry. A capacitance meter or the capacitance function of a DMM will identify an unknown cap, and not much else. If you're talking about properly measuring electrolytics of that size, than the proper meter is called an ESR meter, (equivalent series resistance). That can show if a cap is leaky, they're fail safe, but big money. They can test in circuit so there is no need to remove the cap in the first place. I thought you were speaking of testing much larger capacitors.
> ...


 
Harry, I appreciate the back, seriously, that's pretty damn stand up-ish of you, but I don't think Nemo was literally bashing me, he was just addressing me because I'm an hvac tech and use Fieldpiece, why nemo said sorry to me. 


I hope that that is right anyways. If not then someone really has some issues. :laughing:

It's all good guys, no need to argue.

I for one have never come across a cap that tested good and wasn't good though so this topic is pretty damn interesting.


----------



## Jlarson (Jun 28, 2009)

An ESR meter is on my list to add to my personal test equipment, as if I need more test equipment. 

My usual MO is to test with a DMM and if nothing else looks wrong I replace the cap even if the DMM says it ok.


----------



## HARRY304E (Sep 15, 2010)

Jlarson said:


> An ESR meter is on my list to add to my personal test equipment, as if I need more test equipment.
> 
> My usual MO is to test with a DMM and if nothing else looks wrong I replace the cap even if the DMM says it ok.


When are you going to post all your test equipment on the tool thread..:thumbup:


----------



## Jlarson (Jun 28, 2009)

HARRY304E said:


> When are you going to post all your test equipment on the tool thread..:thumbup:


I don't have years of spare time to collect it from all its various locations and photograph it all :laughing:


----------



## HARRY304E (Sep 15, 2010)

Jlarson said:


> I don't have years of spare time to collect it from all its various locations and photograph it all :laughing:


Same here..:laughing:


----------



## retiredsparktech (Mar 8, 2011)

Big John said:


> Well, it turns out this is all for nothing anyway:
> 
> Motor started acting up again today even with the new capacitor, so I probably got a correct reading because the old cap was actually good.
> 
> ...


Nothing was mentioned, regarding the motor's application. You'll probably have to take to motor apart and inspect the start contacts and the centrifical device.


----------



## oldtimer (Jun 10, 2010)

retiredsparktech said:


> Nothing was mentioned, regarding the motor's application. You'll probably have to take to motor apart and inspect the start contacts and the centrifical device.


 If it has a centrifugal switch, possibly the contacts are not making.

I have cleaned the contacts, and that has worked.

Sometimes you have to make sure the switch is not binding on the shaft.


----------

