What are failure characteristics of an open run capacitor when the breaker is tripping?

[NickJoh]

In trouble shooting the breaker tripping of a 2 HP 230 volt single phase jet pump motor on our irrigation pump I discovered the "run" capacitor is "open". Is that likely to mean a winding shorted and destroyed the run capacitor? Or could the open run capacitor cause the breaker to trip?

A little more info: When the relay applies the 230 volts to the motor the sound from the motor is a buzzing sound for a second or two before the breaker trips. It does not sound as if the motor is trying to spin at all even though the start capacitor checks out fine.

While I am an engineer I have very little experience in this area and don't want to replace the run capacitor if it makes no sense.

My other problem (but only if you tell me replacing the run capacitor is worth a shot) is this run capacitor is so old and discolored I can't tell what its value was. I realize they should be the value originally specified but if I can 't find that value is there a range that "should" work for a 2 HP 230 volt single phase motor?

 

[NickJoh]

Its probably not relevant but it is a Burks 20WA6 pump with a 2 HP Magnetek Century jet pump duty motor. Also, the pump is still installed so my observation that it makes no sound of attempting to spin even though the start capacitor is fine might be because the pump is still under load.

 

[hbiss]

I realize they should be the value originally specified

It doesn't need to be exact, -2+10% or so of the specified value should be OK. AND, while you are at it, replace the start capacitor too!

How did you test the run capacitor to see if it was open? Does your meter have a capacitance function?

-Hal

 

[NickJoh]

Thanks. I used an EXTEXH EX655 's capacitance scale and got 138 micro farads several times in a row for the start capacitor (which I had suspected was bad) but when I checked the run capacitor I couldn't get anything. To be sure I sanded its terminals to guarantee I was making contact. No difference. I switched to its resistance scale and got the same reading (0L) regardless of whether I touched the leads to the sanded terminals or left them to the open air! Not what I expected.

Do you think the open run capacitor could cause the circuit breaker to trip or is it more likely a major motor failure (like a shorted winding) caused the run capacitor to open? It trips the breaker in a second or two.


aeding

 

[NickJoh]

I even repeated the capacitance check after that and waited a LONG time and nothing on the display ever changed

 

[Eddie702]

For sure if the run cap is bad or open the motor may fail to start

 

[NickJoh]

Are the other things I described consistent with that? Like the breaker tripping within 2 seconds with only a buzzing sound fro the motor for the two seconds.

If so, I should try replacing the run capacitor. I have been told to replace it with one within 10% of the original but how would I find out what the original was?

 

[hbiss]

how would I find out what the original was?

Didn't you say that you had the (nameplate) spec?

-Hal

 

[NickJoh]

I can't read anything on the capacitor itself. Would it be in the motor nameplate ?
If so I might be able to get to it if I remove an attached electrical box

Photos below are what I can see but perhaps there is more if I remove the box?

 

[Eddie702]

Isn't there a doghouse on the motor with the capacitor inside? If so the UF and voltage rating of the capacitor should be marked on the cap. Other than that you have the motor model or catalog# You could call Century or a local motor shop and they should be able to tell you what cap to use.

 

[ptonsparky]

Any motor shop worth walking into can sell you the proper capacitor. Give them the motor model # etc. pay for the service.

 

[ptonsparky]

A little on line research may get what you need as well.

 

[CoolWill]

I have a 2 HP pool pump that uses a 145 uF start cap and 35 uF for the run. I texted a friend who installs pools and asked him to look at other pumps he has and he had a 2 HP that used 120 uF start and 45 uF run, so I'd bet that anything between 100 and 150 for start and 30 to 50 uF for run would be just fine

 

[NickJoh]

Thank you so much for doing that! I really appreciate it.

 

[ptonsparky]

FWIW, a customer had a chest freezer that would not start. Start Cap tested just a bit low of the 10% value. It made a difference. It passed a typical field test with an ohm meter.

 

[drcampbell]

... could the open run capacitor cause the breaker to trip?

Definitely.

When all is well, there's both a mechanical angular displacement and an electrical phase shift between the start & run windings. One winding will reach peak current (and peak magnetic pull) first. A moment later, the other winding will reach peak pull in a slightly different direction, creating torque.

It's not a balanced rotating magnetic field, as in a three-phase motor, but it's enough to get it started.

When only one winding is energized, it pulls back & forth without developing any torque.

Since the impedance of the start winding is quite low, it opens the breaker pretty quickly.

 

[NickJoh]

Thank you so much for doing that! I really appreciate it.

I have a 30 and a 50 mfd coming in the next two days but "physically" they are too large to fit within the doghouse but I can at least see if they allow the motor to run and then look for capacitors that will fit. I have called Century Electric Motors to find the correct value originally specified and they seemed committed to help me but the motor was made when they were Magnetec (probably around 1993) and the nice lady I spoke with said she needed to do some more research and would call me back. So, I still "hope" to find the correct value.

Is the fact the original run capacitor is so much smaller (physically smaller) than the ones I have on order any indication of its value? The original capacitor measures 1.7" in diameter by 3.5" long whereas the 30 and 50 MFD I have ordered are 2" and 2 1/8 by 4.5". The original 1.7" is a tight fit in the dog house so I am fairly sure neither will fit. I am surprised a 30 year old capacitor would be physically smaller so I am asking if that is any indication it might be a smaller value than we think?

 

[NickJoh]

Definitely.

When all is well, there's both a mechanical angular displacement and an electrical phase shift between the start & run windings. One winding will reach peak current (and peak magnetic pull) first. A moment later, the other winding will reach peak pull in a slightly different direction, creating torque.

It's not a balanced rotating magnetic field, as in a three-phase motor, but it's enough to get it started.

When only one winding is energized, it pulls back & forth without developing any torque.

Since the impedance of the start winding is quite low, it opens the breaker pretty quickly.

Are you referring to run capacitor or just the start capacitor?

 

[NickJoh]

I decided to include photos of the failed run capacitor showing how much physically smaller it is than 30 year newer capacitors I am considering replacing it with. As you see the 1.7" DIAMETER is a tight fit in the doghouse so I am fairly sure both capaciitors I have coming will not fit.

Also, there is nothing that indicates the value on the doghouse. I thought it might be stamped on the capacitor's can and sanded the bottom but no luck. I can read "potected" stamped on the bottom (not protected) which seems odd. Again I am hoping Century will call me back today with a value but if not I am planning to use the 50 mfd to see if will at least start and not trip the breaker.

After that - if it runs - can anyone suggest what I can do to determine if I have too large or too small a value capacitor. It runs a sprinkler pump so I don't think it is heavily loaded.

 

[drcampbell]

Are you referring to run capacitor or just the start capacitor?

Both. And more to the point, the difference between the two.