NickJoh
Member
- Location
- Columbia, SC
- Occupation
- Retired Engineer
Thanks . I re-read your explanation and now think I understand.
What are failure characteristics of an open run capacitor when the breaker is tripping?
- Thread starter NickJoh
- Start date
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
If a motor only has one cap, is a starting cap. If it has two caps, one is starting, the other is running. Jet pumps wouldn’t need a Cap-Start / Cap-Run (CSCR) type motor, they use a CSIR (Cap-Start / Induction-Run) motor type, so that’s a Start cap.
If a Start cap is failed, the motor will not start and will trip the breaker. The question is, why is the Start cap bad? The usual cause is that the centrifugal switch in the motor has welded, so the cap was left in the circuit all the time, and start caps are not rated to do so. This means that if all you do is replace the cap, it will fail in short order. You likely need to fix the centrifugal switch first.
If a Start cap is failed, the motor will not start and will trip the breaker. The question is, why is the Start cap bad? The usual cause is that the centrifugal switch in the motor has welded, so the cap was left in the circuit all the time, and start caps are not rated to do so. This means that if all you do is replace the cap, it will fail in short order. You likely need to fix the centrifugal switch first.
NickJoh
Member
- Location
- Columbia, SC
- Occupation
- Retired Engineer
Sorry I did not make it clear" My start capacitor measures 138 mfd so it is good but the other capacitor -the run capacitor - is open. I confused things by reporting the motor says "jet pump duty" but it is not a jet pump. It is a Burks 20AW6 pump. The run capacitor is so old and totally black with hardened rust or scale I can't read any markings and so far have not been able to get its value from Century, I tried cleaning it with alcohol and finally sanded the bottom looking for stamped imprits to no avail, Thanks
NickJoh
Member
- Location
- Columbia, SC
- Occupation
- Retired Engineer
So far I have had no luck finding the correct value for the run capacitor >
I have both a 30 and a 50 MFD capacitor coming to try.
I assume I should try the larger one (50 MFD) first?
As much as it seems to matter I am very surprised it is not required to be on the motor's nameplate or stamped on the doghouse or "somewhere"
Again here is the motor info if anyone has access to 30 year old data:
MOTOR: Magnetec Century Part 8-164475-01 Serial # BP6-278
The start capacitor measures 138 MFD so it is good.
I have both a 30 and a 50 MFD capacitor coming to try.
I assume I should try the larger one (50 MFD) first?
As much as it seems to matter I am very surprised it is not required to be on the motor's nameplate or stamped on the doghouse or "somewhere"
Again here is the motor info if anyone has access to 30 year old data:
MOTOR: Magnetec Century Part 8-164475-01 Serial # BP6-278
The start capacitor measures 138 MFD so it is good.
ptonsparky
Tom
- Occupation
- EC - retired
30 years old.
Have you used a Megger on it yet?
Bearings good?
Have you used a Megger on it yet?
Bearings good?
NickJoh
Member
- Location
- Columbia, SC
- Occupation
- Retired Engineer
Other than verifying the start cap is fine (138 MFD) and the run cap is open I have done nothing. There was nothing strange - no bearing noise - coming from the pump which started up fine and ran the sprinklers for at least awhile this year but since it runs at night I really can't say it didn't develop. My plan was to replace the open run cap and see if it would start (not trip the breaker) and if that worked then I hoped to come back with the proper value run capacitor. If it doesn't work I will replace the pump and motor.
NickJoh
Member
- Location
- Columbia, SC
- Occupation
- Retired Engineer
I installed a 30 mfd capacitor and the motor started and the sprinkler pump ran. Thanks to everyone who helped me. It won't fit in the doghouse so I have ordered another (a 35 MFD) capacitor that will fit. If any problems surface with this "guessed at" value I plan to report it here.
Thanks again, Nick
Thanks again, Nick
NickJoh
Member
- Location
- Columbia, SC
- Occupation
- Retired Engineer
Thank you again! Your post was by far the most helpful. Definitely 5 Star on a scale of 1 to 5. I couldn't allow it to remain buried among all the others some of which would not even rate a 1,
I followed your advice and ordered a 30 and a 50 MFD capacitor. The 30 arrived first and the motor ran!
Thank you for your advice and the time you took and thank your friend for his input too.
kwired
Electron manager
- Location
- NE Nebraska
My experience has been CSIR is usually 1.0 HP and less, occasionally 1.5 HP. 2.0 HP and up almost always is CSCR.
This is the way it often is with general purpose motors, it can vary with certain definite purpose motors though.
Only one capacitor can possibly be a PSC motor, though I never seen a PSC as a jet pump motor. I'd expect a 2.0 HP jet pump to be CSCR in most instances 1.5 HP could be either CSCR or CSIR. 1.0 and less probably is CSIR.
If start capacitor or centrifugal switch is bad then the motor depends on the run capacitor when there is one to help develop enough torque to start the motor. Loads that don't need high starting torque may start in this situation but will be much slower acceleration than if the start capacitor is working. If it takes too long to accelerate I can see it possibly tripping either breaker or motor overload though.
If run capacitor is bad but start capacitor is good the motor will start but once centrifugal switch kicks out there isn't enough extra phase shift caused by aux winding to keep motor up to speed. It will run as a rather crude CSIR motor but not for very long as it will not be able to maintain normal speed and will draw higher current than usual, not enough to cause rapid trip of overload or circuit breaker but depending on load characteristics can have a pretty wide range of run time before something trips.